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Understanding Heat Index
Heat index, also known as the "apparent temperature" or "feels like" temperature, measures how hot it really feels when relative humidity is factored in with the actual air temperature. When humidity is high, the evaporation of sweat from the body decreases, reducing the body's ability to cool itself.
How Heat Index Works
The human body normally cools itself by perspiration, where sweat evaporates from the skin. High humidity reduces the evaporation rate, which in turn reduces the body's ability to cool itself. This is why it feels hotter on humid days than on dry days with the same temperature.
Heat Index Formula
The National Weather Service (NWS) uses a complex formula to calculate heat index. The simplified version is:
HI = -42.379 + 2.04901523*T + 10.14333127*RH - 0.22475541*T*RH - 0.00683783*T*T - 0.05481717*RH*RH + 0.00122874*T*T*RH + 0.00085282*T*RH*RH - 0.00000199*T*T*RH*RHWhere:
- T = Temperature (°F)
- RH = Relative Humidity (%)
This formula is valid for temperatures of 80°F (27°C) or higher and relative humidity of 40% or more.
Heat Index Risk Categories
Heat index values are categorized by risk level:
| Heat Index | Risk Level | Possible Heat Disorders |
|---|---|---|
| 80°F to 90°F (27°C to 32°C) | Caution | Fatigue possible with prolonged exposure and physical activity |
| 90°F to 103°F (32°C to 39°C) | Extreme Caution | Heat stroke, heat cramps, or heat exhaustion possible |
| 103°F to 124°F (39°C to 51°C) | Danger | Heat cramps or heat exhaustion likely, heat stroke possible |
| Above 125°F (52°C) | Extreme Danger | Heat stroke highly likely |
Heat-Related Illnesses
High heat index values can lead to several heat-related illnesses:
Heat Cramps
Painful muscle spasms that occur during or after intense exercise and sweating in high heat.
Symptoms:
- Muscle pain or spasms
- Heavy sweating
- Fatigue
Heat Exhaustion
A more serious condition that can develop when exposed to high temperatures and inadequate fluid replacement.
Symptoms:
- Heavy sweating
- Weakness or fatigue
- Cool, pale, clammy skin
- Fast, weak pulse
- Nausea or vomiting
- Headache
- Dizziness
Heat Stroke
A severe and life-threatening condition requiring immediate medical attention.
Symptoms:
- High body temperature (103°F or higher)
- Hot, red, dry or damp skin
- Fast, strong pulse
- Headache
- Dizziness
- Nausea
- Confusion
- Loss of consciousness
Hot Weather Safety Tips
When heat index values are high, follow these safety tips:
- Stay in air-conditioned buildings as much as possible
- Drink plenty of fluids, even if you don't feel thirsty
- Wear lightweight, light-colored, loose-fitting clothing
- Schedule outdoor activities carefully, preferably in the morning or evening
- Take cool showers or baths to cool down
- Check on friends, family, and neighbors, especially the elderly
- Never leave children or pets in cars
- Monitor those at high risk: infants, young children, people 65 and older, and people with chronic diseases