<p>The average minimum temperature in Delhi in December was the second-lowest in 15 years, according to the India Meteorological Department (IMD).</p>.<p>Data released by the IMD on Thursday showed that the mean minimum temperature (MMT) this December was 7.1 degrees Celsius. It was 7.6 degrees Celsius last year.</p>.<p>The MMT for December in Delhi dipped below 7 degrees Celsius only once in the last 15 years, in 2018 when it was 6.7 degrees Celsius, it stated.</p>.<p>The average MMT for December was 6 degrees Celsius in 2005 and 5.9 degrees Celsius in 1996, according to the IMD data.</p>.<p>Delhi also recorded eight cold wave days this December. It had recorded an equal number of cold wave days in December 2018.</p>.<p>The city had recorded nine cold wave days in 1965, the maximum so far, the IMD said.</p>.<p>In the plains, the IMD declares a cold wave if the minimum temperature dips to 4 degrees Celsius.</p>.<p>A cold wave is also declared when the minimum temperature is 10 degrees Celsius or lower and at least 4.5 notches below normal.</p>.<p>According to Kuldeep Srivastava, the head of the IMD's regional forecasting centre, clear skies over Delhi-NCR, multiple western disturbances affecting the Himalayan region and the global impact of La Nina were the major reasons behind such low minimum temperatures.</p>.<p>Delhi recorded "near-normal" minimum temperatures till December 12 as a result of clouds and rainfall in the plains under the influence of western disturbances affecting the region, he said.</p>.<p>Clouds trap some of the outgoing infrared radiation and radiate it back downward, warming the ground.</p>.<p>"After December 12, western disturbances mostly affected the western Himalayan region, leading to significant snowfall and rain over Jammu and Kashmir and Himachal Pradesh," Srivastava said.</p>.<p>After the wind system withdraws, cold north-westerly winds blow from Jammu and Kashmir and Himachal Pradesh to Delhi-NCR, bringing the minimum temperature down, he said.</p>.<p>"Uplifted fog" over Punjab and Haryana made the winds even colder, he added.</p>.<p>"Besides, the sky over Delhi-NCR remained clear on most days. The global factor of La Nina further contributed to the fall in temperatures," Srivastava said.</p>.<p>La Niña is characterised by below-normal sea surface temperatures in the Pacific Ocean near the equator, a result of shifting wind patterns in the atmosphere.</p>.<p>It means colder-than-normal winter across the Northern Hemisphere and warmer-than-average temperatures in the Southern Hemisphere. </p>
<p>The average minimum temperature in Delhi in December was the second-lowest in 15 years, according to the India Meteorological Department (IMD).</p>.<p>Data released by the IMD on Thursday showed that the mean minimum temperature (MMT) this December was 7.1 degrees Celsius. It was 7.6 degrees Celsius last year.</p>.<p>The MMT for December in Delhi dipped below 7 degrees Celsius only once in the last 15 years, in 2018 when it was 6.7 degrees Celsius, it stated.</p>.<p>The average MMT for December was 6 degrees Celsius in 2005 and 5.9 degrees Celsius in 1996, according to the IMD data.</p>.<p>Delhi also recorded eight cold wave days this December. It had recorded an equal number of cold wave days in December 2018.</p>.<p>The city had recorded nine cold wave days in 1965, the maximum so far, the IMD said.</p>.<p>In the plains, the IMD declares a cold wave if the minimum temperature dips to 4 degrees Celsius.</p>.<p>A cold wave is also declared when the minimum temperature is 10 degrees Celsius or lower and at least 4.5 notches below normal.</p>.<p>According to Kuldeep Srivastava, the head of the IMD's regional forecasting centre, clear skies over Delhi-NCR, multiple western disturbances affecting the Himalayan region and the global impact of La Nina were the major reasons behind such low minimum temperatures.</p>.<p>Delhi recorded "near-normal" minimum temperatures till December 12 as a result of clouds and rainfall in the plains under the influence of western disturbances affecting the region, he said.</p>.<p>Clouds trap some of the outgoing infrared radiation and radiate it back downward, warming the ground.</p>.<p>"After December 12, western disturbances mostly affected the western Himalayan region, leading to significant snowfall and rain over Jammu and Kashmir and Himachal Pradesh," Srivastava said.</p>.<p>After the wind system withdraws, cold north-westerly winds blow from Jammu and Kashmir and Himachal Pradesh to Delhi-NCR, bringing the minimum temperature down, he said.</p>.<p>"Uplifted fog" over Punjab and Haryana made the winds even colder, he added.</p>.<p>"Besides, the sky over Delhi-NCR remained clear on most days. The global factor of La Nina further contributed to the fall in temperatures," Srivastava said.</p>.<p>La Niña is characterised by below-normal sea surface temperatures in the Pacific Ocean near the equator, a result of shifting wind patterns in the atmosphere.</p>.<p>It means colder-than-normal winter across the Northern Hemisphere and warmer-than-average temperatures in the Southern Hemisphere. </p>