A combination of high day temperatures and humidity is likely leading to high wet bulb temperatures that could put vulnerable populations at risk. IMD said a dip in western disturbances is causing higher temperatures (ANI)

On Monday, Delhi recorded a maximum temperature of 43.4 degree C with a relative humidity of 31% recorded at 2.30 PM. During the morning hours, relative humidity (RH) was higher around 48%. At 2.30pm, Delhi’s heat index (HI) or ‘real feel’ was around 49°C.

Delhi’s wet-bulb temperature may range from 29 to 31 degree C on Monday afternoon depending on the location. For example, Ayanagar in Delhi recorded a maximum temperature of 45.3 degree C which would translate to a wet bulb of 30 degree C or more. Since RH is dynamic, changing with time and location, it is difficult to assess the exact wet bulb temperature that people may be experiencing. Further, people’s personal wet bulb experience may vary significantly depending on their vulnerability, location, activity, access to cooling etc.

Wet-bulb temperature combines heat and humidity measurements to indicate how effectively the human body can cool itself through sweating when outdoors. Unlike regular air temperature readings, wet-bulb measurements reflect the body’s cooling capacity in humid conditions—when the wet-bulb temperature reaches the same level as human skin temperature, sweat evaporation becomes impossible, preventing the body’s primary cooling mechanism from working.

Over Kolkata, the maximum temperature was 37 degree C, with morning RH of 72% and evening 57%; Lucknow recorded a maximum temperature of 42 degree C with morning RH of 51% and evening 36%, Patna recorded a maximum of 40.3 degree C and morning RH of 60%. Rajasthan’s arid Ganganagar recorded maximum temperature of 47.3 degree C with morning RH of 37 % and evening 12%. All of these indicate that wet bulb temperatures may be nearing or crossing 30 to 31 degree C.

“We are expecting heatwave conditions over the entire NW India. The impact of western disturbances is reducing now so heat is increasing. There is moisture over the region, left from previous WDs. This is causing uncomfortable heat. Monsoon has not reached NW India yet and hence heat is expected to rise,” said M Mohapatra, director general, IMD.

“Extreme humid heat is impacting eastern parts of the country where humidity is very high, over 70% in many places. Over NW India, we can expect humidity to reduce gradually,” added Mohapatra.

Heatwaves were recorded in isolated pockets of Delhi as well, on what was the hottest day of the season in the capital. Safdarjung, the base station in the city, recorded a maximum of 43.4°C – three degrees above normal. It went as high as 45.3°C at Ayanagar. Prior to Monday, the highest maximum in the city was 42.3°C which was recorded on May 16.

Heat wave conditions are likely to impact several parts of northwest India during the next 4-5 days, India Meteorological Department has warned.

Heat wave conditions are likely to prevail over Jammu-Kashmir till June 12, Punjab till June 13; Haryana, Chandigarh and Delhi till June 12; Uttar Pradesh, East Rajasthan and north Madhya Pradesh till June 11. Hot and humid weather is likely to prevail over Gangetic West Bengal, Coastal Andhra Pradesh, Yanam and Bihar till June 11.

Warm night conditions are also very likely in isolated pockets over Punjab, Haryana Chandigarh, Delhi, West Uttar Pradesh and Rajasthan till June 11.

There is a red category warning for West Rajasthan for Tuesday. There is an orange category warning for the rest of NW India for Tuesday.

“Monsoon has not advanced over northwest India yet so temperatures are high because of solar insolation. There may be moisture incursion from Arabian Sea also,” said M Rajeevan, former secretary, ministry of earth sciences.

HT reported on June 8 that the monsoon has not progressed for over a week now — its progress had stalled around June 29 because of intrusion of dry air. It will again commence advancement during the week of June 12 to 18, according to IMD’s extended range forecast.

The northern limit of monsoon continues to pass through Mumbai, Ahilyanagar, Adilabad, Bhawanipatna, Puri, Sandhead Island.

Monsoon is likely to be active over south peninsular India with heavy to very heavy rainfall and isolated extremely heavy falls over Karnataka during June 12 to 15 and over Konkan & Goa during June 13 to 15, IMD said but did not mention when it will start advancing.

An upper air cyclonic circulation is lying over northwest Uttar Pradesh in lower tropospheric levels and a trough is running from this cyclonic circulation to Telangana in lower tropospheric levels. A western disturbance is approaching the Western Himalayas but its impact will be limited to higher altitudes, officials said.

An upper air cyclonic circulation is lying over Southeast and adjoining east central Bay of Bengal in middle tropospheric levels. A trough is also running from north Tamil Nadu to south Maharashtra coast in lower and middle tropospheric levels. An upper air cyclonic circulation is lying over northwest Bay of Bengal and adjoining north coastal Odisha and south Gangetic West Bengal in lower and middle tropospheric levels. Under the influence of these systems, thunderstorm activity is likely over several parts of west, east and central India.

HT had reported on April 17 that the widely accepted wet-bulb temperature survival threshold of 35°C is being questioned after recent physiological studies suggest the limit may actually be closer to 31°C, Harvard researchers revealed.

The 35°C wet-bulb threshold was originally proposed by scientists Steven C Sherwood and Matthew Huber from the University of New South Wales in a 2010 study. They theoretically concluded that exceeding this temperature for extended periods should induce heat stroke within just six hours of exposure.

However, more recent physiological research suggests the survival limit is potentially much lower. A 2023 study by the Center for Healthy Aging at Pennsylvania State University examined young, healthy subjects performing minimal activity in warm environments of 40°C with 50% relative humidity, which roughly translates to a wet bulb temperature of 31°C . Researchers observed that core body temperatures began to rise continuously—defined as uncompensable heat stress—during prolonged exposure of over six hours, even while still below heat exhaustion or stroke values.