How cold and long is this cold spell in Delhi
What explains this year’s chill? HT looked at long-term weather data to make sense.
Delhi has shivered over the past week. January 15, for instance, saw the lowest maximum temperature in 51 years, and the 10th coldest day by maximum temperature in the same period. Lack of sunshine has made the cold harsher. Usually, early January is biting cold in the capital, but things start looking up around the middle of the month. What explains this year’s chill? HT looked at long-term weather data to make sense.
Is pollution blocking sunlight?
Delhi and large parts of India’s northern plains are used to waking up to smoggy mornings in winter, thanks to air pollution. Is the recent phase of sunless days – around 100 flights were delayed at the Indira Gandhi International Airport due to low visibility on January 13 – a result of high pollution as well?
By the standard of recent years at least, the air this January has been better, with an average Air Quality Index (AQI) of 267.8, the lowest since at least 2018 for the period up to January 16. The earlier lowest value of AQI for this period was 309.8 in January 2020. It suggests that lack of visibility and sunlight is more because of fog than smog.
Colder days have made winter harsher
January 15 saw a maximum temperature of 13.03 degrees Celsius in Delhi. This is 7.3 degrees colder than normal, which is the average of temperatures from 1981 to 2010. It is the lowest compared to the normal maximum temperature on this day since 1951, the earliest period for which data is available. While such wide deviation from the maximum temperature is not unprecedented – a slightly higher deviation of 7.5 degrees occurred on October 18 last year and such deviations have occurred on 150 days since 1950 – its timing made the day the 10th coldest day by maximum temperature since 1951. While January 15 was an extreme event, maximum temperatures have been lower than normal since the second week of January. Minimum temperatures, on the other hand, have been close to and above normal.
Colder than usual days and just about normal nighttime temperatures mean the relative comfort which warmer days offer during winter has been missing in the past few weeks. The ratio of average maximum and minimum temperature in the first four days of January was 3.30. In the following 12 days, this was 1.78. In the 1981-2010 period, the ratios for these two intervals were not that different at 3.15 and 3.06.
The temperature reading used here has been taken from India Meteorological Department’s gridded temperature dataset. The grids include an area between one-degree longitude and latitude and cover an area bigger than Delhi’s administrative boundaries.
Is this the longest spell of low maximum temperatures?
Daytime temperature has been below normal on all days since January 5. However, it has been substantially and consistently lower since January 8. In the nine-day period from January 8 to 16, it has been 10% or more less than normal. Its maximum value was 16.9 degrees on January 12 and the minimum value was 13.03 degrees on January 15.
Have there been similar spells of low daytime temperatures? Allowing for slightly more variation, HT searched for all continuous periods of nine days or longer when maximum temperature was less than normal by 10% or more. So that such spells during summer are excluded, only those spells were considered when the temperature was under 20 degrees on at least one day.
There are only 23 such spells since 1951, of which only five, including the current one, are nine days long. The current spell is therefore not the longest. That distinction is held by a 25-day period from December 8, 1997, to January 1, 1998. The average maximum temperature of the current spell (15.76 degrees) is also the 6th lowest, about 1.73 degrees higher than the spell with the lowest average (December 30, 2010, to January 10, 2011). However, this spell has lacked any mild relief in between. The maximum value of daytime temperature for this year’s spell is the lowest among all such spells, and the only one under 17 degrees. Fourteen of 23 spells had a maximum value above 19 degrees.
Rain and overcast skies reasons for spike in cold
The trends above are characteristic of rainy days. Clouds and rain prevent long exposure to sun during the day, reducing daytime temperature. Whatever heat does get through during the day, however, is trapped by clouds and keeps night warmer than usual. Western disturbances this month have ensured this situation prevails. Rainfall this January in Delhi is the 5th highest since 1951 for the period up to 8:30 AM on January 16. The 44% share that heavy rainfall – more than 35.5 mm in a day – has had in this interval is the 3rd highest since 1901. Even when it was not raining in Delhi, it was raining in neighbouring states or in the hills, keeping the capital dark and cold.
Is the situation likely to improve soon? The 7-day forecast of the regional meteorological centre in New Delhi published on January 18 does not suggest so. Maximum temperature is expected to rise to 19 degrees by January 20, but will remain in the 16-17 degrees range thereafter, with skies remaining cloudy until January 24. Minimum temperature is likely to be 13 or 13 degrees from January 20 to 24.