Ch 7 Frontal Depression

8/12/2019 Ch 7 Frontal Depression

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Chapter 7

THE FRONTAL DEPRESSION

Defnition

A frontal depression is a low pressure centre with two frontal sur-

faces, a cold frontal surface to the left and a warm frontal surface

to the right (in the northern hemisphere). Between the two frontal

surfaces lies the warm sector, which is an air ow of tropical airmasses. To the left and right of the frontal surfaces there is an air

ow of cold polar air masses.

Around the low pressure centre there is a complete wind circula-

tion around it.

In Chapter 4 we saw how frontal depressions are formed.

Figure 7.1 shows a completely formed frontal depression. Such a

depression is normally characterised by a number of varying natu-

ral elements such as:

Pressure

Wind

Temperature

Sky (clouds)

Precipitation Visibility

Humidity

Upper air

All these elements determine the type of weather one will experi-

ence. This type of weather will be different for every observer in adifferent spot of the frontal depression.


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Figure 7.1 should actually be seen in 3D because in reality this

depression with its frontal surfaces and fronts looks more like an

open ower, the petals being the frontal surfaces.

In Figure 7.1, dotted line AB is the path the frontal depression fol-lows for an observer situated on AB.

As often depressions follow a NE-ly course, this observer will see

the depression approaching from his left (facing North). After some

time, the warm front will pass over him (or her) and some time

later he (or she) will nd himself in the warm sector of the depres-sion. Some hours later the cold front will pass over the observer

and nally he will have the cold front (and in fact the whole depres-

sion) to his right.

As we shall see, the weather will than improve considerably unless

a new depression is on its way which will normally bring a new de-

terioration of the weather.

The bottom of Figure 7.1 shows a cross-section of the frontal de-

pression with all its weather changes from right to left.

Table 7.1 shows the weather changes in a frontal depression mov-

ing in a NE-ly direction.

Most depressions in Europe move in a NE-ly direction but nowa-

days, depressions seem to be moving in a great number of other

directions.

Table 7.1 is based on Notes on Meteorology by Kemp and Young.


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WARM FRONTWARM

SECTOR

COLD FRONTOCCLU-

SIONApproach-

ing Passing Passing At the rear

PRES-

SURE

Steady fall Stops fall-

ing

Little

change

Sudden

rise

Rising

steadily

Falling

than rising

WIND Veers from

S-SW and

increases

Veers with

possible

increase in

velocity

Steady,

possibly

backing as

cold front

approaches

Sudden

veer from

SW-W or

NW with

squalls

Veloc-

ity tends to

decrease,

steady in

direction

From S

to NW,

Showery

TEMPER-

ATURE

Rising

slowly

Slight rise Little

changewith rela-

tive high

tempera-

tures

Sudden

drop

Little

change buttendency

to fall

Nearly no

change

CLOUDS Becoming

overcast

Ci, Cs, As,

Ns

Ns Overcast

with St

turning to

Sc

Cb Cb, Ac and

Cu with

blue sky

Cs, As, Ns

followed

by Cu and

Cb

PRECIPI-

TATION

Continuous

from driz-

zle to rain

or snow

Rain stops

but eventu-

ally slight

drizzle

Intermit-

tent slight

rain or

drizzle,

possibly

fog

Heavy

rain, thun-

der and

possibly

hail

Possibly

a narrow

belt of

continuous

heavy rain

turningto heavy

showers

later

Rain with

heavy

showers

VISIBIL-

ITY

Deteriorat-

ing

Poor Poor Great im-

provement

Excellent

except in

showers

A little bet-

ter than by

the warm

front

HUMID-

ITY

Increasing Rapid rise Very high Rapid fall Fairly low

but vari-

able in rain

Slightly

deteriorat-

ing

Table 7.1 - Weather changes in a frontal depression


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Comment

Warm front approaching

Figure 7.2 shows the cross-section of a warm front surface.

Figure 7.2

The cirrus cloud is the rst cloud that is observed when a frontaldepression is approaching. It is also the highest cloud.

As the depression moves to the right, the warm air slides down

along the cold air and becomes colder. When the dew point is

reached, clouds are forming. The highest clouds are made out of

crystals and the lower ones are made of water droplets with driz-zle or rain, sometimes heavy rain from the nimbostratus. When

the temperature is sufciently warm, the rain drops falling from an

altostratus may evaporate before they reach the ground.

The pressure decreases and the temperatures rises slowly as the

warm sector approaches.

When passing the warm front, the wind will veer from S to SW.


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The visibility is deteriorating to become poor during drizzle or rainy

periods. The humidity is also increasing and can be high at times.

Warm front

When the warm front of the depression is passing, the pressure will

stop dropping to become stable in the warm sector.

The nimbostratus clouds will still be predominant with no rain or

slight drizzle. The wind will start to blow from the SW and the tem-

perature will slightly rise.

The Visibility is rather poor with a high humidity.

Warm sector

In the warm sector the isobars run parallel with the direction in

which the depression moves and the pressure will remain constant.

As the air is warm the humidity will be high as well, with stratus

clouds and poor visibility or even fog. The sky is normally covered

with stratocumulus clouds with drizzle at times.

Cold front

Figure 7.3 shows a cross-section of the cold front surface.

At the cold front, the cold front surface scoops the warm air into theatmosphere like a bulldozer.

That warm air can reach an height of 6500 m and the quick con-

densation of its humidity will form heavy cumulonimbus clouds with

heavy rain and gusts of strong winds. In the heavy rain the visibility

may be very poor.


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The barometer will rise suddenly and the wind will suddenly veer

from SW to NW. The temperature will also suddenly drop.

Once the cumulonimbus as dropped all of its rain, there will be a

sudden improvement of the visibility and a rapid fall of the humidity.

Figure 7.3

At the rear of the warm front

At the rear of the warm front, the pressure will rise steadily with lit-

tle change in temperature.

As an observer moves farther away from the depression, the wind,

which is blowing from a NW-ly direction will decrease steadily.

The humidity of cold air is fairly low so that altocumulus and cu-

mulus clouds appear with lots of blue sky. The weather is fairly

good with some possible rain. As one is now moving towards a

high pressure, the weather still improves unless a new depression


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starts to deteriorate the weather again.

The visibility is excellent, except in showers.

Occlusion

For the weather changes in an occlusion, se Table 7.1

IMPORTANT REMARK

The best way the follow the evolution of a passing depression, isto observe a barograph.

The graphic of the barograph will give the observer a clear indica-

tion of the passage of the warm and cold front (Figure 7.4).

Often, when the warm front is passing the observer can clearly feelan increase in temperature and when the cold front passes, the

observer will clearly experience a drop of the temperature.

Figure 7.4

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Ch 7 Frontal Depression