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In the temperate latitudes (between the tropics and the polar circles), warm tropical air meets cold polar air. The boundary between warm and cold air masses is called the polar front. As with any boundary between two air masses of different densities, the boundary will not be a straight line. At some points along the front there will be

“kinks” or small irregularities. In these kinks, the warm tropical air intrudes into the cooler polar air, as depicted in Figure 5.9.

The lighter, warmer air in the kink replaces the colder, heavier air, so the weight of the overlying air is reduced, leading to a fall in the surface pressure.

As the surface pressure falls, air will be drawn in towards the area of low pressure.

However, because the air movement is on a large scale, this displacement of air is deflected by the rotation of the Earth.

In the Northern Hemisphere, the deflection of the moving air mass, as shown in Figure 5.10, is to the right, causing winds to blow anti-clockwise around the depression. In the Southern Hemisphere the deflection is to the left, and so, South of the Equator, the wind blows clockwise around a depression. This deflection of the moving air mass is caused by a force known as the Coriolis Force. Coriolis Force will be covered in more detail in Chapter 12, Low Level Winds.

Figure 5.10 In the Northern Hemisphere, air is drawn towards the Low, and is deflected to the right by Coriolis Force, causing the wind to blow anti-clockwise around the depression.

Figure 5.9 Along the Polar front, kinks develop. This is where warmer, less dense, air intrudes into colder air, reducing the weight of the air and creating a centre of low pressure at the

Earth’s surface.

CHAPTER 5: PRESSURE SYSTEMS

There are two types of interaction between the warm and cold air masses which are fundamental to the understanding of frontal weather systems.

The Warm Front.

Along the section of the front depicted by the red semi-circular symbols in Figure 5.11, warm air is being forced against the cooler air.

The warmer air, being less dense, will ride up and over the colder air. The weather feature created in this way, and depicted in Figure 5.12, in cross section, is called a warm front.

Because the warm air is being forced upwards into lowering atmospheric pressure, the air expands and cools. Water vapour in the air, therefore, condenses, creating cloud along the frontal boundary. The cloud takes on a horizontal, layered appearance since the slope of the warm front is about one in one hundred and fifty, much shallower than the slope indicated in the diagram at Figure 5.12. This layer-type cloud is called stratiform cloud.

Figure 5.11 The red semi-circular symbols indicate a warm front, where warm air along the polar front rides up over cold air.

Figure 5.12 The warm front - warm air being forced over cooler air.

CLOUD TYPES ASSOCIATED WITH THE WARM FRONT Ci = Cirrus Cs = Cirrostratus As = Altostratus Ns = Nimbostratus St = Stratus

The Cold Front.

The other main feature of the polar front is depicted in Figure 5.13. Along the line marked by the blue triangles, the blue colder air is being forced against the red warmer air. The colder, heavier air will undercut the warmer, lighter air in the form of a wedge creating a cold front (see Figure 5.14). The blue triangles, in Figure 5.13 are the standard symbols denoting a cold front.

As the cold air advances, it forces the warm air upwards causing the warm air to cool. The water vapour in the warm air mass, consequently, condenses, and clouds are created. The slope of the cold front boundary is steeper than that of the warm front, with a gradient of approximately one in fifty. If we consider the whole extent of the cold front, cloud will still take on a general stratiform appearance. However, there is a fundamental difference along the cold front compared to the warm front. Notice that the cold front slopes forwards first, then slopes backwards creating a wedge shape. The wedge shape is formed because the portion of the front in contact with the ground will slow down due to friction as the front advances, and, as a result, will lag behind the air immediately above it. This phenomenon creates instability in the warm air which is in direct contact with the wedge.

Figure 5.13 The cold front. The blue triangle symbols denote a cold front, where cold air pushes itself under the warm air.

Figure 5.14 The Cold Front - cooler air undercuts warmer air, forcing it to rise. Just ahead of the cold front, vigorous vertical ascent of the warm air creates cumuliform cloud.

CLOUD TYPES ASSOCIATED WITH THE COLD FRONT Cs = Cirrostratus

As = Altostratus Ns = Nimbostratus Cu = Cumulus Cb = Cumulonimbus

CHAPTER 5: PRESSURE SYSTEMS

The unstable air just ahead of the cold front is, therefore, forced to rise vigorously.

This vigorous vertical ascent creates vertically developed cumuliform cloud. The cumuliform clouds formed in advance of the cold front are potentially hazardous, often becoming storm clouds, called cumulonimbus which may often be embedded or concealed within the stratiform cloud. (See Figure 5.14.)

Note that the cold and warm fronts, depicted on weather charts by the semi-circular and triangular symbols, mark the surface position of each of the fronts.

Isobars.

Isobars on weather charts are lines of equal surface pressure. A typical pattern of isobars around a polar front depression is shown in Figure 5.15.

You should note that Polar Front Depressions move from West to East, so that, in a typical frontal system, the warm front precedes the cold front. Therefore, as the warm front approaches from the West, pressure falls. Between the fronts, the pressure falls slightly, but after the cold front has passed, pressure will start to rise again. (See Figure 5.15.)

Figure 5.16, is a typical UK Met Office chart showing Polar Front Depressions with their associated cold and warm fronts and isobars. In Figure 5.16, there is, however, a feature that we have not yet mentioned. This is the occluded front.

Figure 5.15 Isobars on weather charts show the fall and rise in pressure as a depression moves from West to East.

Polar front depressions are formed by kinks

which develop in the boundary between tropical and polar air.

Warm air is forced upwards by the cold air and cloud is formed.

Figure 5.16 Occluded front - the merging of a cold and warm front.

The Occluded Front.

If you examine the highlighted area in Figure 5.16 you will notice that the warm and cold front symbols are found together along one line. When the symbols are arranged in this manner, they indicate the presence of an occluded front. Occluded fronts are created when the cold and warm fronts merge. Occluded fronts will be explained in more detail later on, in Chapter 15, “Air Masses and Fronts”.