How does a depression bring heavy rains

A circular low pressure area roughly 300 Km wide with a surface wind of 25 Knots (45 Kmph) is defined as a “depression.”

The circulation of air, anticlockwise in Northern hemisphere, provides sufficient updraft to generate all types of clouds and the depression could be seen like a full moon in a satellite photograph.

The circular mass of clouds called “Central Dense Overcast” or CDO in meteorological parlance is a confirmation of the formation of a depression.

With a base 5,000 feet or less, the CDO vertically extends up to 18,000 feet or so. The rains that occur release the latent heat of condensation.

A small part of it, say 10 to 20 per cent is enough to increase the circulation in the form of increased wind speeds at lower levels of the atmosphere. The depression intensifies into a deep depression.

Unless the sea is cold (sea surface temperature less than 28 Deg C) or the depression crosses a coast, this intensification keeps on going in this fashion as if there is no end to it.

Around 8,000 to 10,000 feet the circulation generates a white sheet of cloud, known as altostratus (As) with a thickness that could be 5 to 8 thousand feet. This can extend to hundreds of kilometres from the centre of the depression.

This is what is seen by a common man when continuous rain of moderate intensity starts occurring over very large areas and he calls it as “depression rain” by his experience.

The area covered by the rains will depend on the season. In July, when monsoon conditions are very favourable, a depression in head Bay, south of Bangladesh could bring rains of such intensity over Mumbai that it could paralyze the city. A cloud belt 200 Km wide extending from Mumbai to Kolkata too will give widespread light to moderate rains.

On the other hand, during the northeast monsoon period, a depression 100 Km off Karaikal will give heavy rains over the east coast from Kakinada to Kodikarai and some rains over areas south of Karaikal latitude.

The As will not easily dissolve for one to see the sky as the moist air incursion due to the depression will be sufficient to replenish the water vapour that is lost in the form of rains.

The low clouds that surge towards the depression will partially rise up to merge with As as they approach the depression and the rest will reach the depression area.