E-Paper
Capital swelters in second most humid monsoon in 53 years
Data shows that the average relative humidity in Delhi this monsoon, which starts officially in India on June 1, is 76.5%. This is the second highest average after 2008 for the 79-day period from June 1 to August 18 when it was 78.1%.
If it feels wet it is because it is; this is the 18th rainiest monsoon in Delhi since 1901. It has also rained more; thus far, the Capital has received 524mm rain (till August 19), 40% more than normal. It is also extremely uncomfortable, and more people seem to be falling ill.
One underlying cause could be that this is Delhi’s second most humid monsoon season in at least past 53 years according to data analysed by Hindustan Times. And the past week has been the most humid in the same period. Thus, while most of this departure from usual humidity is on account of June and July, the past five days in August (14-18) have set a record.
While long term data on relative humidity – the water vapour in the air relative to the amount needed for saturation at a particular temperature -- is not publicly published by the India Meteorological Department (IMD), HT acquired this data for the Safdarjung weather station in Delhi from meteologix.com, a private weather company. This data shows that the average relative humidity in Delhi this monsoon, which starts officially in India on June 1, is 76.5%. This is the second highest average after 2008 for the 79-day period from June 1 to August 18 when it was 78.1%. The average humidity this season in Delhi is also much higher than the city’s usual monsoon humidity. The 1991-2020 average of humidity for the 79-day period is only 67.2%.
Most of the departure from usual humidity this monsoon in Delhi is because of June and July. The average humidity in June and July was the third and second highest since 1973, while that in the first 18 days of August was the tenth highest. This can also be seen in the ratio of humidity this year and the 1991-2020 average. This number is 1.23 for June, 1.12 for July, and 1.05 for August. However, the last five days in August were indeed unusually humid. The average humidity for the five days ending August 18 was 89.2%, 1.14 times the 1991-2020 average, and the highest for these five days of August. Moreover, even usual humidity in August is expected to be high. July and August are Delhi’s rainiest months and provide enough moisture for air to be saturated with. The 1991-2020 average of humidity in Delhi is 55% for June, 73% for July, and 78% for the first 18 days of August.
See Chart 1
What explains the unusually high humidity this monsoon in Delhi? A combination of factors. While June was not rainy in Delhi (there was no rain on 19 days), an early onset of the monsoon season over peninsular and north-eastern India led to some flow of moisture inland. By the third week of June Delhi was surrounded by the monsoon system, resulting in a spike in humidity, although the system itself arrived in the city only on June 29. July and August on the other hand have been very rainy in Delhi. Delhi received rain on all but eight days in July and has got rain on all but four days in August so far.
As the accompanying chart shows, humidity’s departure from usual spiked on days of intense rain, when temperatures also decrease. It is when the rain is absent or too little to substantially decrease temperatures that the lingering moisture creates discomfort. This is because humidity causes discomfort when it prevents the body’s ability to lose heat from perspiration, which also needs a somewhat high temperature.
See Chart 2
