Capturing Delhi’s unusual weather patterns this summer
The maximum temperature recorded in Delhi on June 30, 2021 was 43.73 degrees Celsius. This is the second highest maximum temperature on this day since 1951, less than a tenth of a degree lower than the 43.76 degrees Celsisus recorded on June 30, 1987. That Delhi is experiencing a heatwave when it should have been raining is yet another example of unusual turn in Delhi’s weather. This newspaper has also, earlier, reported that Delhi climate created a new record every month since August 2020.
An HT analysis of Indian Meteorological Department’s (IMD) gridded database on rainfall and temperature confirms the deviant trends in Delhi’s climate this summer. Here are four charts summarising the break in the pattern:
It was not hot in Delhi when it was supposed to be
Delhi’s maximum temperature, according to IMD’s gridded database, was 43.73 degrees Celsius on June 30, 2021. This is the second highest maximum temperature on this day since 1987. However, what is even more unusual is the fact that June 30, 2021 was the hottest day in Delhi this summer, something which has never happened since 1951, the earliest period for which we have temperature data in the IMD’s gridded database. To be sure, the month of June is one of the hottest in the March-June period, but usually in its first half. In the 71 years from 1951, the hottest day of March-June period has come 38 times in June. On 23 of these 38 times, the hottest day has been on or before June 12, and only 15 times on or after June 16. The last week of June or the June 24-June 30 period, when monsoon generally arrives in Delhi, has recorded the hottest summer day only twice before 2021: in 1987 and in 1991.
The gridded database temperature is different from the ones recorded as a particular station because it is based on interpolating data from weather stations that have been working long-term and whose historical data has been checked for quality, while IMD bulletins might include data from newly added stations too. A grid in this dataset is a box bound by two latitudes and longitudes. While the rainfall grids are 0.25 degree apart in IMD’s dataset, the temperature dataset grids are 1 degree apart. Delhi is not captured by the temperature dataset. The temperature statistics for Delhi here therefore also include the seven districts bordering Delhi: Sonipat, Baghpat, Ghaziabad, Gautam Buddha Nagar, Faridabad, Gurgaon, and Jhajjar.
Data also shows that average maximum temperatures in Delhi in the months of April, May and the first half of June this year have been either close to or significantly lower than the 1981-2010 period average. IMD uses the average of temperatures in the 1981-2010 period to check whether temperatures are higher or lower than what is expected for an interval, with a deviation of 1 degree Celsius considered as normal. This proves that Delhi had a far more pleasant summer or at least a normal summer until the last week of June.
It rained before it was supposed to rain
Delhi received a cumulative rainfall of 111.1mm in May this year. This is 400.4% more than the long period average (LPA) for the month. The LPA of rainfall is defined as the average of rainfall for an interval in the 1961-2010 period. To be sure, bulk of the cumulative rainfall in May was on account of the disturbance created by Cyclone Tauktae. 80.2mm or 72% of the 111.1mm rainfall took place on May 20, which is the highest since 1901 for this day. The LPA figure for rainfall on May 20 is just 0.87mm. May 19, May 21 and May 22 were also high rainfall days, recording the 11th highest, eighth highest and the highest rainfall for these days since 1901.
Moreover, small amounts of rainfall took place once every week in May, making sure that temperatures stayed much lower than what they have been historically. Delhi’s maximum temperatures from May 18 to May 20 were ranked lowest for those days since 1951. From May 21 to May 24, the maximum temperatures for the day were ranked second lowest, sixth lowest, third lowest, and seventh lowest since 1951.
What followed has been an exceptionally dry monsoon in Delhi
The period between June 1 and September 30 is officially considered to be the monsoon season in India. According to IMD’s gridded database, Delhi’s cumulative monsoon rainfall as of 8.30am on June 30 was 23.32mm. This is 61% lower than the LPA and the 39th highest deficiency since 1901, the earliest period for which data is available. The deficiency is more acute if we look at cumulative rainfall in just the second half of June. Against an LPA of 44.7mm rainfall from June 16 to June 30, this year’s rainfall is just 6mm or 87% deficient, making it the 22nd highest rainfall deficit since 1901.
Monsoon has halted at all-India level too
Part of the reason for a drier monsoon in Delhi could be that the monsoon has weakened before it was scheduled to enter north-western India. Rainfall at all-India level was above the LPA on all days except two from June 1 to June 20. After June 20, it has consistently remained below the LPA. Cumulative rainfall was 83.7% above the LPA on June 1. This surplus dropped to 23.7% of the LPA by June 10, rose again to reach 46.83% on June 20, and has since dropped consistently. The overall 188.9mm rainfall in June is only 15% above the LPA of 164mm for June.