Concrete evidence

Extreme heat softens the road surface outside the BMC headquarters in Fort, leaving visible tyre impressions on the tar. Pic/Ashish Raje

When it feels unusually hot, the natural instinct is to check the recorded temperature. However, it is often observed that it feels hotter than what the thermometer shows. What truly matters is not just the air temperature, but also humidity, wind, direct sunlight, and the heat stored within the surrounding urban environment.

Air temperature is typically measured using a dry-bulb thermometer, which is kept in shaded and well-ventilated conditions so that it records the temperature of the surrounding air without being affected by direct sunlight or moisture. Human beings cool themselves mainly through the evaporation of sweat from the skin, which accounts for most of the body's cooling process. When humidity levels are high, sweat evaporates much more slowly. As a result, the body struggles to release heat efficiently, making conditions feel far hotter and more uncomfortable than the recorded air temperature.

Coastal location plays a role

Mumbai's coastal location plays a major role in this process. The city receives a constant supply of moisture from the Arabian Sea, especially during the pre-monsoon and monsoon months, leading to persistently humid conditions. This is why Mumbai's weather can feel exhausting even when the recorded temperature is lower than that of many inland cities.

Scientists often describe this type of heat stress as humid heat stress. High humidity significantly increases the risk of dehydration, heat exhaustion, and heatstroke because people continue sweating without receiving enough cooling relief. Vulnerable groups such as outdoor workers, children, the elderly, and people with existing medical conditions face greater health risks during such conditions. Unfortunately, humid heat stress in India is not monitored or reported as extensively as dry heat stress, despite its significant impact on human health and comfort.

Urban heat island effect

Another important reason why the Mumbai Metropolitan Region (MMR) feels hotter is the urban heat island effect. Rapid urbanisation across the region has replaced open spaces, wetlands, and vegetation with concrete buildings, asphalt roads, and dense construction. These surfaces absorb and store heat during the day and slowly release it at night, keeping temperatures elevated even after sunset.

Research has shown that some parts of the MMR can become several degrees warmer than nearby rural areas during summer. The structure of the city further worsens discomfort. Closely packed buildings and narrow streets trap heat within neighbourhoods. Vehicles, industries, air conditioners, and other human activities also release additional waste heat into the atmosphere. Reduced natural ventilation means that hot and humid air remains trapped over the city for longer periods.

Direct exposure

Direct exposure to sunlight also makes a major difference. People standing under strong sunlight may experience temperatures several degrees higher than what a dry-bulb thermometer records.

Mumbai-MMR usually benefits from the cooling influence of the sea breeze blowing from the Arabian Sea. During the daytime, land heats up faster than the sea, causing relatively cooler air from the Arabian Sea to move inland and lower temperatures. However, there are occasions when winds blow from the east or northeast instead. This can weaken or delay the onset of the sea breeze, allowing heat to build rapidly across the city.

Both dry heat and humid heat stress are projected to increase across the Mumbai-MMR in the coming decades. However, awareness about urban heat island effects and humid heat stress remains relatively low, and monitoring of such conditions is not yet carried out in a comprehensive or systematic way. In addition, there are currently no well-developed early warning systems specifically designed for high-humidity heat stress, even though such conditions can be more dangerous for the human body than dry heat alone.

Strengthening monitoring networks, improving public awareness, and developing dedicated heat-health warning systems are therefore essential for building climate resilience.

Dr Akshay Deoras, Senior Research Scientist, National Centre for Atmospheric Science and the Department of Meteorology, University of Reading, UK