New Delhi, After reeling from February 25 till possibly May 1, one of the longest dry spells of 65 days, the plains of northwest India -- including Delhi-NCR -- are expecting a reprieve due to a Western Disturbance (WDs) on May 2.

Incidentally, there were six WDs in April -- on April 1-4, 7-9, 13-15, 20-22, 23-25 and 28-29 -- that have had an impact on northwest India.

But most of them were feeble and dry and moved across the higher ridges of the Himalayas.

"Only last three systems caused gusty winds and dust rising winds and dry thunderstorms at several places, including Delhi NCR, and also dust storms over Rajasthan on April 14, 22 and 25," said India Meteorological Department (IMD) director general (meteorology) Mrutyunjay Mohapatra.

Because of these dry WDs, since 122 years, April has become the hottest months for northwest and central India and fourth hottest.

So what caused these WDs to be dry?

As the name suggests, WDs are extra-tropical systems that originate in the Caspian Sea, Red Sea area and move towards east and traverse over Iran, Afghanistan and Pakistan to reach India.

"If the WDs move across southern latitude, it gets moisture from Arabian Sea, which in turn leads to rainfall activity over northwest India. If it does not, its impact will be dry," Mohapatra explained and added: "Of the six WDs in April, only one had southerly trough but the rest moved east-northeast wards."

The reason why these systems were feeble lies in its extra-tropical origin, the IMD DG explained.

"Unlike the tropical systems that are dependent on convective phenomenon, the extra-tropical systems depend upon temperature gradient as determined by temperature difference between north and south in the middle latitude area. If it is more, then WD will have higher energy, it will be more intense and vice versa," he said.

That explains why winters have stronger WDs when the temperature gradient in the Caspian Sea, Red Sea area is higher.