🌡️Heat Index Calculator
Calculate the heat index (feels-like temperature) from air temperature and relative humidity or dew point. Includes NOAA safety levels and heat risk guidance.
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Heat Index (°C)
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Heat Index Calculator
The heat index — also called "feels-like" or apparent temperature — measures how hot it actually feels when humidity is factored in. High humidity prevents sweat from evaporating effectively, reducing the body's ability to cool itself. This calculator uses the NOAA Rothfusz equation and includes a dew point input option for a more intuitive humidity measure.
Valid range: air temperature ≥ 27 °C (80 °F) and relative humidity ≥ 40%. Below these thresholds, the simpler Steadman approximation is used.
The heat index was developed in the 1970s by Robert Steadman and later refined into the Rothfusz regression equation used by NOAA. It fills the same role as wind chill in winter — communicating the real physiological impact of weather conditions rather than just the thermometer reading. A temperature of 35 °C (95 °F) at 70% relative humidity has a heat index of approximately 46 °C (115 °F), well into the danger zone for heat illness.
How Humidity Causes Heat Illness
The human body cools itself primarily through sweating. When sweat evaporates from skin, it carries heat away with it. This evaporative cooling is highly effective in dry conditions — a reason why desert heat at 40 °C often feels more tolerable than humid heat at 35 °C. High relative humidity means the air is already nearly saturated with water vapor, leaving little capacity to absorb additional moisture from sweating skin. Without effective evaporative cooling, core body temperature rises, and heat exhaustion or heat stroke can develop rapidly.
NOAA Heat Index Risk Levels
| Heat Index | Category | Risk |
|---|---|---|
| Below 27 °C (80 °F) | Normal | No heat index effect; safe for most activities |
| 27–32 °C (80–90 °F) | Caution | Fatigue possible with prolonged sun exposure |
| 32–39 °C (90–103 °F) | Extreme Caution | Heat cramps and exhaustion possible |
| 39–51 °C (103–124 °F) | Danger | Heat cramps/exhaustion likely; heat stroke possible |
| Above 51 °C (124 °F) | Extreme Danger | Heat stroke imminent; immediate risk to life |
Using Dew Point Instead of Relative Humidity
Relative humidity is temperature-dependent: 70% RH feels very different at 15 °C versus 35 °C. The dew point is often a more intuitive discomfort indicator because it reflects the absolute moisture content of the air regardless of temperature:
- Below 10 °C (50 °F): Dry and comfortable
- 10–15 °C (50–59 °F): Comfortable for most people
- 16–18 °C (60–65 °F): Slightly humid, noticeable to some
- 18–21 °C (65–70 °F): Noticeably humid and somewhat uncomfortable
- 21–24 °C (70–75 °F): Oppressive; most people feel uncomfortable
- Above 24 °C (75 °F): Extremely humid; dangerous during exertion
Heat Illness: Recognition and Response
Heat illness follows a spectrum of severity:
- Heat cramps: Painful muscle spasms, usually in legs or abdomen. Caused by electrolyte loss through sweating. Treatment: rest, hydration, electrolyte replacement.
- Heat exhaustion: Heavy sweating, weakness, cold/pale/clammy skin, weak pulse, nausea. Core temperature normal or slightly elevated. Treatment: move to cool environment, loosen clothing, apply cool wet cloths, hydrate if conscious.
- Heat stroke: High body temperature (above 40 °C/103 °F), hot dry skin (if classic type) or hot wet skin (if exertional), confusion, unconsciousness. This is a medical emergency. Call emergency services immediately. Cool the person rapidly by any means available.
Heat Safety Practices
- Check the heat index forecast before outdoor activities, not just the air temperature
- Drink water consistently throughout the day — do not wait until you feel thirsty
- Wear lightweight, light-colored, loose-fitting clothing that allows air circulation
- Schedule outdoor work and exercise for early morning or evening when temperatures are lower
- Never leave children or pets in parked vehicles — temperatures can reach fatal levels in minutes
Frequently Asked Questions
What is the heat index and how is it different from air temperature?
The heat index combines air temperature and relative humidity to describe how hot it actually feels to the human body. High humidity prevents sweat from evaporating, reducing the body's primary cooling mechanism. A 35 °C day at 70% humidity has a heat index of about 46 °C — it feels 11 degrees hotter than the thermometer reads. Air temperature alone does not capture this physiological impact.
At what heat index does it become dangerous to exercise outdoors?
NOAA's "Extreme Caution" level begins at a heat index of 32 °C (90 °F), where heat cramps and exhaustion are possible with prolonged exertion. At 39 °C (103 °F) the "Danger" category begins, where heat stroke is possible. For vigorous outdoor exercise, most sports medicine guidelines recommend taking precautions starting at a heat index of 27 °C (80 °F) and restricting intense activity above 35 °C (95 °F).
What is the dew point and is it a better comfort indicator than relative humidity?
The dew point is the temperature at which the air would become saturated with water vapor and dew would form. It directly indicates the absolute moisture content of the air, independent of temperature. Relative humidity is temperature-dependent — 50% RH on a cold day feels dry, while 50% RH on a hot day feels quite humid. Most meteorologists and comfort researchers consider dew point a more intuitive and consistent indicator of thermal discomfort than relative humidity.
How does heat index affect people differently?
The elderly, young children, people with cardiovascular disease, and those taking certain medications (including diuretics, antihistamines, and antidepressants) are significantly more vulnerable to heat illness. Physical fitness level also matters — well-acclimatized athletes can sustain activity at higher heat indices than sedentary individuals. Acclimatization to heat takes 10–14 days of gradual exposure and substantially improves heat tolerance.
Can heat stroke occur without prior warning signs?
Yes, particularly exertional heat stroke during intense physical activity. The classic form of heat stroke (affecting sedentary people, especially the elderly, during heat waves) typically progresses from heat exhaustion. But exertional heat stroke in athletes or outdoor workers can develop rapidly without clear prior warning. Any confusion, disorientation, or loss of consciousness in a hot environment should be treated as a heat stroke emergency — call emergency services immediately and begin cooling.
Does the heat index apply at night as well as during the day?
Yes. The heat index is calculated from temperature and humidity regardless of the time of day. Nighttime heat index values matter greatly for health outcomes, particularly during heat waves when temperatures do not drop below 27 °C overnight. The body uses nighttime temperatures to recover from daytime heat stress. Sustained warm nights (above 24 °C) are associated with significantly higher heat-related mortality during heat waves because the body never gets adequate recovery time.
What is the difference between heat index and humidex?
Both are apparent temperature indices that combine temperature and humidity, but they use different formulas and are used in different countries. The NOAA heat index is the standard in the United States. The Humidex is used by Environment and Climate Change Canada. For the same weather conditions, the Humidex typically reads slightly higher than the US heat index. Neither index accounts for wind speed, sun exposure, or individual physiology.