ppl_04_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S E

formations associated with Returning Polar Maritime air are, in general, stratiform in the lowest layers, with convective cloud developing in the unstable middle layers.

POLAR FRONTS.

Fronts are transition zones between air masses of different temperature, namely polar and tropical air masses. In Chapter 5, Pressure Systems, the frontal boundary, which exists between Polar Maritime air and Tropical Maritime air, was described. Known as the Polar Front, this boundary between Polar Maritime and Tropical Maritime air is fluid in nature. When warm air moves northwards to replace heavy cold air, a kink

Fronts are

transition zones

between

different air masses.

A classic

Polar Front depression

is caused by

warm Tropical Maritime air intruding into Polar Maritime air causing surface pressure to decrease.

Figure 15.4 A kink in the Polar Front leads to a reduction in surface pressure and the formation of a Warm Sector Depression.

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Customer: Oleg Ostapenko E-mail: [e-mail]

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is formed in the Polar Front and surface pressure is reduced, creating a polar front depression, otherwise known as a warm sector depression.

As air moves towards the low pressure area in the Northern Hemisphere, it is deflected to the right by the Coriolis Force. This flow of air creates two unique frontal features; a cold front and a warm front.

T h e P o l a r F r o n t .

Along the part of the Polar Front identified by blue triangles in Figure 15.4c, cold Polar Maritime air is being forced against warm Tropical Maritime air, and under-cutting the warmer air. By this mechanism, a cold front is formed as depicted in Figure 15.5.

CLOUD

TYPES

Cu = Cumulus

St = Stratus

Ci = Cirrus

Cs = Cirro - Stratus

As = Alto - Stratus

Ns = Nimbo - Stratus

Figure 15.5 A cross section of a Cold Front. Cold air is replacing warm air and undercutting it.

Along the part of the polar front identified by red semi-circles in Diagram C of Figure 15.4, warm Tropical Maritime air is being forced up and over the colder air, on the other side of the front. By this mechanism, a warm front is created as depicted in

Figure 15.6.

Figure 15.6 A cross section of a Warm Front. Warm air is replacing cold air and overriding it.

Polar Frontal systems, of the type described above are found between latitudes 35°

N and 65° N. They move from West to East across the Atlantic and Pacific. The

Atlantic fronts will normally arrive over the British Isles in sequence: the warm front first, followed by the cold front.

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S E

T h e W a r m F r o n t .

Figure 15.7 shows a cross section across a polar front depression, depicting both the warm and the cold fronts. The warm front, on the right hand side of the illustration, will be the first of the fronts to influence the weather over the British Isles as the depression moves from West to East.

CLOUD

TYPES

Ci = Cirrus

Cs = Cirrostratus

As = Altostratus

Ns = Nimbostratus

St = Stratus

Cb = Cumulonimbus

Cu = Cumulus

Figure 15.7 The slope of the warm front is 1:150. The cloud sequence seen as the warm front approaches is Ci - Cs - As - Ns. Precipitation starts 200 - 300 nm ahead of the front.

The slope of a typical warm front is a very shallow gradient of about 1 in 150, and the clouds that develop along the front are mainly of stratiform type. The first clouds noticed by an observer on the ground in Britain, as a warm front approaches, will be very high altitude cirrus, then cirrostratus. As the front gets closer, thicker, lower altostratus will be observed. The clouds will continue to thicken and lower as the surface front gets nearer, until, eventually, where the warm front meets the Earth’s surface, nimbostratus cloud will prevail.

The slope of a warm front will typically be

1:150.

The whole extent of the warm front may take up to 12 hours to pass over a location, with the cloud ahead of the warm front extending for about 600 nautical miles (nm) ahead of the surface position of the front. The main area of precipitation starts some 200 to 300 nm ahead of the surface front, as the base of the cloud gradually lowers. Precipitation may persist for several hours as the front gets closer. As the precipitation increases in intensity, closer to the ground position of the front, visibility will reduce, and eventually, frontal fog may occur.

Precipitation

associated with a warm

front may

begin 200 to 300 miles ahead of the surface front.

Figure 15.8 As the warm front approaches, the wind will back and strengthen. The blue arrows in the diagram depict the initial backing of the wind.

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Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S ES A ND F R O NT S

V e e- windr direction changes in a clockwise direction.

B a c- windk direction changes in an anticlockwise direction.

As the warm front

approaches, the wind will back and increase but at the

passage of the front it will veer sharply.

C o n d i t i o n s a s t h e W a r m F r o n t A p p r o a c

The wind ahead of a warm front is fairly light, coming from about 250°, but, as the front approaches, the character of the wind begins to change. The wind slowly increases in speed and blows from about 200°; so, with the approach of the warm front, the wind backs slightly and strengthens. Ahead of the front, the visibility will deteriorate, and atmospheric pressure will fall steadily. The temperature, however, will remain at a fairly steady, low value. The dew point temperature will usually be steady, and fairly low, too.

C o n d i t i o n s a t t h e W a r m F r o n t .

At the warm front, itself, the cloud will be low-level, extensive and stratiform, while precipitation will be continuous and moderate, perhaps even heavy. From Figure 15.9, it can be seen that the wind changes direction quite markedly, from approximately 200º, just before the front, to approximately 260º just after the front has passed. The wind, therefore veers sharply.

Figure 15.9. Conditions at the warm front are characterised by thick, low level cloud, continuous heavy precipitation, with a sharp veer in the wind.

When planning a flight, pilots should always bear in mind the effect on wind strength and direction of a passing front. Visibility will be poor, with widespread hill fog developing, over high ground.

As the warm front passes, pressure will continue to decrease slightly; however, the temperature and dew point will suddenly rise in the warm air behind the front itself. The warm air between the cold and warm fronts is called the warm sector.

The cloud in the warm

sector will be extensive, low level stratus or strato-cumulus;

there may also be precipitation in the form of drizzle.

C o n d i t i o n s i n t h e W a r m S e c t o r .

The warm sector lies between the warm and cold fronts; it is a stable, warm, air mass, with fairly consistent weather throughout. The weather conditions associated with the warm sector are characterised by a fairly low, uniform base of extensive stratus or stratocumulus cloud (see Figure 15.7), with the possibility of light drizzle. The wind will be steady, and often strong. The visibility can be poor, with extensive fog over high ground.

By examining the plan view in Figure 15.9, you may be able to deduce that, in the warm sector, as the cold front approaches, along the line AB, the pressure continues to fall slowly. However, the temperature and dew point will remain fairly high and steady, throughout this warm moist air mass, as depicted in Figure 15.7.

T h e C o l d F r o n t .

The cold front, a cross section through which is depicted in Figure 15.10, follows behind the warm sector as the system moves from West to East. Note that the average slope of a cold front is steeper than that of a warm front, usually about

1 in 50.

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S E

In the cold front, cold air advances, and undercuts the warm air, forcing it to rise. This uplift causes the air to cool adiabatically to its dew point temperature, at which stage the water vapour within the warm air mass starts to condense and form cloud. Over the whole extent of the cold front, the main cloud type will be stratiform, since, though steeper than the warm front, the slope of the cold front remains fairly shallow. However, the wedge of cold air, caused by friction holding back the air mass in direct contact with the surface, creates very unstable conditions. The wedge forces air ahead of the cold front to rise rapidly, creating active cumuliform clouds, which sometimes develop into cumulonimbus, ahead of the surface position of the front. This makes the passage of the cold front appear to be quite sudden, with little warning that it is approaching.

Cumuliform clouds on a cold front can create dangerous squall-line thunderstorms, developing rapidly into cumulonimbus cloud which may be hidden inside stratiform cloud layers. Pilots should give cold fronts a wide berth, and keep a watchful eye on the weather immediately ahead of any cold front in their vicinity.

The slope of a cold front will typically be about 1:50.

The strong

convection at the cold front

caused by the

w e d ofgadvancinge cold air is the most common cause of frontal thunderstorms.

Figure 15.10 The Cold Front. Embedded cumulonimbus (CB) cloud inside stratiform cloud layers.

C o n d i t i o n s a t t h e C o l d F r o n t .

An observer on the ground in the British Isles, seeing an approaching cold front, would expect to observe cumuliform cloud or cumulonimbus cloud which may, however, be embedded within the stratiform cloud to give both steady and showery precipitation. The wind will veer sharply, with gusts, and possibly squalls.

At the passage

of a cold front, the w i nwilld

v e esharply.

In p r e c i p thei tva itsi io bn i, will be poor, but will improve

quite rapidly. The pressure will begin to rise, while the temperature and dew point will fall.

Figure 15.11 As the cold front passes, the wind will veer sharply from about 260° to 330°.

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Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S ES A ND F R O NT S

Before the cold front arrives, the surface pressure will fall, but will rise once the front has passed. In precipitation, visibility will be poor, but will quickly improve. The temperature and dew point will usually fall sharply as the cold air behind the front moves in. Figure 15.11 shows the change in wind direction associated with the passing of a cold front. The wind will veer sharply.

C o n d i t i o n s b e h i n d t h e C o l d F r o n t .

Once the cold front has passed, weather will be dominated by the sector of cold Polar Maritime air. Because the Polar Maritime air will have been moving South, and, therefore, travelling over warmer surfaces, it will be unstable. Consequently, isolated cumuliform clouds will develop behind the cold front.

The cumulous clouds in the cold sector will produce moderate to heavy showers of rain, or hail. Pressure will continue rising with the passing of the front, and visibility will be good, though it will deteriorate temporarily in showers. The atmosphere will be unstable behind the cold front.

T h e S p e e d o f W a r m a n d C o l d F r o n t s .

The cold front moves faster than the warm front.

The speed of movement of a warm front may be estimated if you have access to a weather chart incorporating a geostrophic wind scale, as shown in Figure 15.12.

Figure 15.12. Calculating the speed and direction of movement of fronts.

From Figure 15.12, measure the isobaric spacing on the warm front as shown by the orange arrow marked ‘A’. Apply that measurement to the geostrophic wind scale, measuring from the vertical axis, at the appropriate latitude (see orange arrow marked ‘B’), in this case, 60º N; then multiply the answer by 2/3. This gives the approximate speed of the warm front. The front will move in the direction of the symbols, parallel to the isobars behind the front.

The speed of movement of a cold front can be estimated in a similar manner. Again, measure the isobaric spacing on the front, and use the geostrophic wind scale, but do not multiply the answer by 2/3.

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S E

Because a warm front moves more slowly than a cold front, the warm front will eventually be overtaken by the cold front, leading to the creation of an occluded front.

T h e O c c l u d e d F r o n t .

Where the cold front overtakes the warm front, an occluded front is formed, as depicted in Figure 15.13. When the two fronts meet they merge, and, so, to identify this new front, both the warm and cold front symbols appear together on the frontal line of an occluded front.

Figure 15.13 An occluded front is created when the cold front starts to overtake the warm front.

The warm and cold fronts can merge to form an occluded front in two ways.

The cold front

moves faster than the warm

front. The

speed of the warm front is, approximately, two thirds that of the cold front.

The cold front

eventually overtakes the

warm front to

form an occluded front.

There are

two types of o c c l u d

f r o an t :

w a r m o candc lauc so ilod n o c c l u s i o n .

Figure 15.14 A Warm Occlusion: the cold front over-rides the warm front.

If the cold front rides up and over the warm front, as shown in Figure 15.14, the front so formed is called a warm occlusion. If, however, the cold front undercuts the warm front, as shown in Figure 15.15, overleaf, a cold occlusion is created.

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If there is little interaction between the warm Tropical Maritime and Polar Maritime air, no kinks or disturbances will occur at the front boundary. In such circumstances, a quasi-stationary front is formed. Figure 15.16 depicts how the cold and warm front symbols are used to signify the quasi-stationary front. Note that the winds will blow parallel to the frontal position line, as depicted by the blue and white arrows.

Figure 15.16 The Quasi-Stationary Front. This is a polar front which has no observed kink or disturbance along its axis. Winds are parallel to the front axis.

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C H A P T ER 1 5 : A IR M A S S ES A ND F R O NT S

In a flight across an

occlusion, a pilot would

encounter severe weather and a significant change in wind speed and direction.

				Figure 15.15 A Cold Occlusion: the cold front undercuts the warm front.
		O		The weather associated with an occlusion is a combination of warm and cold front
			c c l u s i o n
		w	e a tish e r weather. Typically, occlusion weather is characterised by long periods of continuous
		characterised		rain, interspersed with heavy showers. Occlusions in the British Isles have been
		by long		responsible for most of the major flooding over recent years.
periods of continuous rain,
interspersed with long				T h e Q u a s i - s t a t i o n a r y F r o n t .
showers.				Figure 15.16 returns to the illustration of the polar front, from which we began the

description of frontal systems.

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S ES A ND

R	e p r e s e n t a t i v e	P P L
k n o w l e d g e o f A		i r M

- t y p e q u e s t i o n s t o t a s s e s a n d F r o n t s .

1.The extent of rainfall ahead of a typical warm front may stretch up to distance of:

a.50 miles ahead of the surface position of the warm front

b.200 miles ahead of the surface position of the warm front

c.500 miles ahead of the surface position of the warm front

d.2 miles ahead of the surface position of the warm front

2.Seen from the surface, the passage of a warm front is characterised by which of the following cloud types, seen in sequence:

a.Embedded cumulonimbus within dense nimbostratus

b.Cirrostratus, cirrocumulus, nimbostratus and finally cumulus

c.Cirrus, cirrostratus, altostratus, nimbostratus and finally stratus

d.Nimbostratus, altostratus, cirrostratus and finally cirrus

3.What will normally happen to the surface wind direction following the passage of a warm front?

a.Stay constant

b.Back

c.Veer

d.Veer then back

4.What is the general speed of a warm front?

a.Approximately 1/3 of the speed of the cold front

b.Approximately 2/3 of the speed of the cold front

c.Approximately 1/2 the speed of the cold front

d.The same speed as the cold front

5.Which air mass arriving over the United Kingdom would be characterised by cold, moist, unstable air?

a.Polar maritime

b.Tropical maritime

c.Polar continental

d.Tropical continental

6.Your planned flight lies within the warm sector of a polar front depression.

What would you expect the cloud to be?

a.A fairly low, uniform base of stratus or stratocumulus cloud

b.Low, medium and high level cloud

c.Only high level cloud

d.Clear skies

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Customer: Oleg Ostapenko E-mail: [e-mail]

C H A P T ER 1 5 : A IR M A S S ES A ND F R O NT S Q U ES T IO NS

7.You observe the passage of a frontal system and notice this sequence of clouds: cirrus, cirrostratus, altostratus, nimbostratus and stratus. What kind of a front are you observing?

a.A cold front

b.An occluded front

c.A stationary front

d.A warm front

8.When two air masses converge in a depression, and warmer air replaces the colder air at the surface, the front is known as:

a.A cold front with a typical slope of 1:150

b.A warm front with a typical slope of 1:150

c.A warm front with a typical slope of 1:50

d.A cold front with a typical slope of 1:50

9.The conditions most likely to be encountered when following an East/West track that crosses an occluded front lying North/South, would be:

a.Thick stratus and drizzle ahead of the occlusion, with a light wind generally backing across the occlusion

b.Calm conditions before the occlusion, with considerable backing of the wind across the occlusion

c.Extensive rain, interspersed with heavy showers

d.Calm, settled weather with poor visibility

10.When dry air flows over a warm water surface, the air in contact with the surface will absorb water vapour and become:

a.More dense and stable

b.Less dense, colder and remain at the surface

c.Less dense and unstable

d.More dense and cool

11.What are the characteristics of the passage of a cold front?

a.A dew point rise, a temperature fall and wind backing

b.A dew point rise, a temperature fall and wind veering

c.Steady dew point and temperature but a sharp backing in the wind

d.A dew point fall, a temperature fall and the wind veering

12.Tropical maritime air that affects European weather originates from:

a.The Azores

b.The Indian Ocean

c.The Mediterranean

d.The North Sea in summer

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