They are formed in the temperate latitudes (i.e. 60° N and 60°S) when humid tropical air meets cold polar air.
Westerly winds meet polar winds.
The zone were these two different currents meet is called the polar front.
It is in this zone where depressions are formed.
Stages in the formation of a depression.
1. Embryo stage.
The first stage is known as the embryo stage.
Cold air moves in a general westerly direction along the polar front.
Warm tropical air moves in a generally easterly direction.
The frictional effects of the two air flows causes a wave to develop in the front as shown.
2. Mature depression
In a mature depression the cold front starts to catch up with the warm front.
The wave bulges into the colder air and gets larger.
Pressure falls at the tip of the wave and due to the Coriolis force the wind blows around the low pressure point in clockwise direction in the southern hemisphere and anti-clockwise in the northern hemisphere.
As the bulge described above develops, the warm air rises up over the colder air at the front of the bulge.
This front is called the warm front.
At the rear of the cold air forces its way under the warm air.
The rear is called the warm front.
The result is what is shown in the first diagram above.
A cold front at the rear and a warm front at the fore.
Weather associated with depression.
The first diagram above shows a cross section of a depression and weather associated with their passage.
Whenever you are asked to describe the weather associated with depressions in an exam it is always a good idea to draw a labelled diagram like this one first before using words to describe the relevant weather patterns.
The weather associated with the passage of a depression is as follows.
Passage of the warm front:Clear weather with a few high cirrus clouds. Winds will blow from the SE for a while. As cloud cover grows light shows begin and these grow heavier.
The warm sector-the warm sector is the area between the two fronts when it approaches and passes a place the rain stops, the weather clears, temperatures rise, the air is humid and the wind changes from SE to SW.
Passage of the cold front-The weather changes rapidly as the cold front passes, the wind blows from the NW,temperature falls, there is heavy rain and thunderstorms, with cumulonimbus clouds.
Passage of the depression-the sky clears and the temperature remains cool.