Early Tuesday morning, the minimum or the night time temperature recorded in the city was 13.7 degrees Celsius, which is four degrees below normal for this time of the year.
This was also the coldest night time temperature recorded in over a decade in the month of October in Delhi, as per data from the India Meteorological Department (IMD).
The lowest minimum temperature recorded before this date in October at the IMD’s Safdarjung observatory, which is representative of the city, was on October 26 and 28, 2009, with a reading of 13.5 degrees Celsius.
What is causing the dip in temperature?
There are two main reasons behind the fall in mercury, scientists at the IMD say. One is that the sky over the city during the night has been clear over the past few days — meaning there is no cloud cover.
The second reason is that the wind at night becomes calm, its speed dropping to almost zero, which is characteristic of the post-monsoon period in Delhi.
These two factors result in higher radiation from the Earth’s surface at night time, which cools the surface and the air close to it, IMD scientists say.
A better way to explain this is that during day time, the Earth’s surface heats up from solar radiation, or energy from the Sun. At night time, the Earth’s surface emits this energy back into space in the form of radiation.
If there was cloud cover over Delhi at present, it would absorb radiation emitted by the earth’s surface and re-emit a portion of it to outer space and another portion back towards the Earth, which would increase the temperature of the surface and atmosphere.📣 Click to follow Express Explained on Telegram
Is the dip in temperature unusual?
The minimum temperature so far this month has been largely below normal. It started dipping October 1 onward, when it was 21.9 degrees Celsius and was recorded as 19.8 degrees on October 13.
The temperature then rose on October 14 to 23.7 degrees Celsius and started dipping again. Between October 16 and 19, it remained between 16 to 16.8 degrees Celsius and on October 20, there was a sudden dip to 13.7 degrees.
As per a climatological table prepared by the IMD, based on 30 years data between 1981 and 2010, the normal minimum temperature between October 18 and 22 is 18.1 degrees Celsius.
However, IMD scientists say that as minimum temperature in this period has dipped below 14-15 degrees Celsius in the past, Tuesday’s drop in mercury is not unusual.
Although, between 2007 and 2019, the night time temperature has gone below 14 degrees Celsius only twice — on October 26, 2012, when it was 13.9 degrees and on October 26 and 28, 2009, when it was 13.5 degrees.
Along with the drop in minimum temperature, the maximum or day time temperature has been dipping too, but not as significantly as night time temperatures and with frequent fluctuations.
On Monday, the maximum temperature at the Safdarjung observatory was recorded as 33.9 degrees Celsius, lowest for the month so far.
What lies ahead?
IMD scientists say that conditions that are causing the night time temperature to dip are likely to continue for around three days.
The minimum temperature is forecast to be around 14 degrees on Wednesday, and could slightly rise in the days after that, but the city can largely expect there to be a nip in the air in the evening and the early morning hours from this point onward.
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