High humidity pushes heat index of world's most-visited city into 'danger' zone

Bangkok's air temperature stayed below 40 degrees Celsius on June 25, but soaring humidity pushed the "feels like" heat index into the official "danger" zone, prompting city authorities to warn of a rising heatstroke risk.

The Bangkok Metropolitan Administration's (BMA) Environment Department, which publishes a daily heat-index forecast, said the June 25 reading fell within the "danger" category, defined as 42 to 51.9 C.

The Nation reported that officials urged residents to cut outdoor activity between 11 a.m. and 3 p.m. and to watch for symptoms of heat illness.

The actual air temperature was far lower. The Thai Meteorological Department forecast highs of about 34 to 39 C with a 20% chance of afternoon thunderstorms, Bangkok Post reported.

The difference is humidity. The heat index, often labeled the "feels like" temperature, combines air temperature with relative humidity.

When the air is saturated, sweat evaporates slowly and the body struggles to shed heat. Air in the mid-30s C can feel like the high 40s once humidity climbs.

The "danger" alerts are not new this week. Bangkok's heat index first entered the band on April 1 and held there for nearly three weeks, according to Bangkok Post.

The capital briefly touched the top "extreme danger" level, at or above 52 C, in early May.

The BMA grades the index on four tiers: "watch" covers 27 to 32.9 C, with advice to stay hydrated; "warning," from 33 to 41.9 C, prompts the call to avoid the midday peak; the "danger" band of 42 to 51.9 C, in force on June 25, tells the general public to monitor their symptoms and at-risk people to seek care if they feel unwell; at or above 52 C, the "extreme danger" level, authorities tell everyone to stay out of the sun entirely.

Heat illness often starts quietly, with fatigue, dizziness, skin rashes, redness or swelling, and cramps.

Left untreated, it can progress to heatstroke, a condition in which the body's core temperature climbs past 40 C and which can be fatal.

A man holds up an umbrella as temperatures hit a record 45.4 degrees Celsius (113.7 Fahrenheit) in Bangkok, Thailand, April 21, 2023. Photo by Reuters

Authorities singled out eight groups for extra care: children under 5, people 60 and older, pregnant women, those with chronic illness, people with obesity, those who drink alcohol, outdoor workers such as motorcycle-taxi drivers, delivery riders and construction crews, and people who exercise outdoors, along with visiting tourists.

The warnings come as the BMA rolls out a 2026 heat plan developed with the World Bank, branded "Shaping a Cooler Bangkok."

The plan has mapped 379 heat-risk zones across the capital, concentrated around construction sites, markets, parks and motorcycle-taxi stands, The Nation reported.

The city has also opened free cooling rooms, with locations mapped on the Greener Bangkok site.

Bangkok was ranked the world's most-visited city in 2025 by Euromonitor International's Top 100 City Destinations Index, drawing 30.3 million international arrivals, well ahead of Hong Kong and London.