Weather Forecasting Explained: An Easy Reference Guide (England's Living History) (Countryside Books Reference) 1846741971, 9781846741975

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Introduction Section 1: he Weather Basic Rules Global Weather Patterns Clouds and Rain How Our Weather Moves Section 2: Forecasting he Met Oice and TV Forecasts DIY Forecasting Unusual Clouds and Efects Old Tales Past Extremes he Future Back Matter Bibliography and Useful Websites Also Available

WEATHER FORECASTING MADE SIMPLE

STAN YORKE

COUNTRYSIDE BOOKS NEWBURY BERKSHIRE

Publisher Information First published 2010 © Stan Yorke 2010 Digital edition converted and distibuted in 2011 by Andrews UK Limited www.andrewsuk.com All rights reserved. No reproduction permitted without the prior permission of the publisher: COUNTRYSIDE BOOKS 3 Catherine Road Newbury, Berkshire To view our complete range of books, please visit us at www.countrysidebooks.co.uk

Dedicated to the male members of the Yorke family who spend far too much time shouting at the TV weather forecasts and their ever patient wives who put up with it! Photographs and drawings by Margaret & Stan Yorke except those on page 24 (top), page 27, page 31 and page 55 (bottom) which are courtesy of Shutterstock Cover pictures (except bottom right) courtesy of Kevin FitzmauriceBrown, www.the-picture-collection.eu Designed by Peter Davies, Nautilus Design Produced through MRM Associates Ltd., Reading

Weather Forecasting Made Simple

Introduction

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ankind’s concern with the weather goes far back in time and for good reason: if the weather washed out your crops or baked them dry, you starved. It was truly a matter of life or death to most of the world’s population, so it is easy to imagine the appeal of being able to forecast what the weather was going to bring. Over the centuries this has p ro d u ced a q u an t i t y o f al m o st unbelievable nonsense interwoven with some quite shrewd observations.

As science grew in understanding it was inevitable that man would study the weather and try to understand it better. What we did learn during the 20th century was that it is a very complex subject and it wasn’t until the advent of satellites and computers that we really started to appreciate the size of the problem. Even today, using some of the most powerful computers available, we still struggle to achieve accurate forecasts. But why, oh why, can’t we get it right?

Introduction I’m afraid that part of the answer is that we are still dealing with predictions. One could ask our forecasters, ‘Why, if you can’t get it right, do you constantly pretend that you can, that’s what fortune tellers do, isn’t it?’ Just as with a fortune teller, if some of their predictions come true we smile and think ‘That was lucky’, but we don’t put money on it! The forecasters are, alas, burdened with well remembered mistakes which overshadow our attitude to their constant claims of ‘We’re getting better now’. In fact they are, but they still have a long way to go. So what can this little book do to help? Well, first I will try and explain what the weather actually is, knowledge that will enable us to get the most from Meteorological Office and television forecasts. And here, first of all, a few definitions: weather is the state of the atmosphere at any

particular time and location; climate is the average typical weather for a particular area (usually based on records of 30 years or more); and lastly, meteorology is the scientific study of the weather. In the second half of the book, I’ll look at what we can do ourselves to forecast the weather, which turns out to be quite a lot. How far you want to take it depends on you, of course, but the following pages might just set you off along the amateur meteorology path. One unexpected by-product for me has been to discover just how attractive the sky and clouds can be and I hope some of the pictures in the book will tempt you to look upwards more often. And if it soothes the cries of ‘Why can’t they get it right?’, I shall feel well rewarded. Stan Yorke

Section 1

THE WEATHER Basic Rules

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early all the energy on Earth comes from the sun, a typical m i d d l e - a g e d s t a r, w h i c h contains 99.9% of our solar system’s mass. Some 46% of its radiation is light and a similar amount is near infra red, which we perceive as heat. The rest is in the ultra violet region, which causes us sunburn. The sun also sends out random solar winds, vast eruptions of protons and electrons which are deflected around the Earth by our magnetic field and which we sometimes see as auroras. For our purposes, what all this means is that our weather and our seasons are initially determined by factors well outside the Earth’s atmosphere, and some knowledge of what happens ‘out there’ is helpful in trying to understand the complexities of weather forecasting.

The Seasons The Earth travels around the sun in an elliptical orbit taking 365 days to complete one circuit, our year. The Earth also spins on its axis once every 24 hours, giving us night and day. However, this axis is tilted at 23.5° to our orbit around the sun, which puts the sun over the northern latitudes in summer and over the south in winter, producing our seasons. At any one time there is always an area of the Earth that is receiving the

full energy radiated from the sun – but as the Earth revolves and moves along its annual orbit this area is also moving in a giant spiral between the Tropic of Cancer (summer in the northern hemisphere) and the Tropic of Capricorn (summer in the southern hemisphere). These changes give us night and day and the steady change from summer to winter and back that we are familiar with, but the effect of this ever-changing radiation on the seas and atmosphere is far more dramatic. Something like half of the sun’s radiation is absorbed by the land masses and the sea, the rest is absorbed by the atmosphere and cloud systems or reflected directly back into space. Of the energy absorbed, all is eventually radiated back into space either directly from the Earth’s surface (normally at night) or via the clouds, which are made of warmed-up water vapour from the oceans. Over the long term we lose the same amount of heat that we gain from the sun, so that the Earth, as a whole, stays basically constant. It is the atmospheric conditions that control this delicate balancing act and it is man’s ability to disturb the atmosphere that is at the root of our current concerns over global warming. It is easy to forget just how thin a

Basic Rules

Seasons diagram. If you follow the earth's orbit you will see how, in winter, we are in sunlight for a shorter period than we are in summer.

layer of atmosphere we inhabit. Whilst technically our atmosphere extends upwards above us for over 100 miles, most of this is quite devoid of air. Man can breath in only the first 2 miles of our atmosphere, Exosphere Thermosphere Mesosphere Stratosphere Troposphere

Over 300 miles 50 to 300 miles 30 to 50 miles 6 to 30 miles 0 to 8 miles (UK)

which also contains the vast majority of our weather. The next drawing shows the first 15 miles and below are the names given to the layers, their height above us, and what they contain: Satellites Auroras Spacecraft & meteors Ozone layer Most of our weather

Weather Forecasting Made Simple The top of the troposphere is called the tropopause and varies in height around the world from around 12 miles at the equator to just over 4 miles at the poles. Note the steady drop in temperature as the altitude increases, but which reverses above the tropopause due to the absorption of ultraviolet radiation by the ozone layer. Most of our clouds form below

20,000 ft, but given the right conditions massive cumulonimbus clouds can climb up to the tropopause. The air pressure also drops with altitude as there is less and less air pushing down. The average air pressure at sea level is 1013.2 mb (milli-bar) and at around 20 miles the air pressure is almost zero.

Simplified slice through the first 15 miles of our atmosphere.

Global Weather Patterns

Global Weather Patterns

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espite massive local disruptions to the weather caused by land masses and mountain ranges, there are basic underlying weather systems that dominate the Earth’s weather patterns. The driving force for these is the central ring around the Earth that receives the maximum solar radiation. It is easiest to refer to this just as the equator though, in practice, the ring moves further north in summer and towards the south in winter.

The Winds This band of high, received radiation causes the air to heat up and rise – like nearly all materials air expands when heated and thus becomes less dense and lighter. So, this air naturally starts to rise and at the surface, where we are, we see this reduction in air pressure as a series of ‘lows’.

The rising air eventually reaches the tropopause and having lost much of its heat it has also lost its enthusiasm for climbing and so it spreads out to the north and south. Eventually it drops back towards the Earth and then turns to run over the Earth’s surface back towards the equator where the heat will set it off again. These bands of falling, cooler air produce areas of high pressure around the world. The returning winds heading towards the equator are the famous Trade Winds. As with the centre of any low pressure system, there is very little wind crossing the surface at the equator (the air is simply rising) and this is the cause of the, so-called, doldrums, when ships are becalmed at sea. This rolling wind system extends around the Earth and is relatively stable and constant.

Basic Equatorial winds, the engine of the world's weather.

Weather Forecasting Made Simple

Basic theoretical ground level wind movements in the absence of any disturbances from land masses.

We now know there are four further rolling wind systems, two to the north and two to the south of the equator. And there is one other factor that affects these general winds, called the Coriolis effect. Due to the rotation of the Earth’s surface these general surface winds are steered away from a simple, straight north-south direction. In the northern hemisphere the north to south Trade Winds in fact run northeast to south-west. The North Atlantic surface winds, which dominate our weather in the UK, belong to the next band of wind systems and at low altitudes are theoretically south to north in direction but due to the Coriolis effect they actually run south-west to north-east.

In Theory If there were no land masses or variations in the temperature of the sea, these wind systems would be dominant and constant. However, if you travelled around the equator you would find land beneath you for around a quarter of the journey. Going south to a latitude of 30°S, the proportion drops to around an eighth, but head down the globe to 60°S and there is no land mass at all. This relative freedom from land masses gives the southern hemisphere a generally more reliable weather pattern than the northern hemisphere, where the proportions of land mass to ocean rise to a half at 30°N and 60%

Global Weather Patterns

Movement of the main Atlantic surface currents. There is an uninterrupted flow around the Antarctic which our two main flows join.

at 60°N (the UK sits between 50° and 58°N). This extra land mass contributes to our more volatile weather patterns.

The Oceans The sea also has a system of generalised flows though, unlike the air, its boundaries – the land masses – are real and very solid. Water is also denser than air and so takes longer to heat and cool, creating much larger, slower moving systems. The equatorial band still provides most of the heating but due to the absence of land masses at around 60°S the sea is able to flow in a cold, uninterrupted easterly circle.

The largest of these flows – and the one which affects our weather – is the North Atlantic Gulf Stream, which moves an incredible 30 billion gallons of water every second! Less water vapour rises from cold currents and the air above these is generally dry, whereas the warmer seas provide the bulk of the water vapour taken up by the atmosphere which in turn produces clouds and rain. Water vapour is pure water, free from salts, and except for passing through man’s polluted skies where it becomes very slightly acidic, it remains pure until it trickles over the land on its journey back to the sea. About onethirtieth (3%) of the Earth’s water is held in a pure form in the ice caps, glaciers and snowfields. Surprisingly, another 7% of the stored pure water resides beneath the surface as ground water. The quantity of water vapour taken up from the sea is amazing. In temperate climates an area of just 2 square miles evaporates 2 million gallons of water into water vapour every day! We must remember that we have only looked at the air flows at the surface, and whilst these are the ones that we feel and see, the higher driving air currents, including the jet streams, also influence our weather. This, then, has established the very general patterns of water and air movements around the Earth, which will do for now, because next I want to look at water vapour and clouds.

Weather Forecasting Made Simple

Clouds and Rain

Cumulus forming over Henley-on-Thames during a sunny morning showing the critical height at which condensation has started due to the lower temperature a few hundred feet up. The still rising air plus the internal warming is allowing these clouds to grow upwards. The wind blowing from left to right has steered these clouds into rows or avenues.

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ater vapour, the basis of clouds and rain, is the gaseous form of water; it is invisible and is produced from water when certain conditions of temperature and pressure are met. When you watch the pavements dry after rain, have you ever wondered where the water has actually gone? Well, it’s turned into water vapour and risen and mixed with the air. If you boil a kettle on one side of your kitchen on a cold day, you will soon

see water droplets condensing on the w i n d o w s . Wa t e r h a s t r a v e l l e d throughout the room as invisible vapour and is condensing back to its liquid form on cold surfaces. Don’t, though, confuse vapour with steam, which is visible and composed of relatively large droplets of water. Watch and you will see that any steam slowly disappears – it is ‘drying’ just as the pavements did, by changing into water vapour. This raises the interesting question

Clouds and Rain of how much liquid water in its invisible vapour form can we get into a fixed volume of air? The amount of water vapour held in the air is its humidity, and the upper limit is called the dew point, above which the vapour starts to reform, or condense, back to water droplets. How much water vapour is held and the actual dew point, depend on the air temperature. Please don’t panic: the simple idea that water vapour will reform as water, dependent on its temperature, is all we need to know to understand why and when it will rain. This change from vapour to liquid is very subtle and produces minute water droplets which we see as mist. Even close to saturation (above 90% relative humidity) the air is still only about 4% water and these first droplets are extremely small. They actually depend on microscopic particles being present in the air which act as seeds around which the water droplets form. These particles are usually dust, pollen or even bacteria. It is only when the rate of condensing increases that the droplets start to bump into each other, forming larger drops until they reach a size that is too big to be held up in suspension. Then they fall as rain. A single raindrop contains around a million ‘first stage’ mist droplets. This change from one state to another is not free of cost. To change from water to vapour takes heat – absorbed from the original surface and put into the resultant vapour as latent heat. This is why we sweat: the evaporation of the liquid on our skin cools our skin by removing heat into the invisible but ‘warmer’ water vapour.

Clouds When water vapour rises into ever cooler air, it will eventually reach its dew point and then condense into extremely small water droplets. It is because this dew point temperature tends to lie at one specific height that we see the cloud base at the same height, giving a strange layer effect. These initial droplets are easily kept aloft by the same gentle updrafts that lifted the vapour in the first place. The latent heat trapped in the vapour is released, however, when the water vapour condenses back into water and this extra heating within a cloud mass is very important in keeping the cloud warmer than its surrounding air – thus it will continue to rise. This rising internal air and vapour is what causes the buoyant, ever-changing fluffy tops of cumulus clouds. The more heat from the sun, the more vapour rises, and the more vigorously the cloud will grow.

It’s All Relative It is common practice to refer to ‘warm air’ or ‘cold air’ and in forecasts you will often hear talk of warm or cold fronts. This can be misleading, because although to us humans ‘warm’ and ‘cold’ mean quite specific temperatures, in the physics of gases and vapours it is the difference between them that matters. As we can easily imagine, air warmed to 25°C will happily rise through an air mass that is only 20°C; but so will air at an icy -10°C when surrounded by air at an even colder -15°C, though neither temperature sounds very warm! None of the constituent gases that make up our air will freeze or condense even at temperatures as low

Weather Forecasting Made Simple as -60°C, so the air itself will always obey the ‘rise and fall’ temperature rules virtually anywhere on Earth. Water vapour will also obey these basic rules until it condenses at its dew point. So, if the air within the cloud is rising, it takes a lot of tiny water droplets to join up before they are heavy enough to oppose this upward air direction and start to fall as rain. The base area of a cloud is dark simply because the cloud itself is preventing sunlight from penetrating down to its base. In temperate climates, like the UK, most of the rain starts as snow crystals forming at the top of the clouds (the High Clouds 0 Cirrus 1 Cirrocumulus 2 Cirrostratus

clouds that have a layered appearance even above the 6,500 ft limit. These three height ranges are generally referred to as high level, medium level and low level. Two further words are used to add a description of the clouds: Cumulus refers to a shape that is round, fluffy and looks rather as though the cloud has been piled up into a heap, and Nimbus simply refers to rain-bearing clouds. At the 1896 International Meteorological Congress the following ten cloud classifications were established, though not quite in the order we use them today, as shown below:

Medium Height clouds 3 Altocumulus 4 Altostratus 5 Nimbostratus

coldest part), but as they fall and grow the snow melts and leaves the bottom of the clouds as rain.

Cloud Types Clouds form at different heights and in many different shapes and sizes. As long ago as 1803 a Quaker chemist and amateur meteorologist by the name of Luke Howard wrote a paper setting out the basic classifications. There are three names used to indicate the height of the clouds. Cirrus refers to high clouds above 16,500 ft, Alto to heights of between 6,500 and 16,500 ft, and Stratus to clouds below 6,500 ft. As this last name implies it can also be applied to

Low clouds 6 Stratocumulus 7 Stratus 8 Cumulus 9 Cumulonimbus

The phrase, ‘being on Cloud Nine’, is believed to have been inspired by this listing – No. 9, Cumulonimbus, being the massive clouds that can grow from low levels right to the top of the troposphere. Each of these ten classifications is subdivided into species and varieties, giving over 50 individual types! In this book I have chosen familiar examples of the ten basic forms as seen in the UK and only where it seems to be appropriate have I used the more detailed name. It can be very difficult to judge the type of cloud one sees. Within these very basic categories there are many variations and subdivisions.

Clouds and Rain Unfortunately, there is a fair amount of personal interpretation involved too, as each group can display a wonderful variety of shapes. Basic cumulus and cirrus clouds are by far the most readily recognised. It can also be difficult to judge the height of clouds, particularly on

photographs where one loses the intuitive ability to scan around and look up and down to get some perspective. Some of the following pictures of cloud formations are often associated with particular types of approaching weather.

Cirrus Very high clouds with fine, teased out filaments of ice crystals often called ‘mares’ tails’. Generally, they indicate a spell of fine weather.

Weather Forecasting Made Simple

Another good sign for continued fine weather is when aircraft condensation trails (contrails) which form from the water vapour given out by a jet engine, fade quickly behind the aircraft.

Contrails that slowly spread out and stay, sometimes for hours, are not good news. A rain filled warm front is probably only 12 to 18 hours away.

Clouds and Rain

Another clue to an approaching low pressure area and rain is when cirrus spreads out into wide layers often joining up into large areas as here.

Jet Stream Cirrus Occasionally one sees the direct effect of the jet stream blowing cirrus clouds across the sky. Many suggest that this too, forewarns of rain within 12 hours.

Weather Forecasting Made Simple

Cirrostratus Cirrus that has grown and spread to cover large areas. The dark line through the cloud is a distrail (dissipation trail – the opposite to a contrail) where an aircraft has flown through the cloud layer, leaving a ‘gap’.

Cirrocumulus Referring to high clouds that have developed a thicker or lumpy appearance.

Clouds and Rain

Altostratus Mid level cloud that has formed white or grey layers; the sun's position can still just about be seen above the photographer. It rarely produces much rain but if followed by a cold front, it can develop into thicker, darker nimbostratus within 6 to 12 hours, which is not good news.

Altocumulus Mid level cloud that has developed into separate clouds, allowing the sun to shine through the gaps. This type of cloud only occasionally produces rain but it can herald a change to wetter weather within 12 to 24 hours.

Weather Forecasting Made Simple

Altocumulus This group of clouds is one of the largest, with many variations. This version is known as Statiformis and produces a wide range of mid height shapes, all made of small clouds tightly packed in a layer.

Nimbostratus Where altostratus has thickened and grown higher, producing rain. These clouds in the lower to mid height range provide most of our long-lasting rain and drizzle – possibly the most depressing clouds of all.

Clouds and Rain

Stratus Simply low level altostratus through which the sun or moon can still be clearly made out.

Stratocumulus Separate low level clouds where the cumulus has stopped growing, giving a definite height and thickness. The most common cloud type on earth but they rarely bring rain.

Weather Forecasting Made Simple

Cumulus Fractus The smallest of the cumulus cloud group, individual fluffy clouds which carry no rain at all.

Cumulus Separate small fluffy clouds starting to come together and often referred to as ‘fine weather clouds’.

Clouds and Rain

Cumulus Congestus where individual turrets grow upwards on their own. These turrets can often exceed one mile in height, dependent on the sunlight and the moisture within the base cloud.

Cumulus Humilis If there is continued warmth, cumulus clouds will start to grow in overall size and height. Provided they don’t grow any higher, the weather should stay fine for at least 12 hours.

Weather Forecasting Made Simple

Cumulonimbus The final form of ever-growing cumulus clouds. These are the classic clouds of heavy rain and thunder which can grow to enormous heights, often arriving in the evening, having spent all the afternoon growing ever higher.

Confusion! It is fairly easy to identify high cirrus clouds by their thin wispy shape and low down are the unmistakable signs of cumulus clouds bubbling up from near ground level. It’s the mid level clouds that are difficult to judge.

Clouds and Rain Fog and Mist

Rain

Fog is, in effect, cloud that has formed near the ground, helped by the plentiful dust particles that seed the water droplets. Mist is simply a less dense version, which by definition does not limit visibility to below 1 km. Fog and, particularly, mists show just how delicate and small the differences in temperature are that affect the air’s ability to retain water vapour. Both occur when the temperatures near to the ground are lower than the air above, typically after a clear, cold night during which the ground has lost its warmth by radiation but before the sun has risen sufficiently to inject meaningful quantities of heat at ground level.

Clouds, formed of the minute droplets of water from condensing vapour, are held aloft by updrafts. The higher the vapour rises, the colder the surrounding air and the more condensing will take place. These are the rising air currents that form low pressure areas where warmer air naturally rises. If the clouds are not too deep but are still condensing large quantities of vapour (nimbostratus), the inevitable contact between the crowded water droplets will form a larger size (typically ¼ to ½ mm in diameter) which will eventually become too heavy for the updraft to support. These will fall gently from the cloud as fine rain or drizzle.

Early morning fog over the River Severn with thinner mist drifting towards us across the fields.

Weather Forecasting Made Simple If the cloud structure is much taller (cumulonimbus), then the descending small droplets usually form as ice crystals. These spend much more time descending within the cloud, which in turn means they collide with more and more droplets before reaching the base of the cloud, by which time the ice has melted. These larger droplets (typically ½ to 2 mm in diameter) are weightier and fall faster as heavy rain. In winter when the cloud itself is cold and the surrounding air even colder, the descending ice crystals will

continue to build but will not melt, and so arrive as snow. Snow - great fun for the children yet it somehow always manages to catch the rest of us out! There are many types of snow depending on the shape and size of the original crystals. These factors in turn depend on the conditions inside the clouds when they were formed. Very fine, dry snow known as ‘diamond dust’ brought chaos to our rail network in 1991 and again to the Euro Star trains in France in 2009.

Clouds and Rain

Hail, fortunately rare in the UK but very dramatic and painful!

Hail

Ice

Sometimes the droplets will freeze solid within the lower parts of the cloud and can be carried back up inside the cloud by turbulent updrafts. They then repeat their ‘falling through the cloud’ journey, collecting more water en route, which again freezes. Dependent on how vigorous the air updrafts are, the frozen droplets will eventually fall from the cloud as sleet or hail. Though rare in the UK, hailstones can repeat their circuits through the cloud many times, reaching alarming sizes. The largest recorded hailstones fell in Bangladesh in 1986 and weighed 1 kg each.

Yet another variation on rain occurs when the ground level air is well below freezing. If the air is particularly clean, the rain droplets will not change to solid ice drops but will become super-cooled to temperatures well below 0°C. On contact with ground level objects like trees, railway lines and electric cables, these droplets immediately turn to ice. It is this ice that causes chaos, pulling down cables and power lines. Snow which has passed through warmer layers of air on its descent can be slightly wet as it reaches the ground and again may freeze and turn to ice on contact. This is what is happening

Weather Forecasting Made Simple Ice visits us virtually every year either on very cold road surfaces or, as here, as dripping water frozen in time on a canal lock gate. Ice expands when it forms and it is this effect that breaks open road surfaces by filling small crevices as water and then freezing and expanding.

when we hear the authorities complaining of ‘the wrong kind of snow’. We can now perhaps sense the way that small changes in the air and cloud temperatures which we are normally unaware of, can cause completely different results.

Frost When cold air lies over surfaces such as grass and plants it can lead to frost. Because the tiny hairs on blades of grass and other A cold December morning where moisture drifting over very cold grass has frozen onto the fine hairs on the edges of low lying vegetation.

Clouds and Rain

Hoar frost on the same morning showing the delicate build-up of ice crystals. Note how nearby leaves just a few inches further into the bushes have not cooled quite so much and are showing almost no frost at all.

fine plant structures cool quickly, any water vapour in the air condenses out onto these ‘nuclei’ first, forming an attractive coating of ice. If the ground temperature is just above zero, dew will form instead. (Right) Exceptional hoar frost caused by very cold water vapour being gently moved by the wind and meeting just one side of the tree.

Weather Forecasting Made Simple Rainbows One charming effect of rain is the rainbow. When sunlight passes through a raindrop it is refracted into its component colours which, in turn, are scattered. This effect is, however, dependent on the light rays entering and leaving the raindrop at quite specific angles (as indeed it is in a glass prism). Thus, when we look into rain with sunlight passing through it we see the refracted colours at one particular angle. This constant angle to the rain droplets means the rainbow describes a circle, but we never see the bottom

of the circle because the ground gets in the way, leaving us to view the top half – the rainbow. A secondary, much fainter rainbow occurs at a slightly larger angle, outside the primary one. You will see that the secondary rainbow has its colours in the opposite order to the main one. The classic rainbow with its much fainter secondary rainbow slightly higher. The brightness of the red edge is determined by the size of the rain droplets and can vary widely.

Clouds and Rain

Thunderstorms A striking contrast to the gentle rainbow is the rage and raw energy displayed in a thunderstorm. The thunder is a by-product of the real action, which is the lightning. Lightning is still not fully understood but is the result of massive electrical charges building up in different parts of large, deep clouds (cumulonimbus). Eventually this charge difference can reach up to 100 million volts and the intervening air breaks down, allowing the vast energy to discharge. Discharges within the clouds themselves produce sheet lightning whilst discharges to earth are generalised as fork lightning. The lightning is not a single flash as our eyes tend to see it but instead is a series of very rapid discharges. Some move down from the cloud to earth and others run from earth towards the cloud.

A splendid display of lightning formed from a great number of strikes which occur too quickly for our eyes to separate but have been caught here by a long camera exposure.

The effect of these discharges on the air through which they travel is to produce astonishingly high temperatures, sometimes up to 30,000°C. It is this that causes the nearby air particles to literally explode, producing the noise we hear as thunder. Sound travels much slower than light which is why we see the flash of lightning before we hear the thunder. The lightning is roughly 1 mile away for every 5 seconds of delay. Incidentally, lightning does strike the same place twice. Skyscrapers regularly get hit several times a year and in one famous storm the Empire State Building was hit 15 times in as many minutes!

Weather Forecasting Made Simple

How Our Weather Moves

S

ome time back I said ‘that will do for now’, always an ominous sign that life will soon get more complex! We have seen that there are areas where energy from the sun warms the air, which in turn starts to expand and rise, creating an area of low pressure. Similarly, where the same air flow sinks back towards the Earth’s surface it causes high pressure. Unfortunately these highs and lows and the wind systems between them are rather complex and variable, particularly in the northern hemisphere. If we look again at our sketch of the Earth but now add more realistic detail, we see that the highs and lows do not follow simple lines around the Earth but instead form great, vaguely circular areas. These in turn differ from season to season, and indeed from year to year. The weather charts on the opposite page show typically where major high and low pressure centres form, but between these there is a constant flow of local highs and lows that move in the general direction of the dominant winds and it is these that bring our ever-changing weather. Due in part to the Coriolis effect (see page 10), in the northern hemisphere the air associated with a high pressure region rotates clockwise and around a low pressure area it moves anticlockwise. However, over land the low level winds are affected by friction

with the Earth’s surface, particularly by contact with hills and mountains, which creates considerable turbulence. One visible effect of this is that low level and medium level clouds may be seen moving in different directions, which can be confusing. All charts showing highs and lows are, in fact, maps of the air pressure, where each line represents a constant pressure in milli bars. The Met Office draw these lines at 4 m bar spacing, working up or down from 1,000 mb.

Fronts The other activity is concerned with the temperature of the air masses moving around under the influence of the major highs and lows. It is inevitable that between these systems the air from one will meet the air from the other, causing a ‘front’. The basic rule is that the cooler air will stay near the ground (being heavier) whilst the warmer air will rise over it. This is, however, no ‘gentlemen’s excuse me’. The warmer air is thrust upwards into ever cooler air and, as we have seen, clouds quickly form. The clashes often form local lows which then increase in size and severity. It is these fronts that produce most of the bad weather associated with a low, with great arms of cloud being flung out from the centre. In a warm front the warmer air is catching up with colder air, over

How Our Weather Moves

This shows the generalised distribution of highs and lows for January. The pattern stays fairly true to our basic chart with lows forming along 60°S, the equator and 60°N. Highs form along 30°N and 30°S. The surprise is the Asian land mass which has drawn the 30° high far to the north. Below is the picture in July and again the same vast Asian land mass has now drawn the Equatorial low further north.

Weather Forecasting Made Simple which it gently rises. A cold front, usually associated with a depression, invariably moves faster and has a steeper rise which thrusts under the warm air. A depression is an alternative name given to lows that form between the 30° and 60° latitudes and can be up to 2,000 miles wide.

high has formed it tends to move very slowly and can become a ‘blocking high’ with approaching lows forced to skirt around the outside edges. The severe cold in the winter of 2009/10 was caused by a series of blocking highs which settled over Scotland and lasted for many weeks.

Inversions

On The Move

Another result of masses of warm air being lifted over cold air is that this forms a local, low level ‘inversion’. Though fronts are always drawn as fairly steep slopes, in reality this slope is very gentle and it can take between 1 and 2 miles for the warm air to be lifted just 1,000 ft. This cold/warm interface thus spreads out over a large area, and having the warm air on top of the cold means that this is opposite to the normal higher equals colder rule and is called an inversion. Now as relatively warm, moist air rises from the ground, it will reach the inversion where the air above it is also warm, so the natural rising stops. Unless the rising air is very vigorous it will be prevented from rising any further, forming low, boring stratus clouds. Highs, on the other hand, are much more well behaved. Here the air is falling down to Earth, creating higher pressure as it piles up on top. This compression also causes heating which helps evaporate any moisture present, giving the familiar clear weather of a high. Unfortunately this same falling air and rising vapour can also form an inversion, usually in winter and at around the 6,000 ft level, which traps a layer of dull cloud. Though no rain is formed these dull highs can sometimes last for days. Once a

The next sketches show the progress of a typical ‘low’ across the UK. The low

Our story starts with the low just off south-west Wales. Note the wind directions over Cumbria (shown as a white star). The two bands of rain associated with the warm and cold fronts are bringing rain from the south coast, through London, the Midlands and north Wales, whilst the warm front is soaking Cumbria and Newcastle.

How Our Weather Moves can be looked on as a self-contained system which is being pushed slowly north-east by the basic North Atlantic south-westerly winds. Whilst a very simple example, it shows how the winds can sometimes seem to be changing in a quite illogical way compared to the weather. In practice, the Atlantic is full of highs and lows, all usually progressing in a general northeasterly direction.

Some three hours later and the low is centred north of Anglesey. The rain which was falling over the Midlands is now over the Scottish lowlands and Kent. Note the wind over Cumbria.

The Jet Stream There are several around the world – narrow ‘rivers’ of fast moving air sitting around 10 miles above us – but it is the one that flows from America across the Atlantic to Europe which adds energy to and helps to steer the Atlantic weather. The jet stream varies from complete absence to an ever-changing twisting flow. Sometimes it flows smoothly for weeks, even months and if we are lucky

Another three hours and the low has moved to the north-east coast with eastern Scotland, Sussex and Kent under rain. Again, note the wind direction over Cumbria – it has turned through 180°! Yet all the time the weather system has gently moved north-east in a constant direction.

Weather Forecasting Made Simple it steers most of the lows away above Scotland, giving the UK a good spell of sunshine. Some years it drops south and we have a very wet and disturbed season. Just another input to add to the

bewildering problems of meteorology! Anticipating the position of the jet stream has proved very difficult and is often the root cause of apparent errors in forecasting.

A superb display of cirrostratus being driven east across the sky by the jet stream. If you can clearly see this type of cloud moving just by looking at it, then it is probably travelling at least 80 mph, often much more.

Section 2

FORECASTING The Met Office & TV Forecasts

T

he desire to forecast the weather has always been the driving force behind the study of our weather systems. As science gained ground during the Renaissance many famous names applied their minds to the study of the weather. Galileo led the interest in studying our Earth – he invented the thermometer and greatly improved telescopes. Convinced that much of what happened could be explained by science and not by faith, he was duly accused of heresy and was forced to end his years under virtual house arrest (he was formally pardoned by the Roman Catholic Church in 1992!). Around 1600, European explorers were venturing far and wide, and men like Columbus and Magellan developed the first useful records of the weather they encountered on their voyages. Torricelli invented the barometer to measure air pressure, Gabriel Fahrenheit developed his temperature scale and in 1742 Anders Celsius introduced a new scale which had 0° as the boiling point of water and 100° as the freezing point. Just three years later, this was reversed by Carolus Linnaeus but is still called the Celsius scale to this day. By the 1800s weather stations were

becoming widespread, permitting maps to be made which enabled the first serious study of the systems that govern our weather. Just as knowledge of the trade winds was once a guarded commercial secret, the latest weather knowledge was always treated as a military secret. We tend to forget that HMS Beagle was on a governmentsponsored trip to study the weather – Darwin was merely a secondary passenger taken along to help with observations and to gather botanical information.

The Met Office The Meteorological Office was set up in London in 1854 as a department of the Board of Trade, under Captain Robert FitzRoy, who had been the commander of HMS Beagle on Darwin’s famous expedition. Its prime target was to create forecasts for shipping, particularly for the Navy. By 1861 it was sending telegraph warnings of bad weather to harbourmasters, who would raise warning signs on a pole near the harbour entrances. By 1879 it was supplying forecasts to the press, who have published them ever since. High level balloons and early aircraft enabled us to see the clouds

Weather Forecasting Made Simple from above for the first time, as well as allowing high level measurement of winds and temperature. Today we can get this same thrill as we fly abroad, though turbulence reminds us of just how much activity there is high above the earth! Weather forecasting on the radio started in 1922 and apart from a very short-lived test transmission, television forecasts began in July 1949, using simple charts. On 11 January 1954 George Cowling of the Met Office made the first five-minute live weather forecast, a job he shared with T.H. Clifton. The first computer started work in the Bracknell HQ in 1962 and a steady flow of ever more powerful computers has increased prediction accuracy and improved the graphics ever since. There are two fundamentally different approaches at work behind the modern forecast. The reason for the massive computer power is that the Met Office has long believed in trying to create a mathematical model of the weather systems, which would then show how the weather would develop over the coming days and weeks. It has proved a vastly more difficult task than expected and many believe it may never be possible, so complex are the random elements that affect the weather. The other approach is a development of the ‘it looks very dark and it’s moving this way – it’s going to rain’ school. By using an ever-expanding range of monitoring stations plus satellite pictures, this observational approach provides very good accuracy for a day or two ahead and indeed provides most of the TV forecast information. It has been remarked that with the vast amount of observed information

available almost anyone could make accurate forecasts for 12 or even 24 hours ahead without a single jot of computing. There is also yet another approach used by commercial companies to give medium and long term forecasts, involving study of external influences such as the behaviour of the jet stream and sun spot activity. This is coupled to a large library of weather development records and a little bit of ‘when the world’s weather looked like this, this is what happened next’. Advocates of this approach claim much better medium and long term accuracy than is achieved by the Met Office.

Reading Between the Lines So how do we get the most from the television service? The first step is to accept that because of our position between the Atlantic Ocean and the world’s largest land mass, our UK weather systems can change at an alarming rate, particularly when there are rain-bearing depressions (lows) passing through. The next step is to excuse the awful politically correct approach that demands that every area is mentioned each time. Even Scotland was redrawn larger on the weather map a few years ago in response to complaints that it was being neglected in the forecasts. This wastes so much time which would be better used for the forecasters to qualify and expand their predictions. Slightly more to the point is the way that the maps depict rainy/sunny/ frosty areas with apparent phenomenal accuracy, a BBC feature endorsed by the constant smiling confidence of the presenters. Our own confidence is encouraged by the maps being left fixed at one time, often 9 am or 3 pm,

The Met Office and TV Forecasts long enough to identify where we live and to then assume the depicted cloud pattern is exact and literally what we will have. The current fad is to give a two or three word summary at the beginning – such as ‘TODAY Cold Showery’ – which is invariably correct and probably all most people want, and indeed the entire ITV forecasts are kept fairly generalised. We must also remember that the television presentations are made by the television companies using their own staff to interpret the Met Office data, and the media have never let facts get in the way of presentation! So, try not to read too much into the hour-by-hour maps but instead look at the overall movement of the lows, highs and fronts. Often the local forecast is more realistic if you concentrate on what is said rather than the computer graphics. Incidentally, if you have a few minutes the Met Office’s own website is very good and contains a wealth of information including forecasts and pressure charts (see ‘Useful Websites’). I casually use the term ‘local forecast’, but in fact local knowledge can be essential as hills, mountains and valleys all affect the weather. It is information that the national forecasters rarely have but which local forecasters are able to take into consideration. What I hope has become apparent is how relatively small the differences in temperature, winds and humidity have to be in order to cause what we humans perceive as major changes. Just two or three degrees can be all it needs for annoying rain to become dangerous ice, for light mist to become lethal fog or for heavy rain to become

a flood. Something as simple as a small drop in wind speed can mean a low becomes almost stationary rather than drifting along at 5 or 6 miles per hour. Often it is the occluded fronts that cause the problem. This is where the encroaching cold air has lifted the warm air completely off the ground, encouraging it to rise still further. Rising warm air means moisture and condensation which equals rain. (We must remember that rising air is what causes lows in the first place.) Warwickshire has had two dreadful examples of this effect in the last decade where heavy rain fell for eight hours rather than just one or two, simply because the low had become almost stationary. The Boscastle floods in 2004 were caused by an almost stationary low system which dropped rain for hour after hour. In the northern Atlantic a cold front often drags south-west from the centre of a low. Unfortunately, the low is often moving gently north-east which means that the entire cold front, maybe 200 miles long, can pass over the same ground giving perhaps 20 or more hours of rain without a break. Today such situations are always flagged up in the forecasts but perhaps because of our inherited disbelief in the forecast accuracy, we tend not to give these warnings the credit we should.

Coming to a Location Near You Local knowledge can come into use when the geography of an area affects the rain patterns. You may have noticed how often following a flood one of the locals will be interviewed wryly saying, ‘Well, it often floods here. Mind you, I haven’t seen it this

Weather Forecasting Made Simple

Flood marks from the last 350 years recorded below Worcester cathedral. The highest marks feature, in order, 1670, 1770, 1947, 1886, 2007, 2000 and 1924.

bad before and I’ve lived here for 20 years.’ This is small consolation to those whose belongings are ruined but it does beg the questions, ‘When was it this bad before?’ (the answer is often 25 or 30 years ago) and ‘If this road

The Met Office and TV Forecasts regularly floods why on earth was it developed for housing?’ A few years ago the Met Office published maps showing areas potentially at risk from flooding. The reaction was indignation from house owners who seemed to think that they had not been at risk before the maps were published. This last point causes the Met Office great pain. It is not a statutory body and it does not have to be consulted over matters such as town developments. Some places have records of flooding that go back centuries. Bewdley, Worcester, York, Lewes and Tewkesbury spring to mind as recent victims of floods. So check your local history!

Low and Behold the Front Before we look at how clouds and the long-forgotten mahogany mounted barometer in the hall might help us, I want to look once again at the plague of our lives – the low pressure depression. If you cast your mind back to the first part of the book, we saw how our weather system is based on a set of great rolling air systems. The UK sits near the 60º interface which like the equator, has air arriving from north and south, but these currents are now very different in temperature and the overall result is a very energetic region of lows that track generally north-east across the mid-Atlantic and into the British Isles.

A simple diagram of how a low quickly forms along the warm/cold meeting line, complete with rotating winds and rain.

Weather Forecasting Made Simple

Cross sections through cold (left) and warm fronts. In a warm front it is the warm air from the south that is doing the pushing. In a cold front it’s the cold air from the north that’s driving the action.

An occluded front occurs when the two fronts meet, lifting the warm air completely off the ground. The result is that all the clouds bunch together, giving a longer lasting band of rain.

Despite these lows being of different sizes and intensity, they do have some basic characteristics that enable us to anticipate their arrival and to understand the sequence of weather that they cause. If we now slice through the two fronts, we can see the warmer air being lifted, which means it gets colder and thus rain clouds are formed.

This knowledge gives us an idea of what may pass over us. Both fronts are drawn moving from left to right. In a warm front the cloud sequence starts with high cirrus above cumulus, followed by nimbostratus under altostratus giving rain. The front peters out with simple cumulus and the temperature goes from cool to warm as

The Met Office and TV Forecasts it passes. A warm front is a relatively gentle affair and can be from as little as 6 miles to perhaps over 100 miles wide. A cold front is always steeper and more turbulent and tends to move faster than the warm front. The warm air is forced upwards by the advancing cold air mass and as it mixes with the cold air condensation starts, giving a typical sequence of clouds and rain. Starting with stratocumulus and cirrus, then cumulonimbus gives heavy rain and it peters out with low cumulus and showers. This time the temperature drops as the front passes over. A cold front is typically 50 to 100 miles wide and much more vigorous and active. An occluded front combines both rainproducing areas into one broad rain band. We now know roughly what to expect as a low moves past us but we still need to know where the low is and where it is going! This can be judged from the pressure charts which unfortunately have been almost completely dropped by the television companies (though see the excellent forecasts in the Countryfile TV programme) but are still supplied by the Met Office website. If we haven’t got these (or just don’t believe them!) then we have to use the wind information aided by the clouds to work out where the low is. Do remember that we are discussing a generalized, typical situation and as the Met Office will tell you, this can sometimes be completely upset by Nature’s tantrums. I have concentrated on lows so far as they bring our rain and it’s rain we normally worry about, but before proceeding I must mention highs. These form where air is falling, warming up in the process (as it compresses) and

giving dry and often fine weather. Like lows they form in a vaguely circular shape with winds flowing around them but this time in a clockwise direction. There are no obvious fronts within a high pressure area and the whole system moves slowly – an altogether more benign affair.

The three main stages of a cold front. stratocumulus, cumulonimbus and low showery cumulus.

Weather Forecasting Made Simple

DIY Forecasting

W

e can identify much of our approaching weather without depending on the television or web forecasts, by using our own local observations. So let’s see what information we can assemble to help us determine the coming weather.

The Tools We have five basic parameters: temperature, pressure, humidity, wind and cloud type, none of which need to be measured with any great accuracy. These will give us a lot of weather information but to look ahead we also need to know how they are changing with time. Now this can seem like a lot of information to collect but if we are methodical it’s not too difficult. Pressure can simply be read from our faithful old barometer, relative humidity is provided by a hygrometer, often combined with a barometer (you can find barometers for sale on the Web for quite reasonable prices). Temperature and humidity need to be measured outdoors or possibly in a room devoid of any heating. Keep your thermometer out of sunlight and winds and not inside a shed that gets lovely and warm in the sunshine! Take two readings, at least three hours apart, so that you can see whether the parameters are rising, staying the same or falling. Assessing the wind and cloud type is much more subjective. Low level wind direction can be taken

from a weathercock or by watching smoke from a chimney. More likely you will have to simply look at the lower clouds and work out in which direction they are moving. For this we need to know where North or South is – either use a compass or the sun’s direction at noon (11 am in British Summer Time) to identify a feature that is due south which you can use as a reference. The tricky bit is watching the clouds for a few moments and working out in which direction they’re going; again, a small compass can make this easier. Always record the direction from which the lower clouds (and thus the wind) are coming, not the direction they are moving towards. There’s no magic in this, it’s simply that winds are always recorded this way. Personally I find it easier to locate the direction into which the clouds are departing and then simply record the opposite, i.e. if the clouds are drifting away to the south then it’s a north wind. It is very difficult to estimate the wind speed from watching the clouds so for our simple exercise I suggest just four speed bands based very loosely on the Beaufort scale. First, calm for when there’s no noticeable wind. You may see smoke drifting gently from a fire or chimney, and this covers speeds from zero to 3 mph. Next, gentle breeze where we see twigs and leaves moving and we can feel the wind on our faces, indicating speeds between 4 and 10 mph. Next,

DIY Forecasting strong breeze where large branches are in motion and it can be difficult to use an umbrella, indicating speeds between 10 and 30 mph. Lastly, gale for speeds above 30 mph where whole trees are in motion and care is needed to walk into the wind. Cloud type identification does not have to use the Latin names, instead simply record what you see. This feature is only used to confirm and add confidence to our conclusions about the location of the low or high. If you can, look at the clouds above you as well as into the distance but remember clouds can form and dissipate in a few minutes. Indeed, 20 minutes is a good life for an individual cloud. Set out these readings and observations in a simple table:

Time/Date Pressure mb Humidity % Temperature °C Wind Strength Wind Direction Cloud Type

First reading 11 am, 13 Dec 1030 69 3 Gentle breeze North Small fluffy

the wind rotates clockwise around a high, the wind direction will give a rough idea of where the centre is. The lower the wind speed and the higher the pressure, the closer we are to the centre. If the pressure is dropping then the high is moving away and we are heading towards a more unsettled period. The readings in Table 1, with high pressure and a northerly wind, would put us near the star symbol. The slight fall in pressure implies the high is moving away from us and no change in the wind direction suggests that the high is slowly moving north. This illustrates one of the confusing effects of weather systems like lows and highs. The wind is coming from the north so it seems obvious to think of the weather coming from the same direction but in fact the weather

TABLE 1 Second reading 1 pm, 13 Dec 1029 67 9 Very gentle breeze North Small fluffy

So what does it mean? The first thing is to look at the pressure: anything above 1020 mb means we are under a high pressure area. High pressure systems usually last for a few days or more so the pressure trend will give a hint of whether we are entering (rising trend), under a fairly stable high (no change) or leaving the high (dropping). As

Trend Slow fall Slow fall Rise Dropping No change No change

system is moving away to the north. Because of the relatively slow movement of highs it is likely that the weather will stay dry and fine for at least another 24 hours and quite probably 48 hours, though the low winter temperatures and the fairly high humidity make mists and fog probable. If the jet stream has settled to the north of the UK then it is quite

Weather Forecasting Made Simple possible that the high might ebb and flow for a week or more. All we can do is to keep an eye on the pressure readings each day which will quickly show if we are heading towards a low and more changeable weather. The rise in temperature between our two readings is quite natural under well spread cumulus, but by the same token the fairly clear skies will give another cold night. Both highs and lows vary in their strength or vigour, a really strong high may reach a pressure of 1045 mb and a really deep low may sink to 950 mb. The average pressure at sea level is 1013 mb, anything above this is regarded as high and below it as a

low. We could thus have a weak high which may reach barely 1020 mb, or a weak low at 1005 mb. The only way we can anticipate the probable strength is to look at the speed with which the pressure is changing and to be aware of the wind speeds. The faster these two parameters are changing, the more extreme the high or low will be. At other times of the year the principles still apply. The temperature will, hopefully, be higher and the humidity lower but the winds and pressure trends still hold true. Do remember that real lows and highs are not perfectly round which will make the location technique less accurate.

DIY Forecasting

This is the Met Office pressure chart for noon on 13 December and indeed there is a circular high (1032 mb) over Scotland. In the Mediterranean there is a small weak low with an occluded front (purple), a short warm front (red) and a longer cold front heading into Spain (blue). There is another low off the top of the chart in Russia with its occluded front hanging down before splitting into a cold front to the north-west, and a long fragmented warm front reaching right down into the UK which brought us unexpected light rain for a while. Effects like these are impossible to forecast from our simple local readings. Note the fun and games going on over the Atlantic being blocked from us by the high. (© Crown copyright 2009, the Met Office)

Low and Behold So let us now look at lows. There are two changes in the interpretation of our readings. Firstly the pressure will be lower – usually heading below 1010 mb, and it will also probably

be changing faster than in a high pressure area. The other factor is that the winds circulate around a low in an anti-clockwise direction. Again using our table layout:

Weather Forecasting Made Simple

Time/Date Pressure mb Humidity % Temperature °C Wind Strength Wind Direction Cloud Type

First reading 9.10 am, 1 Nov 988 84 10 Strong breeze South Heavy rain

TABLE 2 Second reading 1 45 pm, 1 Nov 994 87 12 Breeze South-West Showers

Here in the UK the majority of lows approach from the south-west, with the warm front arriving first and this deep low was no exception. With a pressure of 988, this is a fairly deep low and we are probably quite near the centre – the strong winds and rain reinforce this. Loooking at the ‘Winds blow into a low’ drawing below, a strong south wind would put the centre of the low to the northwest of us. The second set of readings show a rise in the pressure indicating that the low is moving away from us,

Trend Slight rise Slight rise Slight rise Drop Getting better

again confirmed by the improving rain situation and less strong winds. The south-westerly wind implies the low is now north of us meaning the whole low system is moving away to the north-east. The pressure should continue to rise with less rain over the next 12 hours, though at this time of the year it is likely that another low system will follow within 24 hours. In fact, this low deepened as it moved over northern England and into Scotland where its rain caused considerable flooding.

DIY Forecasting

Time/Date Pressure mb Humidity % Temperature °C Wind Strength Wind Direction Cloud Type

First reading 8.50 am, 28 Jan 1018 88 5 Light breeze North-East Overcast

TABLE 3 Second reading 1.15 pm, 28 Jan 1014 86 8 Breeze North-East Dull

Table 3 shows an in-between situation which is fairly common, particularly in winter. Note the pressure, not really one thing or the other and staying fairly steady. As the figures suggest, the high (over the Atlantic) was more dominant. With the wind from the north-east, we are Typical pressure change as a ‘low’ passes over us. This down and then up trend helps to identify where we are relative to the low.

Trend Slight fall Slight fall Slight rise Slight rise No change

under the blue arrows in the sketch below. We can see how the natural winds from the two pressure systems are assisting these long, gentle ‘inbetween’ winds. Often there will be trailing fronts from the low which bring dull and drizzly weather. If you look at central France on the Met Office chart (page 47), you will see that it is between the high over Scotland and the low over the Mediterranean. The winds will be as our blue arrows, bringing cool air from Germany and Russia, which

Weather Forecasting Made Simple may well last for two or three days, even though neither the high or low are anywhere near France. These descriptions are intended to simply illustrate how, by noting the winds and pressures a few hours

apart, one can estimate where the low (or high) is and at what sort of speed it is travelling through your region. But do remember, the real weather is usually more complex so don’t expect too much!

The much neglected but very useful barometer. Not as common as they used to be but search the Web and you will find plenty from around £30.

Section 2

Unusual Clouds and Effects

Altocumulus stratiformis is a medium height cloud made of individual cloudlets set out in lines – easy!

Crepuscular rays where the sunshine lights up dust and pollen in the atmosphere. This remarkable display looks like the start of a sci fi film.

Weather Forecasting Made Simple

No, not a tropical river, just one that is warmer than the near zero air temperature above it, causing ‘steam fog’.

The trailing edge of a weather front slowly moving to the right (west).

Unusual Clouds and Effects

An unusual and beautiful lenticular cloud formed by moist air being lifted by a mountain range. It then condenses to form cloud but once over the mountain the air falls and returns to vapour, leaving this UFO-shaped cloud.

Sunset approaches on a cold November afternoon, looking down onto a sea of mist lying over Gloucestershire.

Weather Forecasting Made Simple

A rarely seen form of cirrocumulus called ‘herringbone’ where ice crystals mix with supercooled water droplets.

These clouds carry the delightful title of ‘Altocumulus of a chaotic sky’ caused by clouds at different levels and nicely placed sunshine.

Unusual Clouds and Effects

Stratocumulus Lenticularis - the low ‘lens’ shaped clouds which can form in moist air as it rises over hills. These are the least common form of the vast range of Stratocumulus clouds.

Cirrus fibratus produce a wide range of delicate fibrous shapes and patterns.

Weather Forecasting Made Simple

Snow at its prettiest. A thin layer has gently settled on this magnificent sycamore tree.

Delicate cumulus humilis clouds wandering along a valley have literally bumped into Snowdon watched from above by some altocumulus.

Section 2

Old Tales

O

ur ever-changing British weather is such an obsession with most of us that we have grown up with common sayings that seem part of our heritage. Some of them are surprisingly accurate too! There are also numerous old tales related to weather forecasting which often started as very local sayings and seem reasonably helpful for just one area – due, perhaps, to the local valleys or mountains, or proximity to the sea. ‘If the rain starts whilst the wind is still very low, beware.’ This indicates that the high level winds are strong – they have dragged the rain ahead of the low level winds, but these strong winds will soon work their way down to lower levels. Should the winds pick up well before any rain arrives then the front will pass with relatively gentle winds. ‘When the wind is in the East,’Tis good for neither man nor beast.’ So said Izaak Walton in the Compleat Angler. Easterly winds have always had a bad reputation, there’s even reference to them in the Bible. These are not the short-lived winds of a low pressure depression moving across but a long term general east wind. The main complaint appears to be that Easterlies vary in speed and direction in an alarming and sporadic manner. ‘Long foretold – long hold’ is part of a very old saying which basically says

that a wet weather system which takes its time arriving, going through the various stages one at a time, will in turn last a long time. Should a set of early warning clouds arrive all bundled together then the approaching system is probably fairly old and will pass quickly. ‘Red sky at night, shepherd’s delight, Red sky in the morning, shepherd’s warning.’ Probably the best known old rhyme linked to the weather and one that is often correct. Our sun always sets in the west and what we see is the sinking sun (thus the red colours) reflecting on the underside of high clouds. This means there are no middle level or low clouds to the west but there are high clouds – a quite specific set of conditions which tend to occur when the air is fairly stable and dry. A calm night can be expected, often along with a pleasant morning the next day. A red sky in the morning is again the low red sun reflecting on high clouds but being in the east these are often the cirrus of an approaching warm front and thus potential bad weather. Research in London in the 1920s showed this rhyme to be correct 70% of the time. July 15th is ‘St Swithin’s Day’ and a 16th-century rhyme declares that if it rains then it will continue for 40 days. A dry St Swithin’s Day will bring 40 dry days. Again this old saying is based on well observed weather patterns. In most summers the path

Weather Forecasting Made Simple of the jet stream settles down by mid July, as indeed does the weather and this sets the mood until well into August.

people are about to note the start of rain in the night and rain rarely lasts more than 7 or 8 hours.

‘Rain before 7, clear by 11’ is another oft quoted saying that can often seem apparently correct, but then few

Red sky in the evening, particularly with this warm peach-like colouring, is often a good omen for the next day.

Section 2

Past Extremes

O

ur weather is controlled by many variables and from time to time there are moments when all the variables are bad at the same time. Such moments are notoriously difficult to predict since they often involve local circumstances. Our history records very many floods, freezes and high winds that caused chaos and loss of life, going back to the earliest times. Poor summers have been blamed on all manner of things. Some 200 years ago it was cannon fire from the Napoleonic Wars; in the First World War it was artillery fire on the Western Front; in the 1950s it was nuclear testing; and today it is global warming. One slightly encouraging thing I found whilst researching for this book was that the dates when major worldwide weather records were set, are all more than 39 years ago, so perhaps we shouldn’t panic because

Rain -

Snow -

Temperature -

of just one bad winter. Closer to home, if we look at the dates on the flood wall at Worcester we find the years marked are 1852, 1886, 1910, 1924, 1939, 1940, 1946, 1947, 1948, 1960, 1965, 1990, 1998, 2000 and 2007. Here we can see there were seven floods in 26 years between 1939 and 1965, but we’ve only had four since 1965 – that’s 45 years. So maybe the weather’s actually getting better! The problem is that in our small country most of our extreme events are very local. Dreadful for those affected, of course, but just a few days in the news for the rest of us and so easily forgotten. So here then is a selection of UK events taken from the last 50 years or so; most are local but very dramatic. The thing to remember is that these kinds of events have always occurred, though global warming may make them happen a little more often.

Most intense (38mm in just 1 minute) Most in one day (1.87 metres) Most in one year (264 metres) Greatest in one day (19 metres) Greatest in one year (311 metres) Greatest depth (114 metres) Highest recorded (58°C) Greatest rise (27°C in 2 minutes) Greatest drop (56°C in 24 hours)

1970 1952 1860 1921 1971 1911 1922 1943 1916

Weather Forecasting Made Simple 1962 Fierce Atlantic gales blew across the Pennines and funnelled down onto Sheffield, damaging some 100,000 homes. The following winter was so bad that there was talk in the papers of the world drifting into another ice age. It took three months for a thaw to set in. All caused by the massive Asian high which had drifted west and settled over the North Sea. 1968 January and Glasgow was struck by 100 mph gales, 19 people died and 100,000 homes were damaged. In July we had a spell of blistering hot weather with 32°C recorded in London. Tower Bridge jammed, it had expanded so much. At Wimbledon 550 people needed treatment. 1975 Between 26 March and 10 April there was widespread snow, followed by more early in June which came as far south as East Anglia and London. A few days later a heatwave struck and was followed by a glorious summer. 1976 The hottest, driest and sunniest summer since records began. It covered the whole of the British Isles. 1978 For Cambridge, the Boat Race ended when they were sunk by strong winds (the fourth sinking in the 20th century) 1981 Over Easter a deluge of rain fell on Horsham in Sussex, followed two weeks later by the worst late April snowfalls in the 20th century. 1982 January brought extreme cold, Scotland saw -20°C, even Glasgow

hit -14°C. Wales had its worst snow falls on record. Whirlwinds lifted straw and dropped it on Devizes (Wiltshire) from a clear blue sky. One of the wettest Wimbledons followed, with 3 inches of rain in one day. 1983 The most disruptive Easter, when snow fell over most of England. In June the coastline of Dorset and Hampshire was hit by tremendous thunderstorms; hailstones the size of golf balls smashed windows. 1987 The Boat Race was blighted by gales and lightning nearly struck the Cambridge boat. In October a ferocious storm moved northwards over the Channel and southern England. The devastation was the worst to date: 19 million trees were flattened, 3 million homes lost electricity and 19 people died. At the time it was the most expensive single natural disaster in the world. 1990 In January came a slightly less severe re-run of the 1987 storms, which brought down fewer trees but being a daytime event killed 47 people. 1994 Early April brought 100 mph gales, then snow storms followed by floods. In July, South Wales had intense thunderstorms. During a five hour period of mayhem, lightning struck a gas valve at the Milford Haven oil refinery, setting off an enormous explosion and starting a fire which took two days to extinguish. 1996 The famous Cliff Richard community singing at Wimbledon when rain poured down for three days.

Past Extremes 1998 Torrential rains and floods across central England again at Easter. 2000 Yet again, Easter brought rain and floods to large parts of England. 2003 August brought a long, settled heatwave, taking temperatures to record highs: 38.5°C in Kent and 32.9°C in Scotland (101°F and 91°F). 2004 In August, London had exceptional downpours which sent hundreds of thousands of gallons of sewage and surface water into the Thames as the drains couldn’t cope. On the 16th the Boscastle disaster struck, when 3½ inches of rain fell on the area in just one hour. The whole 24 hour period dropped 8 inches of rain on north Cornwall, that’s three months’ rain in a single day! 2005 The West Midlands get around half of the 60 or so tornadoes in Britain each year – this year it was more serious with 100 mph winds which caused extensive damage to roofs and chimneys in Birmingham. 2006 May time in eastern England

brought a covering of yellow pollen which had been carried aloft from birch trees in Denmark. It was London’s turn for a tornado, which left hundreds of properties damaged. 2007 Following a lovely spring, summer turned into a disaster. It was the wettest Wimbledon for 25 years and severe floods left 13 dead and 4,800 homes destroyed. Eight motorways were closed, railways were washed out and Yorkshire and Gloucestershire were particularly hard hit. At £3 billion, it became the biggest single event insurance claim ever. 2008 Easter again widespread snow.

brought

2009 This year managed several records. Following the coldest winter in a decade, February brought snow and disruption across Europe. After a beautiful May and June, July was wet and, in November, Cumbria had eight days of flooding. Both months broke the UK records for rainfall. 2010 Started with the coldest December/January for 25 years, with snow continuing to cause problems into February.

Weather Forecasting Made Simple

The Future

I

t would be foolish to pretend that forecasting the UK’s weather will ever be easy. The Met Office is, at last, getting better in the 24 to 60 hour time period and eventually will no doubt get better over the medium range as well. Hopefully, the suggestions in this book on doing it oneself will add authority to your own predictions, if only over a pint in the local pub! So what of the future? On the personal front, don’t buy property on a flood plain nor below 25 metres above sea level nor at the bottom of a narrow valley. Then there’s global warming. It’s not that the jury is still out, it’s that the jury has been confused by vested interests and politics. You may well have noticed how those in authority have concentrated on how to generate ever larger quantities of electricity albeit in a ‘greener’ manner but no

one has seriously suggested using less energy. But of course there’s no money in that! It was calculated some years ago that if everyone in America simply turned their computers off when they weren’t using them, the drop in consumption would allow a whole power station to be closed. So maybe we should be concerned, but perhaps if we replanted the great forests we’ve savaged (which absorb CO²), maybe halved the number of bovine animals we raise (which produce methane equivalent to all the CO² produced by our cars and airplanes) and stopped wasting energy, we could worry just a little less. If we stopped building in silly places and checked our weather history a bit more, then perhaps the impact of global warming itself might also be a lot less.

Bibliography and Useful Websites

Bibliography Buckley, B., Hopkins, E., Whitaker, R., Weather, Readers Digest Dunlop, Storm, Guide to Weather Forecasting, Philips Hamblyn, Richard, The Cloud Book, David & Charles Hodgson, Michael, Basic Essentials – Weather Forecasting, Morris Book Hook, Patrick, Weather Watching, Collins McIlveen, Robin, Fundamentals of Weather and Climate, Chapman & Hall Pretor-Pinney, Gavin, The Cloudspotter’s Guide, Hodder & Stoughton Simons, Paul, Since Records Began, Collins Watts, Alan, Instant Weather Forecasting, Adlard Coles Nautical Watts, Alan, Weather Wise, Adlard Coles Nautical

Useful Websites BBC television: www.bbc.co.uk/weather ITV: www.itv-weather.com Meteorological Office: www.metoffice.gov.uk UK Weather Information: www.weather.org.uk; www.metcheck.com

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