Depression
Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Class 7, Ch 2 "Understanding the Weather"; Class 7, Ch 3 "Climates of India"
Meaning
A depression is an area of low air pressure surrounded by closed isobars (lines on a weather map joining places with the same pressure). Normal sea-level pressure is about 1,013 mb (millibars). Winds blow inward towards the low and turn around it, and the rising air brings cloud and rain. On the India Meteorological Department (IMD) scale, a depression has winds of 31–49 km/h. It becomes a deep depression at 50–61 km/h and a cyclonic storm at 62 km/h or more.
Example
During the south-west monsoon, depressions form over the head of the Bay of Bengal. They move west-north-west across Odisha and central India and bring heavy rain to the plains.
Don't confuse with
- Low-pressure area: a weaker stage, with winds below 31 km/h, before a system becomes a depression.
- "Depressions" of north-west Europe: there, the word means extra-tropical cyclones. These are frontal lows of the middle latitudes.