About Heat Index Calculator
The heat index calculator determines the apparent temperature (how hot it actually feels) by combining air temperature and relative humidity. Also known as the "feels like" temperature, the heat index is crucial for understanding heat-related health risks and making informed decisions about outdoor activities, work safety, and health precautions during hot weather.
What is Heat Index?
The heat index is a measure that combines air temperature and relative humidity to calculate how hot it feels to the human body. When humidity is high, the body's ability to cool itself through perspiration is reduced, making the air feel hotter than the actual temperature. This calculator uses the NOAA (National Oceanic and Atmospheric Administration) formula, which is the standard for weather services worldwide.
How Heat Index Works
The human body cools itself primarily through evaporation of sweat. When humidity is high, sweat evaporates more slowly, reducing the body's cooling efficiency. The heat index accounts for this effect:
- Low Humidity: Sweat evaporates quickly, providing effective cooling
- High Humidity: Sweat evaporates slowly, reducing cooling and making it feel hotter
- Temperature Impact: Higher temperatures combined with high humidity create dangerous conditions
- Wind Effect: Wind can enhance cooling, but heat index focuses on still air conditions
Heat Index Categories
- 80-90°F (27-32°C) - Caution: Fatigue possible with prolonged exposure and physical activity. Stay hydrated and take breaks in shade.
- 90-103°F (32-39°C) - Extreme Caution: Heat cramps and heat exhaustion possible with prolonged exposure. Limit outdoor activities and stay well-hydrated.
- 103-125°F (39-52°C) - Danger: Heat cramps and heat exhaustion likely; heat stroke possible with prolonged exposure. Avoid strenuous outdoor activities.
- 125°F+ (52°C+) - Extreme Danger: Heat stroke highly likely with continued exposure. Stay indoors in air conditioning. Outdoor activity extremely dangerous.
NOAA Heat Index Formula
This calculator uses the Rothfusz regression equation developed for the National Weather Service:
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²
Where: T = temperature (°F), R = relative humidity (%), HI = heat index (°F)
The formula is most accurate for temperatures above 80°F (27°C) and relative humidity above 40%. For lower values, the calculator uses simplified equations or returns the actual temperature.
Health Effects by Heat Index
- 80-90°F: Fatigue during prolonged exposure or physical exertion
- 90-105°F: Heat cramps, heat exhaustion possible; heat stroke possible with prolonged exposure
- 105-130°F: Heat cramps or exhaustion likely; heat stroke probable with prolonged exposure
- 130°F+: Heat stroke imminent; extreme danger to life
Safety Recommendations
- Stay Hydrated: Drink plenty of water before, during, and after outdoor activities
- Limit Outdoor Activity: Avoid strenuous exercise during peak heat hours (10 AM - 4 PM)
- Seek Air Conditioning: Spend time in air-conditioned spaces during extreme heat
- Wear Appropriate Clothing: Light-colored, loose-fitting, breathable fabrics
- Take Breaks: Rest frequently in shaded or cool areas
- Never Leave Children or Pets in Vehicles: Interior temperatures can become deadly within minutes
- Check on Vulnerable Individuals: Elderly, young children, and those with chronic illnesses are at higher risk
- Watch for Heat Illness Symptoms: Dizziness, nausea, headache, rapid heartbeat, confusion
Heat-Related Illnesses
- Heat Cramps: Painful muscle spasms, usually in legs or abdomen. Caused by loss of salt and moisture through sweating.
- Heat Exhaustion: Heavy sweating, weakness, cold/pale/clammy skin, fast/weak pulse, nausea, fainting. Requires immediate cooling and hydration.
- Heat Stroke: Body temperature above 103°F, hot/red/dry skin, rapid pulse, confusion, unconsciousness. MEDICAL EMERGENCY - call 911 immediately.
- Heat Rash: Skin irritation from excessive sweating. Keep affected area dry and cool.
Who is Most at Risk?
- Infants and Young Children: Less efficient temperature regulation
- Elderly (65+): Reduced ability to regulate body temperature
- People with Chronic Illnesses: Heart disease, diabetes, respiratory conditions
- Outdoor Workers: Construction, agriculture, landscaping, delivery personnel
- Athletes: Especially during intense training or competition
- People on Certain Medications: Some drugs affect body's cooling ability
- Overweight Individuals: Body may retain more heat
- Those Without Air Conditioning: Limited access to cooling
Workplace Safety
- OSHA recommends monitoring heat index for outdoor workers
- Implement work/rest cycles in hot conditions
- Provide shade and cool drinking water
- Train workers to recognize heat illness symptoms
- Acclimatize new workers gradually (7-14 days)
- Schedule heavy work during cooler parts of the day
- Use buddy system to monitor coworkers
Heat Index vs. Other Measures
- Heat Index: Combines temperature and humidity; used in hot, humid conditions
- Wind Chill: Combines temperature and wind speed; used in cold conditions
- Wet Bulb Globe Temperature (WBGT): Includes solar radiation and wind; used for outdoor work safety
- Humidex: Canadian equivalent of heat index
Factors Affecting Heat Index
- Relative Humidity: Primary factor; higher humidity = higher heat index
- Air Temperature: Base measurement for calculations
- Sun Exposure: Direct sunlight can add 10-15°F to heat index
- Wind Speed: Light wind can lower apparent temperature
- Cloud Cover: Clouds reduce solar radiation impact
- Time of Day: Peak heat typically occurs 2-4 PM
Global Climate Considerations
- Climate change is increasing frequency and intensity of heat waves
- Urban heat islands can add 5-10°F to city temperatures
- Tropical and subtropical regions experience high heat index year-round
- Temperate regions face dangerous heat index during summer months
- Nighttime heat index is increasingly important as overnight temperatures rise
Using This Calculator
- Select your preferred temperature unit (Fahrenheit or Celsius)
- Enter the current air temperature
- Enter the relative humidity percentage (0-100%)
- Click "Calculate" to see the heat index
- Review the category and safety recommendations
- Take appropriate precautions based on the results
Limitations
- Most accurate for temperatures above 80°F (27°C)
- Assumes shade and light wind conditions
- Direct sunlight can increase apparent temperature by 10-15°F
- Individual factors (age, health, fitness) affect heat tolerance
- Does not account for acclimatization to heat
- Wind chill is more appropriate for cold weather
Emergency Response
If someone shows signs of heat stroke (high body temperature, altered mental state, hot dry skin):
- Call 911 immediately
- Move person to cool, shaded area
- Remove excess clothing
- Cool with water, ice packs, or wet towels
- Do NOT give fluids if person is unconscious
- Monitor until emergency help arrives