Why was winter so severe in the entire North and Central India this January? Why did Mumbai and Kolkata, known for balmy winters, actually shiver at 10 degrees Celsius? The low temperatures were directly a cause of unusual wind patterns this year — impacting not just the Indian subcontinent but paralysing China for about a week under heavy snow.

Between latitudes 30 and 50 degrees North, ridges of high pressure and troughs of low pressure form all the time. They survive in a competitive atmosphere. One forces the other to give way and the pattern changes every few days ushering in normal weather in India, South East Asia and China.

This year, something different happened.

An obstinate ridge (a line of high pressure) formed over the Central Asian range extending up from 25 degrees North to 45 degrees North. “For nearly two weeks, starting January 21, it refused to budge from its position,” says Akhilesh Gupta, advisor to the Minister for Science and Technology.

The result: winds to its west were forced to climb up from lower to higher latitudes. These winds climbed as high as 47 degrees North, the region of erstwhile Russia.

Once the winds went right up and became icy cold, they were forced downhill on the other side of the ridge,right over the Himalayas into the Indo-Gangetic plains.

In the absence of the ridge in this position, the winds would have flowed straight from comparatively warmer Afghanistan and Pakistan into India. Since it would not be so cold, it would also collect moisture over the Arabian Sea. Warm winds laden with this moisture would result in a dense fog blanket over entire North and North West India.

... contd.