EXPLAINED: Factors that led to second-coldest December in Delhi after 1901

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Absence of a strong western disturbance for the past 10 days, which could have disrupted the direction of icy winds coming to the national capital from the hills, resulted in Delhi experiencing its second-coldest December in over a century, weather experts say. Western disturbances in early and mid December led to heavy snowfall in hilly areas north of Delhi and frigid winds have been blowing in from there. This led to a sharp decline in temperatures in the plains, said Kuldeep Srivastava, head of IMD's regional weather forecasting centre.

Another western disturbance could have changed the wind direction away from Delhi, but an absence of such a weather system for 10-12 days aided the conditions for a harsh winter.

"The region has witnessed a long gap of 10-12 days between western disturbances. Usually, there's only a gap of 3-4 days in two WDs, and as a result the wind direction keeps on changing. This time, the cold winds continued unabated from north-northwest for over 10 days in absence of a strong western disturbance," Mahesh Palawat of Skymet Weather, a private forecaster, said.

The India Meteorological Department said the mean maximum temperature for December 2019 will be below 19 degrees Celsius, the lowest since December 1997 (when it was 17.3 degrees Celsius) and the second-lowest since 1901, when IMD started keeping the records.

The factors responsible for such a cold December are a "very long spell" of icy winds and a layer of fog lingering over vast swathes of the northern plains for 10-12 days, the experts said.

The wind speed has remained moderate, dispersing the fog near the ground. But a layer of fog persisted in the upper atmosphere at a height of 2,000-3,000 feet, from Punjab to Uttar Pradesh, which did not let sunlight penetrate through to the ground.

"Due to hazy sunshine and cold winds, the day temperatures remained low," Palawat said.

Srivastava said a change in wind direction on Sunday did not help much as the easterly winds blowing from Uttar Pradesh to Delhi are also very cold.

Usually, easterly winds are comparatively warmer and increase the temperatures, he said.

"So, both northwesterly and easterly winds are cold and causing a drop in the mercury," he said.

Easterly winds have brought moisture which has led to formation of dense fog, further affecting the day temperatures, said both of them.

Since December 14, Delhi has been witnessing "severe cold/cold days". A streak of cold days is known as a "cold spell". This means Delhi has recorded a 16-day December cold spell, the longest after the 13-day cold spell in 1997.

According to IMD, a "cold day" is when the maximum temperature is at least 4.5 notches below normal. A "severe cold day" has the maximum temperature at least 6.5 degrees Celsius below normal.

Delhi experienced its coldest recorded December day since 1901 on Monday, recording a maximum of just 9.4 degrees Celsius.

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