Number Theory: Why Jan 2023 feels colder than last year
Large parts of north India are reeling under numbing cold with the mercury remaining below the freezing point at most places in Jammu and Kashmir. Is this winter really colder than what is normally the case? An HT analysis of the India Meteorological Department’s (IMD) gridded database can answer some of these questions.
Large parts of north India are facing a severe cold wave. Is this winter really colder than what is normally the case? Is it a harsher winter than what we had to experience last year? An HT analysis of the India Meteorological Department’s (IMD) gridded database can answer some of these questions.
India's 4th coldest January since 1951 during the day; 26th coldest at night
India's average maximum temperature, a proxy for temperatures during the day, is 22.67 degrees Celsius this month up to January 8. This is 1.45 degrees below the 1981-2010 average (considered the "normal") and the fourth coldest since 1951, the earliest year for which the IMD has published gridded temperature data. Similarly, the average minimum (available up to January 9) is 0.36 degrees below normal and the 26th coldest since 1951.
These two sets of numbers make it clear that this January so far has been colder than normal and is exceptionally cold during the day. However, as the deviations and ranks show, night temperatures are clearly not as far below normal as day temperatures are. It is here that perception might play a role in how cold this January feels. This is because January 2022, the most immediate reference point, was not cold in the same way. Last year, while maximum temperature during January was much lower than normal (9th coldest for the period up to January 8), minimum temperature was exceptionally high (11th warmest for the period up to January 9).
Parts of north India are colder than what the all-India average suggests
To be sure, the all-India average of maximum temperature understates the extent of winter this January. For instance, although the maximum in India is "only" the 4th lowest since 1951, it is the lowest since 1951 in Bihar, Punjab, Sikkim, and Uttar Pradesh, and second lowest in Haryana.
Delhi on the other hand is ranked 4th coldest just like India, while Jharkhand, Madhya Pradesh, Jammu and Kashmir, and Ladakh are also experiencing their 5th-10th coldest January months. In terms of deviation from normal, Himachal Pradesh and north-eastern states (all except Sikkim and Meghalaya) are the only states in the northern half of the country that are experiencing a warmer January than normal. Most peninsular states on the other hand are experiencing a slightly warmer than normal January. These trends in deviation from normal also largely hold true for minimum temperatures.
Lack of rain explains why January 2023 is cold in a different way than January 2022
As the discussion above shows, there is a broad convergence in the trends of maximum and minimum temperatures in January this year: both are colder than normal. This was not the case last year largely because January then felt cold due to rainfall, which is almost completely absent this year.
In the first nine days of the year, it has rained in 15 of 32 states and union territories for which this calculation is possible using the IMD's gridded dataset. Yet, even the state with the most rain in proportion to the state's Long Period Average (Andhra Pradesh) has a deficit of 85%. In comparison, it had rained in 26 states and UTs last year in the same interval, with 12 of them getting a surplus of over 250%. How does this explain temperature trends? Rain reduces day temperatures because clouds block sunlight to the surface of the earth. Although this is not the only way maximum temperature goes below normal - as the maximum this year shows - this was a big reason behind the low maximum last year. Because clouds also trap heat (and slow down its release at night), night temperatures did not drop fast last year. Lack of rains and cloud cover has been a major factor in lower than normal night temperatures in northern India.
January chill is a departure from 4 the trend for the winter season so far
With the first week of January over, we are also now almost half way (around 43%) through the official 2022-23 winter (which runs from December to February every year).
For the period of winter up to this point, the low temperatures in January have not made the 2022-23 winter as exceptionally cold. By maximum temperature, the 2022-23 winter is 18th coldest compared to January 1-8, which is the fourth coldest. India's average minimum, on the other hand, is 21th hottest for the winter up to now, compared to 26th coldest by just the January 1-9 period's minimum. This is because most of December was warmer than normal.
