The Post-Monsoon Steam Bath
The primary reason for the suffocating heat in September is the combination of resurgent sunshine and lingering humidity. During the monsoon, continuous cloud cover and rain keep surface temperatures down. As the monsoon withdraws, the clouds break, allowing
direct, strong sunlight to heat the ground. However, the atmosphere and the ground are still saturated with moisture from the preceding rainy months. This high humidity is critical. Our bodies cool down by sweating, and for that sweat to be effective, it needs to evaporate. When the air is already full of water vapour, evaporation slows down dramatically. As a result, the body's natural air conditioning system fails, making the air feel much hotter than the actual temperature. This phenomenon is measured by the 'heat index' or 'feels-like' temperature, which can be dangerously high even with moderate thermometer readings.
The Urban Heat Island Effect
This post-monsoon heat is significantly worse in cities due to the Urban Heat Island (UHI) effect. Cities are dense concentrations of concrete, asphalt, and dark-coloured materials that absorb and retain far more solar radiation than natural landscapes like forests and fields. Throughout the day, these urban surfaces act like giant sponges for heat. After sunset, while rural areas cool down relatively quickly, cities begin to radiate all that stored heat back into the environment. This process is intensified by a lack of green spaces and water bodies, which have a natural cooling effect. Buildings also block wind flow that could otherwise dissipate heat. Consequently, a city can be several degrees warmer than its surrounding rural areas, a difference that becomes starkly apparent when the cooling rains of the monsoon cease.
Changing Monsoon Patterns
Climate change is altering the character of the Indian monsoon. Instead of prolonged periods of moderate rain, recent trends show an increase in long dry spells punctuated by short, intense downpours. A fall in consistent rainfall in September means the ground and atmosphere dry out faster. Dry soil heats up more quickly than moist soil, contributing to higher ambient temperatures. Furthermore, forecasts for September 2026 suggest below-normal rainfall for many parts of the country, partly influenced by a strengthening El Niño, a climate pattern often linked to weaker monsoons in India. This reduced rainfall activity means more consecutive sunny days, allowing heat to build up without the interruption of cooling showers. The IMD has noted that subdued rainfall is likely to continue over Peninsular India, which will also contribute to higher daytime temperatures.
The Heat That Doesn't Leave at Night
One of the most challenging aspects of this phenomenon is the rise in nighttime temperatures. The same materials that create the Urban Heat Island effect are responsible for keeping cities warm overnight. Concrete and asphalt slowly release the heat they absorbed all day, preventing any significant drop in temperature after dark. High humidity also plays a role here, trapping heat closer to the surface like a blanket. Warmer nights are particularly dangerous because they deny the human body a chance to cool down and recover from daytime heat stress. Studies have shown an alarming increase in dangerously humid heat days and warmer nights across Indian cities, a trend driven by the combination of ocean warming, which pumps more moisture into the air, and rapid, often unplanned, urbanisation.
