Rising temperatures make hot, muggy conditions a global reality

What makes heat most dangerous is something that is often overlooked: humidity.

In the 1970s, Delhi averaged 96 days of oppressive heat each year. That figure has since climbed to 135 days. Bhopal, long regarded as a city with relatively mild weather, used to experience only 20 such days annually. Today, it faces around 75 hot and humid days a year, according to a Climate Central study released on Wednesday.

Climate Central defines a “dangerous humid heat day” as one with a daily maximum wet-bulb temperature of 25°C or higher. Covering 254 countries and territories and 961 cities, the study found a sharp rise in dangerous humid heat conditions worldwide.

Here are its key findings:

• Dangerous humid heat days have more than doubled globally, increasing from an average of 10 days a year in the 1970s to 23 days a year between 2016 and 2025
• 64 per cent of all dangerous humid heat days recorded since 1970 can be attributed to climate change
• The greatest impact has been felt in tropical, humid regions
• Tropical South America, coastal West Africa, and Southeast Asia now endure hot and humid conditions for six months on average
• Climate change has increased daily maximum wet-bulb temperatures by an average of 1.2°C
• In 2024 alone, climate change contributed approximately three additional weeks of dangerous humid heat days

VULNERABLE COASTS

A review of major Indian cities shows that while the number of oppressive hot and humid days has increased across the board, coastal cities and those with abundant water bodies appear to be particularly vulnerable.

In Mumbai, for instance, the number of dangerous humid heat days has increased from an average of 136 a year in the 1970s to 206 days a year today. In Chennai, the figure has risen from an average of 205 days a year in the 1970s to 257 days annually. For India as a whole, the average number of dangerous humid heat days has increased from 101 days a year in the 1970s to 141 days today.

The report says that what makes heat most dangerous is something that is often overlooked: humidity. “Air temperature only tells one part of the story — the rest is hidden in humidity, which can make seemingly mild days far more dangerous than they appear.”

Singapore has recorded one of the steepest increases in dangerous humid heat days in Asia, with the annual average rising from 151 days in the 1970s to 280 days today. In Vietnam, the figure has increased from 159 days to 199 days a year over the same period. China currently records an average of 53 dangerous humid heat days annually, while Japan experiences 40 and the United States 21.

The findings highlight the growing toll of human-induced climate change, which is driving a sharp increase in dangerous heat and humidity worldwide.

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