Dry heat and humid heat
Dry heat is high temperature with low humidity: sweat still evaporates, but the body soaks up heat fast. It peaks in South Asia’s pre-monsoon months. Humid heat is warm, wet air in which sweat can’t evaporate, so the body can’t cool itself even at lower temperatures. It dominates Southeast Asia and builds through South Asia’s monsoon, and it is rising faster than dry heat. The two need different warnings and different cooling.
Dry heat
Very hot air with little moisture. Sweat still evaporates, but when the air is hotter than the skin it pours heat in and the body loses water fast.
When: South Asia’s pre-monsoon months, roughly April to June.
What helps: water and salts; shade and rest in the hottest hours; evaporative coolers work well in dry air.
Humid heat
Warm, wet air in which sweat can’t evaporate, so the body can’t cool itself even at lower temperatures.
When: much of the year in Southeast Asia; South Asia’s monsoon and coasts. In South Asia it is rising faster than dry heat.
What helps: cool rooms and moving air; rest breaks set by the heat people feel. Misting and evaporative cooling lose much of their power.
Measure what the body feels
| Measure | What it adds | Who uses it |
|---|---|---|
| Air temperature | Heat in the shade only | Most heatwave definitions |
| Heat index | The “feels-like” temperature in the shade, combining air temperature and humidity. In the Philippines, a heat index of 42 °C or more is officially a danger level. | PAGASA, the Philippines |
| WBGT | The workplace measure of heat. It adds sun and wind to heat and humidity, and is used to set rest breaks and stop work. | Singapore’s Heat Stress Advisory |
| Wet-bulb temperature | Heat and humidity combined into one number, read from a thermometer wrapped in a wet cloth. It shows whether sweat can still cool you. A wet-bulb temperature of 35 °C is often quoted as the limit of human survival; recent studies suggest the real limit is lower. | Research and physiology |
Six questions for heat warnings and work rules
- Does your warning use a measure that counts humidity, such as the heat index or WBGT?
- Do thresholds differ for humid coastal areas and dry inland ones?
- Do warnings cover humid heat in the monsoon, not only the pre-monsoon peak?
- Are misting and evaporative coolers used only where the air is dry enough for them to work?
- Are work–rest rules set by WBGT or the heat index, not air temperature alone?
- Do warnings include hot nights, when bodies need to recover?
Watch the 2-minute explainer: beattheheat.asia/#watch-dryhumid
Sources
- Ivanovich et al., Geophysical Research Letters, 2024. agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL107382
- Vecellio et al., Journal of Applied Physiology, 2022, summarised by Penn State. psu.edu/news/research/story/humans-cant-endure-temperatures-and-humidities-high-previously-thought
- PAGASA: heat index. pagasa.dost.gov.ph/weather/heat-index
- Coffel et al., Earth’s Future, 2025: more distinct dry and humid heat. agupubs.onlinelibrary.wiley.com/doi/10.1029/2024EF005770
- Ministry of Sustainability and the Environment, Singapore, 2023: Heat Stress Advisory. mse.gov.sg/latest-news/press-release-heat-stress-advisory