Why humidity makes heat feel worse
The body cools itself mainly by sweating. When sweat evaporates, it takes heat away from the skin. In humid air, sweat evaporates slowly, so the body struggles to shed heat and feels hotter than the thermometer says. The heat index, sometimes called the apparent temperature, puts a number on that feeling.
How it is calculated
The National Weather Service uses a regression equation published by Lans Rothfusz in 1990, fitted to tables developed by R. G. Steadman in 1979. It combines the temperature T in °F and the relative humidity RH in a nine-term polynomial. Two adjustments apply in extreme cases:
- With humidity below 13% between 80 and 112 °F, a small amount is subtracted.
- With humidity above 85% between 80 and 87 °F, a small amount is added.
When the result would be below about 80 °F, a simpler formula is used instead. The equation assumes shade, a light wind and an adult walking outdoors.
Danger levels
| Heat index | Level | Possible effects |
|---|---|---|
| 80 to 90 °F | Caution | Fatigue with prolonged exposure or activity |
| 90 to 103 °F | Extreme caution | Heat cramps and heat exhaustion possible |
| 103 to 124 °F | Danger | Heat cramps and exhaustion likely; heat stroke possible |
| 125 °F or more | Extreme danger | Heat stroke highly likely |
Example
At 90 °F with 60% relative humidity:
- Heat index: 100 °F (38 °C), extreme caution
- Humidex: 43, great discomfort
- Dew point: 74 °F, oppressive
Direct sunshine can raise the heat index by up to 15 °F.
The humidex and the dew point
Canada uses the humidex, which combines the temperature with the dew point. A humidex above 40 means great discomfort, and above 45 is dangerous. The dew point itself is a good guide to how muggy the air feels: above about 65 °F the air feels sticky, above 70 °F oppressive.
Staying safe in the heat
- Drink water regularly, before you feel thirsty, and limit alcohol.
- Schedule hard work and exercise for early morning or evening, with frequent breaks in the shade.
- Wear light, loose, light-colored clothing.
- Check on older adults, young children, people with chronic illness and anyone who lives alone.
- Never leave a child or a pet in a parked car.
Heat stroke is an emergency: confusion, a very high body temperature, hot skin and fainting call for 911 and immediate cooling.