Heat Wave Alert Withdrawn For Delhi, What Reasons

The heat wave alert for Delhi, issued earlier, stands withdrawn. A heat wave was predicted to sweep Delhi/NCR between 16th and 18th April 2025. The temperature has been on the rise for the last two days and staying above normal by 2°-3°C over Delhi and its suburbs. Base station Safdarjung yesterday recorded a day high of 37.9°C and the airport observatory clocked 36.7°C, both about 2°C above the normal. The temperature is likely to rise further but may not amount to heat wave conditions.

Heat wave is declared when the maximum temperature is at least 40°C and the departure from normal is 4.5°C or more. A severe heat wave is announced when the departure from the normal rises to 6.5°C or more. Severe heat wave is also considered when the maximum temperature reaches or exceeds 45°C.

Earlier, Delhi witnessed heat wave conditions for three consecutive days, between 07th and 09th April, when the temperatures stayed above 40°C. The highest temperature during that spell was 41°C on 08th April 2025.  The day temperature was 5°-6°C above the normal during those days and therefore amounted to heat wave conditions.

The temperature may still reach or exceed 40°C between 17th and 20th April 2025. There is a fair amount of chance that the maximum may even exceed 41°C on 18th April. Today itself, the mercury will be close to breaching the 40°C mark. However, all these reads may not qualify for heat wave criteria.

The reasons for rejecting heat waves are the following. The month of April starts with a pentad normal of 34°C and reaches about 35°C by 07th April. Under such conditions, as and when the maximum temperature reaches 40°C or more, the departure from normal exceeds 4.5°C and therefore qualifies as a heat wave. As the month of April proceeds, the pentad normal increases to about 37°C by 20th April and touches 38°C by 26th April. In such cases, the maximum temperature needs to reach or exceed 42°C and 43°C, respectively, to satisfy heat wave criteria.

Western disturbance over the northern mountains will be quite active between 18th and 20th April. There will be fairly widespread rain, thundershowers and hailstorms in the hilly region. Higher reaches will have snowfall over Jammu & Kashmir and Himachal Pradesh. Such a heavy spell will suppress the mercury levels down the slopes of mountains. Plains may not witness any significant weather activity. Under the combined influence of western disturbance in the mountains and cyclonic circulation in the plains, an east-west trough will run in the close proximity of Delhi. The day temperatures are surely expected to rise above 40°C but may not reach or exceed the required grade of 42°C for heat wave conditions. Accordingly, the heat alert for the national capital is ‘held in abeyance.’