CalcHeat Index Calculator
Heat Index Calculator.
Heat Index Calculator: Know the Real Feel of Summer Heat
The heat index calculator computes the apparent temperature, or what the air actually feels like to your body when relative humidity is factored in. Developed by R. G. Steadman in 1979 and refined by the National Weather Service, the heat index tells you how hot it really feels beyond what the thermometer reads. Athletes, outdoor workers, coaches, event planners, and anyone spending time in hot weather can use this tool to assess heat stress risk and make informed decisions about activity levels and hydration.
Heat Index Formula (National Weather Service)
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R² + 0.00122874T²R + 0.00085282TR² − 0.00000199T²R²
Where T is the air temperature in degrees Fahrenheit and R is the relative humidity as a percentage (e.g., 70 for 70%). This complex regression formula was derived by fitting a polynomial to the original Steadman model. It applies only when the temperature is at or above 80°F and the relative humidity is at or above 40%. For conditions outside this range, a simpler approximation formula is used: HI = 0.5 × (T + 61.0 + (T − 68.0) × 1.2 + R × 0.094).
The physiological basis of the heat index lies in evaporative cooling. The human body relies on sweat evaporation to dissipate heat. When humidity is high, the air is already saturated with water vapor, which slows evaporation significantly. This means the body cannot cool itself as effectively, making the perceived temperature higher than the actual air temperature. At 100°F with 60% humidity, for example, the heat index reaches approximately 133°F — a dangerous level for prolonged outdoor activity.
Worked Examples
Example 1: Summer Heat Wave
A summer afternoon reaches 95°F with a relative humidity of 70%. What is the heat index?
Calculation using the NWS formula: T = 95, R = 70
HI = −42.379 + 2.04901523(95) + 10.14333127(70) − 0.22475541(95)(70) − 0.00683783(95)² − 0.05481717(70)² + 0.00122874(95)²(70) + 0.00085282(95)(70)² − 0.00000199(95)²(70)²
HI = −42.379 + 194.66 + 710.03 − 1494.62 − 61.69 − 268.63 + 812.43 + 396.73 − 867.30 ≈ 131°F
A heat index of 131°F falls into the "extreme danger" category. Under these conditions, heat stroke is highly likely with prolonged exposure. Outdoor activities should be avoided, especially during midday hours. This scenario is typical of extreme heat waves in the southeastern United States, where high humidity combines with already high temperatures to create dangerous conditions.
Example 2: Moderate Summer Day
A summer day in a dry climate shows 90°F with only 35% humidity.
Calculation (below 40% humidity, using the simple formula): HI = 0.5 × (90 + 61.0 + (90 − 68.0) × 1.2 + 35 × 0.094) = 0.5 × (90 + 61.0 + 26.4 + 3.29) = 0.5 × 180.69 = 90.3°F
When the heat index equals the actual temperature, evaporative cooling is working efficiently. In dry heat like this, the body can cool itself effectively through sweat evaporation, making outdoor activity safer. This is the difference between humid heat in Miami and dry heat in Phoenix — both may read 100°F on a thermometer, but the heat index and the physiological stress are vastly different.
Common Uses
- Planning outdoor athletic practices and events to determine if heat safety modifications or cancellations are needed
- Assessing workplace heat stress risk for construction workers, agricultural laborers, and other outdoor occupations
- Scheduling outdoor activities such as marathons, festivals, and sports matches during safe temperature windows
- Understanding weather forecasts more completely by looking beyond the temperature to the heat index value
- Monitoring vulnerable individuals (elderly, young children, those with medical conditions) during heat waves for heat-related illness
- Evaluating the effectiveness of cooling strategies like hydration, shade structures, and misting fans based on heat index severity
Common Mistakes
- Assuming the heat index applies only in direct sunlight — it is actually calculated for shaded conditions, and direct sun can add up to 15°F to the apparent temperature
- Using the heat index when temperatures are below 80°F — the Steadman regression model was designed for hot conditions and is not valid below 80°F or below 40% humidity
- Ignoring wind speed effects — the heat index assumes light wind conditions. A strong breeze can provide significant relief by enhancing evaporative cooling, but the heat index does not account for this
- Confusing relative humidity with the dew point — these are related but different. The heat index formula specifically requires relative humidity as a percentage input
- Thinking the heat index is the temperature inside a parked car — car interior temperatures can be 30–50°F higher than the outside air temperature, far exceeding even the heat index
Pro Tip
When monitoring heat stress for outdoor activities, use the heat index in combination with the wet-bulb globe temperature (WBGT) for a more complete picture. A quick field approximation for WBGT is: WBGT = 0.7 × Twb + 0.2 × Tg + 0.1 × Tdb, where Twb is the wet-bulb temperature (easily approximated from a psychrometric chart) and Tg is the globe temperature capturing solar radiation. However, for most people, a simpler rule works: if the heat index exceeds 90°F, take frequent breaks in shaded or air-conditioned areas, drink water every 15–20 minutes regardless of thirst, and watch for symptoms of heat exhaustion such as headache, dizziness, nausea, and cool clammy skin despite the heat. When the heat index exceeds 105°F, consider rescheduling strenuous outdoor activities entirely.
Frequently Asked Questions
The heat index is what the temperature feels like to the human body when relative humidity is combined with the air temperature. Unlike actual air temperature, the heat index accounts for how humidity affects the body's ability to cool itself through sweat evaporation. High humidity slows evaporation, making the body feel hotter than the actual temperature.
The National Weather Service issues heat advisories at different thresholds. A heat index of 80–90°F is caution, with fatigue possible. At 90–103°F, risk of heat cramps and heat exhaustion. At 103–124°F, danger of heat stroke. At 125°F and above, extreme danger with high likelihood of heat stroke.
The heat index is calculated for shady conditions with light wind. If you are in direct sunlight, the apparent temperature can be up to 15°F higher than the calculated heat index. This is why weather forecasts typically state both the temperature and the heat index.
Wet-bulb globe temperature (WBGT) is a more comprehensive heat stress metric that incorporates temperature, humidity, wind speed, and solar radiation. While the heat index only uses temperature and humidity (and assumes shade), WBGT accounts for all environmental factors affecting heat stress.