Much of the highly variable and cloudy weather we come across in the temperate zone is the direct result of travelling cyclones. The terms extra tropical cyclones, temperate cyclones, or depressions are interchangeably used to denote these moving cyclones in the mid-latitude zone.
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Since the middle latitudes are an area of convergence where contrasting air masses generally meet, it is there that the cyclones and anticyclones travel with varying regularity along with the prevailing westerly winds.
As centers for converging and rising air these moving cyclones produce cloudiness and precipitation. They also bring about changes in temperature and air- pressure.
Extra tropical cyclones develop in regions lying between 30° and 65° north and south latitudes in both the hemispheres. It is in these latitude zones that the polar and tropical air masses meet and form what is known as the polar fronts.
Most of these cyclones form at a wavelike twist or perturbation on these fronts. Since a cyclone on a weather map is shown as a low pressure area enclosed by a number of isobars circular or elliptical in shape, it is also referred to as a low or a depression.
When the isobars take an elongated shape, the pressure system is called a trough. Depicts the general lay-out of cold and warm fronts in an extra tropical cyclone as shown on a surface weather map.
Shape and size:
There is a great degree of variation in the shape and size of middle-latitude cyclonic storm. No temperate cyclone is ever exactly like any other. Generally the isobars are almost circular or elliptical.
However, in certain depressions, the isobars take the shape of the letter V. Such storms are called a V- shaped depression. At times, the cyclones become so broad and shallow that they are referred to as troughs of low pressure.
There are occasions when these storms become greatly elongated and lose some of the common characteristics of an ordinary temperate cyclone.
The V-shaped depressions are generally oval-shaped with one part relatively wider than the other. The long axis of this type of depression is aligned in the south-west to north-east direction with its wider part towards the north.
The short axis is arranged in the north-west to south-east direction. The long axis is often twice the short axis in length. These storms vary in size, intensity and other characteristics such as speed, strength of winds, and amount and type of cloud cover.
The diameter of the temperate cyclones may vary from 160 km to 3,200 km. But most of the cyclones have diameters measuring 300 to 1500 km. The average length of extra tropical cyclones in the United States of America is about 1600 km.
The estimated area covered by an average cyclone is about 1.6 million sq. km. The vertical extent of an average-sized cyclone is estimated to be 10 to 12 km.
The air pressure in a cyclone is lowest at the centre and increases towards its margins. The pressure at the centre varies from one storm to another.
A strong cyclonic circulation may have as low as 940 to 930 mb pressure at its centre, while a moderate cyclonic storm may have about 1000 mb pressure.
The pressure difference between the centre and the outer margin of a low may vary from 10 to 20 mb. In a very large and intense cyclone, this pressure difference may be as much as 35 mb.