What is the heat index and how is it different from air temperature?

The heat index (also called "apparent temperature" or "feels-like temperature") combines air temperature and relative humidity to describe how hot the air actually feels to the human body. At high humidity, sweat evaporates more slowly, reducing the body's ability to cool itself, so the perceived temperature is higher than the thermometer reading. At 35 °C (95 °F) with 70% humidity, the heat index is around 50 °C (122 °F) — far above the measured air temperature.

What formula does this calculator use?

It uses the official NOAA/NWS two-step method. First the Steadman (1979) simple estimate is computed and averaged with the actual temperature. If the average is below 80 °F (27 °C) that value is used directly. At or above 80 °F the full Rothfusz regression (a nine-term polynomial in T and RH) is applied, with two optional NWS adjustments for very low humidity (RH below 13%) or very high humidity (RH above 85%) at specific temperature ranges.

When is the heat index valid?

The Rothfusz regression is most accurate when the air temperature is at or above 80 °F (27 °C) and relative humidity is 40% or higher. At lower temperatures or very low humidity the calculated value may be lower than the actual temperature, in which case the ambient temperature itself is the more relevant figure. The calculator flags this by showing no elevated risk category.

What are the NOAA heat index risk categories?

NOAA defines four categories. Caution (27–32 °C / 80–91 °F) — fatigue possible with prolonged activity. Extreme Caution (32–41 °C / 91–103 °F) — heat cramps and heat exhaustion possible. Danger (41–54 °C / 103–125 °F) — heat cramps and heat exhaustion likely, heat stroke possible. Extreme Danger (above 54 °C / 125 °F) — heat stroke highly likely. These thresholds are based on healthy adults at rest in the shade; direct sun, physical exertion, and individual health factors all increase actual risk.

Does direct sunlight affect the heat index?

Yes. The standard heat index assumes the person is in the shade. Direct sunlight can add 8–15 °C (roughly 15–25 °F) to the perceived heat, pushing the effective heat index into a higher risk category than the calculator shows. Always account for sun exposure, especially during outdoor work or exercise.

What should I do if the heat index is in the Danger or Extreme Danger zone?

Move to a cool, air-conditioned environment. Drink water regularly — do not wait until you feel thirsty. Avoid strenuous outdoor activity during the hottest part of the day (typically 11am–3pm). Wear lightweight, light-coloured clothing. Check on elderly neighbours, young children, and pets. Know the signs of heat exhaustion (heavy sweating, weakness, cold/pale/clammy skin, fast pulse, nausea) and heat stroke (high body temperature above 40 °C, hot/red/dry skin, rapid pulse, unconsciousness) — the latter is a medical emergency requiring immediate cooling and emergency services.

Heat Index Calculator

The heat index — sometimes called the “feels-like” or “apparent” temperature — tells you how hot the air actually feels to your body once humidity is factored in. A thermometer measures the air temperature alone, but your body cools itself by evaporating sweat. When the air is already saturated with water vapour (high relative humidity), sweat evaporates far more slowly, trapping body heat and making the environment feel much hotter than the mercury suggests. This calculator combines your air temperature and relative humidity to give the precise feels-like figure using the official NOAA Rothfusz regression, then maps it onto the four-tier NOAA risk scale so you can make informed decisions about outdoor activity, hydration, and personal safety.

How it works

The calculation follows the two-step procedure published by the U.S. National Weather Service (NWS).

Step 1 — Steadman simple estimate. For all inputs the calculator first computes a fast approximation due to Steadman (1979):

HI_simple = 0.5 × (T + 61.0 + (T − 68.0) × 1.2 + RH × 0.094)

where T is air temperature in °F and RH is relative humidity in percent. This result is averaged with the actual temperature. If that average is below 80 °F (27 °C) the simple estimate is used directly — at lower temperatures humidity has a relatively minor effect and the full polynomial is unnecessary.

Step 2 — Rothfusz regression. When the average from Step 1 reaches or exceeds 80 °F the full nine-term polynomial (Rothfusz, 1990) is applied:

HI = −42.379 + 2.04902·T + 10.14333·RH − 0.22476·T·RH − 0.00684·T² − 0.05482·RH² + 0.00123·T²·RH + 0.00085·T·RH² − 0.0000020·T²·RH²

Step 3 — NWS adjustments. Two small corrections refine accuracy at the edges of the scale:

Low-humidity adjustment (RH below 13%, T between 80–112 °F): subtracts a correction proportional to how dry and how far from 95 °F the air is.
High-humidity adjustment (RH above 85%, T between 80–87 °F): adds a small correction for muggy but not-yet-hot conditions.

The “Working” panel in the calculator shows every arithmetic step so you can verify each stage.

Worked example

Suppose you are in a city on a summer afternoon: 35 °C (95 °F), 70% relative humidity.

Steadman simple: HI_simple = 0.5 × (95 + 61 + (95 − 68) × 1.2 + 70 × 0.094) = 97.5 °F
Average with T: (97.5 + 95) ÷ 2 = 96.2 °F → above 80 °F threshold, use Rothfusz.
Rothfusz regression → approximately 124 °F (51 °C).
No RH adjustment needed (RH not below 13% nor in the narrow high-RH band).
Result: ~51 °C / 124 °F — solidly in the Danger category.

Air temp	RH	Heat index	Category
28 °C / 82 °F	50%	~29 °C / 84 °F	Caution
32 °C / 90 °F	60%	~41 °C / 105 °F	Extreme Caution
35 °C / 95 °F	70%	~51 °C / 124 °F	Danger
38 °C / 100 °F	90%	~59 °C / 138 °F	Extreme Danger

The table illustrates why a tropical city at 35 °C feels genuinely dangerous even though the thermometer reading alone might seem manageable — humidity doubles the physiological burden.

Formula note

The Rothfusz regression is derived from Steadman’s original biophysical model and fitted to a large table of feels-like values. It is accurate to within ±1.3 °F (0.7 °C) across the valid range (T ≥ 80 °F, RH ≥ 40%). All arithmetic in this tool is performed in Fahrenheit internally (as per the NWS specification) and then converted to Celsius for display. Reference: NOAA/NWS Heat Index Equation, wpc.ncep.noaa.gov/html/heatindex_equation.shtml.