DEPRESSIONS

DEPRESSIONS Depression is a mass of air whose isobars form an oval or circular shape, with low pressure at the centre. The air converges at the centre and rises to be disposed off. In a depression, the winds rotate anticlockwise in the northern hemisphere. While in the southern hemisphere, the circular movement of winds is in a clockwise direction. Depressions are rarely stationary and tend to follow definite tracks. They are most influential over the ocean spreads and they weaken over land areas. They are of two types.

1. Depression or Temperate Cyclones Also known as extra-tropical or wave cyclones, temperate cyclones are active over mid-latitudinal region between 35° and 65° latitudes in both hemispheres.
Origin and Development There are two theories of origin of temperate cyclones.
(i) Polar front theory by Bjerkenes According to this theory, the warm-humid air masses from the tropics meet the dry-cold air masses from the poles and thus a polar front is formed as a surface of discontinuity. Such conditions occur over sub-tropical highs, sub-polar lows and along the
. tropopause. The cold air pushes the warm air upwards from underneath. Thus a void is created because of lessening of pressure. The surrounding air rushes in to occupy this void and, coupled with the earth's rotation, a cyclone is formed which advances with the westerlies.

(ii) Thermodynamic theory by Lampert and Shaw Accord­ing to this theory, in sub-tropical areas, an overcrowding of vertical currents releases the surplUs energy upwards which, after meeting the upper cool air, converts into an eddy. This eddy tends to settle down as an inverted 'V' shaped cyclone.
The temperate cyclones occur mostly in winter, late autumn and spring. They are generally associated with rainstorms and cloudy weather.
Distribution The favourite breeding grounds of temper­
ate cyclones are listed below.

1. Over USA and Canada, extending over Sierra Nevada, Colorado, Eastern Canadian Rockies and the Great Lakes region.
2. Mexican Gulf.
3. The belt extending from Iceland to Barents Sea and
continuing over Russia and Siberia.
4. Winter storms over Baltic Sea.
5. Mediterranean basin extending upto Russia and even
upto India in winters (called western disturbances).
6. The Antarctic frontal zone.
During summer, all the paths of temperate cyclone shift northwards and there is no temperate cyclone ove sub-tropics and the warm temperate zone, although a hig] concentration of storms occur over Bering Strait, USA ani Russian Arctic and sub-Arctic zone.