Depression or a low is an atmospheric vortex with a central region of low pressure. In the northern hemisphere, the winds blow round the center in a counter-clockwise direction. The intensity of the vortex is measured by the strength of winds. Thus, when the winds round the vortex are strong, the depression is classified as a deep depression; with still stronger winds a deep depression becomes a cyclonic storm and so on.

System Range of wind speed (m/s)

Low
8.5

Depression
8.5 - 13.5

Deep Depression
14.0 - 16.5

Cyclonic Storms
17.0 - 23.5

Severe Cyclonic Storms
24.0 - 31.05

Hurricanes / Typhoons
Greater than 32.0

Meteorologists have often wondered how these depressions form and why do they always move towards the West.

Nearly fifty years ago, an Indian meteorologist, the late V.Doraisami Iyer, examined the tracks of cyclonic storms over the Pacific Ocean and the South China Sea for the years 1884 - 1930. He observed that 135 out of 370 cyclonic storms ultimately moved over India as residual low-pressure systems.

In recent investigations, two Indian meteorologists, K.R. Saha and J. Shukla collaborating with F. Sanders from the Massachusetts Institute of Technology examined 52 lows and depressions in the Bay of Bengal during the ten-year period 1969-78. They observed that 50 of these disturbances were westward propagating systems from the northwestern parts of Thailand. Of this number, 32 were associated with a subsequent development in the Bay of Bengal.

Acknowledgement: Excerpt from 'The Monsoons' by Dr.P.K. Das, former Director General of the National Meteorological Service of the Government of India. Published by the Director, National Book Trust, India. Price: Rs.75/=.