Why cyclones are less common during main monsoon months like July and August?

New Delhi, May 6: The monsoon normally arrives in Kerala around June 1 and covers the rest of India by mid-July and brings about 70 per cent of India's annual rainfall. The rainfall is expected to be mostly uniformly distributed.

Normal to above normal seasonal rainfall is most likely over many areas of northern parts of Peninsular India and adjoining Central India, over foothills of the Himalayas and some parts of Northwest India, the IMD had said last month.

Why Cyclones are less common during main monsoon months like July and August?

According to India Meteorological Department, Tropical cyclogenesis requires several favourable precursor environmental conditions. Warm Ocean waters (of at least 26.5℃ throughout sufficient depth at least on the order of 50 m). Relatively moist layers near the height of 5 km. Nonnegligible amount of Coriolis force, pre-existing near surface disturbance. Low values of vertical wind shear between the surface and upper troposphere.

In July and August winds on the surface are westerly/south-westerly to the south of the monsoon trough and south easterly/easterly to its north and are generally stronger over the seas than the Land areas. The upper winds are westerly/south-westerly to the south and south easterly/ Easterly to the north of this trough region.

Westerly winds increase with height and reach a maximum speed of 20-25 knots between 900 to 800 hPa levels. Easterly winds strengthen with height from 200 hPa reaching a maximum at 100 hPa. Speeds are between 60 to 80 knots over peninsula at 150 /100 hPa level or even at lower height (around 200 hPa) in the southern latitude. This results in high values of vertical wind shear which is unfavourable for Tropical cyclogenesis. So, we don't get cyclones during main monsoon months like July and August.