Perched on the 15th floor of the New Secretariat Building near Calcutta’s Dalhousie Square, a weather radar scans the skies, its tracking beam sweeping Jharkhand’s Chhotanagpur plateau twice every minute.

For radar operators this time of the year, the Chhotanagpur plateau is a zone of special interest — the origin of Nor’westers, the thunderstorms that batter Bengal and eastern India year after year during April through May.

But today there was no sign of a Nor’wester.

“It’s a bit unusual,” said Devendra Pradhan, director of the India Meteorological Department’s radar centre in Calcutta. “We haven’t had a thunderstorm since April 15.”

A wind from the Bay of Bengal is bringing in moisture. “A good sign,” Pradhan said. Conditions are turning favourable for a thunderstorm.

The topography of the Chhotanagpur plateau, the summer heat, and moisture from the Bay combine to produce the Nor’wester. When the conditions are right, hot air rises high into the atmosphere, where a tall column of cloud — sometimes 18 to 20-km high — builds up.

“This is the only region in the world where vertical cloud thickness reaches up to 20 km,” said Pradhan. It’s these systems that evolve into Nor’westers.

“They’re more frequent than cyclones, and can cause much trouble,” said Someswar Das, an Indian meteorologist now on deputation at a South Asian Regional weather centre in Dhaka. Nor’westers cause houses to collapse, bring down trees, and even cause electrocution.

Since March this year, Pradhan and his colleagues have tracked 14 thunderstorms — seven in March and an equal number in April to date.

But over the past week, there’s not been enough moisture flow from the Bay.

Moisture from the Bay would increase the chance of a Nor’wester. But scientists can’t predict Nor’westers.

“They develop fast and decay fast — we’re still trying to understand the mechanisms,” said a weather scientist in Pune trying to model thunderstorm behaviour.

But the weather radar with a range of 300 km allows scientists to track cloud and moisture build-up in the Chhotanagpur plateau — about 250-km away.

Once a thunderstorm begins, the radar readings allow scientists to calculate its speed and path.

“We can predict thunderstorm movement about two or three hours in advance with accuracy,” said a scientist.

Whether a Nor’wester reaches Calcutta depends on its structure.

Thunderstorms may emerge as single storms or appear in clusters. “A multi-cell thunderstorm moves faster than a single cell storm,” said Pradhan.

Bay wind

The Met office said the maximum temperature in Calcutta and the districts was unlikely to increase in the next few days, adds a staff reporter. On Friday, the maximum temperature was 35.5 degrees Celsius.

“The southerly wind is blowing strongly from the sea and the hot wind from the west has shifted course,” said G.C. Debnath, of the Regional Meteorological Centre at Alipore.