Meteorology, Vol. I: Discussion

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or

The fall.

passed on

difference,

accompanied

pressure

falling.

lower curve starts to

blizzard

June 16th

when compared with

and the calms and northerly winds when the curve

Turning back to plate 17 we

convenience

for

example,

last

capriciously

so

related

closely

ca.se,

curve are repeated the winds at

this

occurred

The

Evans one bottom curve

actual pressure at Cape

the

On

station.

independent

entirely

are

Cape Evans and Framheim.

curves.

see

During

another high wind from the

Looking now at the lower curve corresponding to the see

wind changes

for

plates,

period of

on each plate shows the pressure difierence Cape Evans — Framheim, plotted

Cape Evans.

the

Here we

7.

some other motive

of

of

a

all

wave passed over the

single

the

that

instead

pressure between

while the

no obvious reason

for

example plate

by a

tempted to

found on

waves and

southerly

followed

pressure waves,

16th and a trough

.Tune

waves and occur under the influence

examines the difference

be

will

Take as another

a

first

pressure

with an occasional light wind

weather

same

The

changes took place while the crest

such evidence as this one

of the pressure

is

small

the

in

The two remaining

was accompanied by a calm and the blizzards

passed.

a trough on

wave there

this

wave

the

concerned.

is

a large pressure wave between

the

sharply

rise

happened when the large wave passed

at all phases of the actual pressure

pressure alone

the

calm

the reverse of what

This

is

during

passed

from the north.

the

this

The second

to

still

our

the barometer was rapidly falling.

usually associated with a complete reversal of the wind.

is

between

trough

to

fix

blizzard on

fir.st

was no apparent change

this occurred there

blizzards on this plate are associated

to

To

ended at midday on the 16th by the wind suddenly turning completely round and blowing

strongly from the north.

On

while

between

relationship

barometer at Cape Evans.

15th-16th

the

of

Evans the

conditions.

but the blizzard stopped, for no apparent reason, while the barometer was

The third occurred on the night

Barrier.

however, one examines

If,

when the barometer commenced

12th just

of the

seen.

accompanying

sheet occurred on October 10th with a nearly steady

the

the

later.

winds are clearly

northerly

the pressure curve for Cape Evans alone

so

any intimate

is

and on

one examines the wind arrows placed along the pressure curves for Cape

periods

It

to a line joining

produced passes very

this line

intensity (see page 195).

The conditions

the

notice

becomes necessary now to examine the pressure waves to

It

If

to

interesting

is

Pressure

relationship

pressure travelling

of

of the Antarctic Continent along directions parallel

outwards from the centre

near

217

on the

large

28

rises

PRESSURE, WINDS AND WEATHER.

218

occurred

on the bottom curve and therefore blizzards

which

a very

We

one.

rigid

This relationship

passed.

compared with other meteorological

difference between Cape Evans and Framheim increases and high northerly

winds

light southerly

We

have now found that

due to

winds

passage

the

to see

difficult

is

it

any

Melbourne at the top of each the

—

examples

good

On

plate.

Framheim, 300 miles to the

be

case

the

which

of

are

the other hand there

Cape Evans and the changes

winds at

between the winds

relationship

close

waves

the pressure

if

by the

and low pressure systems which are maintained

high

of

them

romid

circulating

calms or

winds,

will occur while the pressure difference decreases.

at Cape Evans and the actual pressure waves, as would

were

all

rules this

conclude that a blizzard will occur when the pressure

therefore

can

they

found to be the general rule throughout

be

will

exceptions, but

There are certainly

the plates. is

on these two plates

clear

so

is

while

pressure

in

be seen in the

to

a

is

curves for

relationship

close

between

between that station and

difference

east.

Theoretical Discussion of the Effects of Pressure Waves.

We

what

investigate

and

distribution

pres.sure

Sea

now

will

area from without

;

supposing

winds this

These waves will be supposed to

and

pressure

sea-level

We

have

waves

the

and

surrounding plateau

to

first

determine

the

at

sea-level

able to stations.

motion.

air

the waves, and

without

distribution

pressure

the

on the Ross

we have been

adding their pressure to the existing

the area

over

travel

imposed

are

view of the fact that

consequence changing the pressure distribution and

in

therefore

that

reasonable in

is

same pressure waves on the

identify the

passage of pressure waves would have on the

the

effect

then to impose a series of waves on this distribution and examine the result. For simplicity in the following discussion we will assume that the three stations Cape

Evans and Framheim are situated at the corners of an isosceles right angle In figure line joining Cape Adare and Framheim is the hypotenuse.

Cape

Adare,

triangle of which the

%4m

three

the

stations

We

will

In the absence of a difference of

be restrained.

pressure

from the

slowly

would increase

Barrier, pressure

increase of

with latitude which

however,

is,

a

large

temperature

were no is,

then

effects

high

give

pressure

motion could

temperature between the Ross

Sea and the

north

the

to

difference

if

all

south owing to the general

feature of

the Antarctic.

have

motion of the

now air

to

examine

how

this

between the

Barrier

the

place

along

direction

shown on as an

of

figure

easterly

and low over the

Barrier like

those

shown on

Sea.

figure

simple

pressure

distribution

will

If

64a.

there

That

Barrier.

be affected when

takes place.

the flow

isobars

and the

64a the whole wind.

This,

act as a wall running north pressure

it.

and the Sea and

it.

over the

In the southern hemisphere, owing to the takes

This would

Sea to the north of

they would run approximately east and west and parallel to the edge of the

We

then

Barrier.

air

motion the isobars would run something

air

A E B

the line

F,

edge of the

distribution

Barrier than over the

the chief pressure difference would be due to

Both

line

common

a

is

cause the pressure to be higher over the

There

and the

what would be the pressure

consider

first

E and E F the

by the points A,

represented

are

represents the line of the Western Mountains

distribution.

It

is

such

in

low air

a

direction

pressure

on

that

the

the high pressure

right.

the air is

on the

is

and south to

not possible owing to at right angles to

see that

the

air

left

of

to the west

the Western Mountains which

the easterly

from

motion

Under the pressure distribution

over the Ross Sea area would tend to flow

however,

easy

influence of the earth's rotation,

wind induced by the

the eastern half of the

Barrier

would flow over to the west and would then be forced, along with the air from the western to travel to the northwards paraflel to the line of the Western Mountains.

half,

THEORETICAL PRESSURE DISTRIBUTION.

219

There would therefore be an easterly wind over the east of the Barrier and a southerly

wind

alonii the line of the

mountains.

It

also clear that the intensity of

is

the southerly wind

LOW

SE.A

BARRIER

HIGH

HIGH

(b)

ia) Theoretical pressure distribution,

Fig. 61.

must increase as

flows towards the

it

the south and from the east of the

motion on the Barrier

it

does

not

In fact

Barrier.

constantly receiving air both from

is

it

practically the whole

of the

This

matter

how

shown diagrammatically by the arrows

is

the motion

of

air

is

deflected

in

velocity will always adjust themselves in time until (neglecting the effect

the air

travelling along the isobars with the high j^ressure on the

have adjusted strong.

that

We

therefore get a

over the

isobars

are

be

themselves to

east

not

of

system

of

but

the

to

near

flow

and

isobars like that

where the

Barrier

the

afiected,

parallel

air

Western

the

near together where the

air

motion

started

the

pressure

at

from figure 646 that when motion occurs

Framheim at

This increase

of

pressure

the north-west of the Barrier, but easterly

wind over the west

consequent

increase

of

of

pressure,

it

is

shown on the same

figure.

will

wind

We

is

is

now

is

see

Whereas before

strong.

in

two ways,

first

the piling up of the moving rapidly from south

see

from

Cape Evans than

appreciably higher at

due to the concentration of the

brought about

the air

is

Moimtains the isobars turn sharply to the

necessary to cross two isobars in going

is

it

Barrier causes also

wind

Framhoim and Cape Evans was the same, we

to Cape Evans, hence the pressure

Framheim.

friction)

can move from east to west the original

north and crowd together near Cape Evans where the air current the

of

and the low pressure

646 has become established the isobars

the wind flow shown in figure

When

left

isobars will be inversely proportional to the

on the right and the distance between the velocity.

figure 646.

restrained the pressure

or

and wind is

set in

air

concentrated in the current which flows across the north-west corner of

is

the Barrier near to Cape Evans.

Now

because

north

the air

to

air

motion over

stoppage of the there

north

and

a

near the

PRESSURE, WINDS AND WEATHER.

220

Western Mountains

pressed to the west

is

by the

force due

to the

Both

produces a pressure gradient away from the mountains.

and

this

pressure

and

earth's rotation raise the

effects

cause the isobars to adjust themselves to the direction of the air movement.

Going further we see that an increase of the pressure difference between the Barrier and the Ross Sea increases air flow over the west of the Barrier with a greater concentration Thus a uniform lowering of the of the motion near to Cape Evans where the pressure rises. pressure

over the Ross Sea or a raising of

southerly wind at Cape Evans and causes to

the

We

the

pressure

the pressure at

have

now

the

investigate

to

result

of

impressing

The required values are found and the theory all essentials the pressure distribution shown on principles

is

the

to

increases rise

the

relatively

isobars

pressure waves on this

travelling

diagi'ammatic. substantially supported

646

figure

by noticing that

which has been derived from

the same as that of the average pressure distribution found from

observations and already given in figure 58, page 176. for

Evans

So far we have not considered the magnitude of the pressure differences,

general distribution.

first

Barrier

the

Cape

pressure of Framheim.

the isobars shown on figures 6ia and 646 being only

in

over

and

therefore

in

the

future

In the latter

discussion

we

the actual

we have numerical values

\vill

use

it

in

place

of

the

approximate diagram of figure 646 distribution have

been entered to

our diagrammatic representation of the geography of the Ross Ssa area.*

The whole area

On fit

figure

65a

the

isobars

from the average

pressure

45°

270*

(a) Fio. 65

(a)

45

(6) Actual pressure distribution.

(6)

Imposed pressure waves.

Hence * Figures 65 to 73 were all prepared before the error described in the footnote to page 166 was detected. the pressures used in these diagrams are all -OS" too low. This, however, is quite immaterial to the present discussion as the pressure distribution

is

unaffected.

THEORETICAL EFFECTS OF PRESSURE WAVES. and

into squares,

has been divided

The

with the isobars.

The wave a,

b,

at

are

fronts

the pressure due to the

angles

to

the

wave

is

according

etc.,

c,

to

Thus the value

of

this

in

of

be

to

the line

having

not

these

waves

affected

the

b,

front the

...

c,

the

to

values of

n, o. letters

a to

squares to which the letter

of

in

one

have

but

only

chosen

been

to

As

o.

a,

b,

c,

attached.

is

i.

two particulars,

in

phases

principle,

has

E

arrives at the three

it

and

amplitudes

different is

a,

given

travelling from

the

may vary

investigated

the difference in phase %^ath which

result

squares

be

will

determined by the position of the wave has to be added to the pressure

i

entered in each of the squares along the line

The wave

Values

line.

wave which is the same along the whole wave pressure

must be added to the pressures

etc.,

be impressed on this pressure distribution travel

in the direction of the line of

i.e.,

right

has been entered in agreement

in each square the pressure

waves which are to

from Framheim to Cape Adare,

221

for

stations.

been investigated,

A

number

large

and

it

is

by changing these

detail

in

discussion

amplitude and

in

(a)

which

here

of

(6)

in

waves

found that the variables.

From

the best general

gives

result.

This

wave

is

Cape Adare

to

given at Cape Evans by dp =-3" cos

a

in

dp=-3" cos

(61+45°)

In

6.56

figure

quarter

of

and

from

travels

it

Framheim

So that the pressure changes at Framheim are

period.

its

9

and at Cape Adare dp=-3" cos (6-45°).

the

wave

be investigated

to

shown

is

in

the same

way

as the

pressure

The abscissa represent equal intervals of time, but as we are not concerned with absolute time, which involves the rate of travel of the waves, but only with time in so far as it affects the phase of the wave when it arrives at the different

Volume

curves on the plates in

stations, the phase of the

wave

The if

the

arrives

phase

at

at

wave

each

Cape

II.

Cape Evans

at each interval at

an

station

Evans

is

e,

eighth of it

is

a

6+45°

period at

is

given instead of the time.

after

Framheim

left

it

and

the

9-45°

last.

Thus

at

Cape

Adare.

The most important curve on the plates in Volume II was found to be the one giving therefore has the difference in pressure between Cape Evans and Framheim, a similar curve been added to figure 656.

We

shall consider the effect of the

from the epoch when the tions of

maximum

wave of

at eight intervals during its

the wave has

the wave at each of the epochs considered

just

reached Cape

passage commencing Adare.

The

posi-

are shown in figure 656 by the vertical

lines at

e

= io",

93°, 135°, 180°, 225°, 270°, 315°

and

360°.

For each position of the wave two diagrams are given. In the first the pressure which In the results by the simple addition of the pressure wave to the normal pressure is shown. second an attempt has been air

made

to

motion would produce, taking into

pressure which the alteration in the for the account the inertia of the air and the obstacles to

allow

each position of the wave there will be two diagrams similar to figures 64a and 646, which are the corresponding diagrams for the conditions in the absence of the

its

motion.

Thus

pressure waves.

for

Pressure, winds and weather.

222

(a) Fig. 66.

9

= 45°,

change figures.

of

figure

pressure

66.

— Starting

due

the

is

is sufficient

with

the

inii>ressed

added

resulting

Isobars through

squares.

added pressure

been the

diagram, figure 65a, with in

the

to

These values have

Modified pressure distribution

these

to

crest

the

of

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