Delhi was not 51°C, but humidity made it feel that way ahead of monsoon

New Delhi: Delhi did not record a temperature of 51.3 degrees Celsius on Saturday. The number that alarmed residents was the city’s heat index, or “feels-like” temperature, which touched 51.3°C as humidity turned a 41.3°C day into a far more oppressive one. The India Meteorological Department said the capital’s heat index touched its highest level of the year on June 28. Safdarjung, Delhi’s base weather station, recorded an actual maximum temperature of 41.3°C, about four degrees above normal. The difference between the two numbers explains why the city felt harsher than the thermometer suggested. Air temperature measures how hot the air is. Heat index measures how hot it feels to the human body when humidity is added to the equation. Humidity slows the body’s natural cooling mechanism. Sweat cools the body only when it evaporates. When the air is already loaded with moisture, evaporation becomes slower and the body finds it harder to release heat. That is why a 41°C afternoon can feel much closer to 51°C. The discomfort was reflected in another measure watched closely by scientists: wet-bulb temperature. Delhi’s wet-bulb temperature peaked at 29.77°C on June 28. Wet-bulb temperature measures how effectively the body can cool itself through sweating. A wet-bulb temperature of 29.77°C is below the most dangerous thresholds, but it is high enough to show why conditions felt unusually muggy. At around 32°C, even healthy adults find sustained outdoor work difficult. At 35°C, sweating no longer cools the body effectively. The current discomfort has been driven by moisture-laden southwesterly winds from the Arabian Sea over northwestern India. Delhi’s summer heat is usually dry. This spell has been different because high temperature has combined with high moisture, creating humid heat. The IMD has issued a yellow alert for thunderstorms, lightning, light rain and gusty winds over Delhi-NCR on Sunday and Monday. Wind speeds may touch 30-40 kmph and gust up to 50 kmph during thunderstorm activity. The weather office expects temperatures to ease after Monday as rain and thunderstorm activity continue through the week. The monsoon is expected to advance further over northwest India in the coming days, bringing relief from the humid heat. Delhi’s air quality remained in the moderate range on Sunday, with real-time AQI trackers placing the city’s AQI around 140. Under the CPCB scale, AQI between 101 and 200 is classified as moderate. The important point for residents is that Delhi did not become a 51°C city. The actual temperature remained above normal but far lower than that. What made the weather punishing was humidity, which pushed the heat index to a level that felt much more severe than the recorded temperature.