Mist covers Darjeeling on Sunday while tourists (below) take a stroll at Chowk Bazaar in town after a heavy rain. Pictures by Suman Tamang

Siliguri, Aug. 7: Heavy rain lashed the Darjeeling hills and Sikkim last night, brought about by a cyclonic circulation with a fast upper air wind movement that results in what weather experts call “orographic effect”.

Heavy rain for high altitude areas and mild showers for plains was the result of the orographic effect as was evident from recordings in the Terai, the Dooars and Cooch Behar in the past 24 hours (see chart below).

While the Darjeeling hills and Sikkim received more than 65mm rain, several places in the plains recorded less than 50mm rain. Rainfall above 65mm is considered heavy by the weather experts.

South Bengal also experienced heavy rain yesterday because of the cyclonic circulation that prevails over Gangetic Bengal and Bangladesh.

Officials of the Jalpaiguri Met Office say the Darjeeling hills and Sikkim have recorded higher rainfall as the upper air wind in a cyclonic circulation moves at high speed and is forced to rise further when it meets a hilly barrier and finally precipitates on cooling.

“South Bengal is experiencing heavy rainfall because of the cyclonic circulation that prevails over Bangladesh and Gangetic Bengal. In such situations, the upper air wind moves fast as it is higher above the ground. The moisture-laden upper air wind travelled to north Bengal where it was obstructed by the hills in Darjeeling and Sikkim,” said Indranil Sengupta, the assistant meteorological officer at Jalpaiguri Met Office.

“When a parcel of warm air reaches a hill, it is lifted up the slope and becomes cool. This process is known as orographic lifting. The cooling of the air often results in large clouds, precipitation and even thunderstorms,” he added.

According to Sengupta, the orographic impact is less in places located away from the hills.

“The orographic lifting has some effect in the foothills. But the effect gradually comes down at locations farther away from the hills. This is the reason why hills received heavy showers, while plains recorded less rainfall,” said Sengupta.

The official said the hills would experience similar conditions as long as the cyclonic circulation persisted over south Bengal and Bangladesh.

“The hills will receive more thundershowers as long as the cyclonic circulation prevails over south Bengal. Heavy rainfall is forecast for some locations in sub-Himalayan Bengal and Sikkim in the next 24 hours,” said the weather official.

Erosion crisis

Irrigation minister Manas Bhuniya said on Sunday that the state government alone could not fight floods and erosion in Malda district.

“The Centre must come up and help the state government meet the crisis. The flood waters, sand and boulders are flowing from north India to Malda through the Ganga. The Kosi river originating in Nepal is also doing similar damage,” said Bhuniya.

The minister reviewed the erosion and rise in water levels of rivers in Malda with the district administration and irrigation officials on Sunday.