Monsoon Has Arrived, So Why Is It Still So Hot? The Science Behind India's New Heat Threat

The arrival of the monsoon is usually associated with relief from India's scorching summer. But for millions living in cities such as Delhi, Mumbai, Ahmedabad and Hyderabad, the rains have brought sticky, exhausting weather instead of cooler conditions.

The reason lies in a combination of rising humidity, warmer nights and urban heat, which together make temperatures feel far higher than what thermometers record. Experts say this growing "humid heat" is becoming one of India's biggest climate challenges.

Why Doesn't Rain Always Bring Relief?

Rainfall lowers the air temperature, but it also increases the amount of moisture in the atmosphere. This high humidity makes it harder for sweat to evaporate from the skin, weakening the body's natural cooling mechanism.

As a result, people continue to feel uncomfortably hot even when temperatures drop after rainfall. According to a September policy brief by the Centre for Science and Environment (CSE), Delhi's heat index frequently crossed 46-50°C during July and August between 2021 and 2025, despite lower actual temperatures. The humid conditions also kept electricity demand high as air conditioners ran for longer hours.

What Is Heat Index And Why Does It Matter?

The heat index is the "feels like" temperature — a measure of how hot the weather feels when air temperature is combined with relative humidity.

When humidity is high, sweat evaporates more slowly, making it harder for the body to cool itself. This means even moderate temperatures can become dangerous, particularly for outdoor workers, elderly people and those with health conditions.

According to Climate Central, India has seen a sharp rise in dangerously humid heat days since the 1970s. Days with a wet-bulb temperature of 25°C or higher have increased from 101 to 141 annually, while cities including Delhi, Ghaziabad, Ahmedabad, Jaipur, Nagpur and Raipur have all recorded significant increases.

Why Humidity Makes Temperatures Feel Worse

Scientists attribute the rise in humid heat partly to warming oceans. The Arabian Sea and the Bay of Bengal are carrying more moisture inland, increasing humidity across much of India.

In cities, this is compounded by rapid urbanisation and the Urban Heat Island effect, where concrete, asphalt and buildings absorb and retain heat. According to the Council on Energy, Environment and Water (CEEW), districts across the Indo-Gangetic Plain are witnessing rising relative humidity alongside greater heat exposure.

Experts also caution that common solutions such as cool roofs and tree planting are not sufficient on their own. Rajan Rawal of CEPT University notes that indoor comfort depends not only on roof temperatures but also on humidity, airflow and building materials. Similarly, studies have found that in humid cities, vegetation must provide shade while maintaining airflow, as trapped moisture can increase heat stress.

Why Indian Cities Stay Hot Even At Night

One of the biggest changes is the rise in unusually warm nights.

High humidity prevents roads, buildings and other surfaces from releasing heat after sunset, keeping temperatures elevated through the night. If nights remain hot, daytime temperatures also tend to stay higher.

A study published this year in Physics and Chemistry of the Earth found persistent nighttime heat waves in 15 smart cities, including Delhi, Agra, Kanpur, Varanasi, Gwalior and Kochi. Another 16 cities, including Bhopal, Nagpur, Raipur and Puducherry, experienced persistent heat during both the day and night.

Is This The New Normal Under Climate Change?

Experts believe Indian cities will need to move beyond seasonal emergency measures and adopt year-round strategies to cope with changing heat patterns.

The CSE recommends stronger heat action plans with passive cooling measures such as better insulation, shading, natural ventilation and reflective surfaces, along with cooling shelters for vulnerable communities. Experts also suggest increasing green spaces while ensuring adequate airflow, expanding reliable electricity access and improving hospital preparedness for both daytime and nighttime heat.

Some cities have already begun adapting. Thane has updated its heat action plan to account for humidity and warm nights, while Bhubaneswar is developing a digital climate-risk dashboard to map heat stress at the building level.

As climate change reshapes weather patterns, experts say tracking humidity and the heat index will become just as important as monitoring temperature. For urban India, the challenge is no longer just surviving hotter days, but enduring increasingly uncomfortable nights as well.