Estimate apparent heat from temperature and humidity using a standard heat-index formula.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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The heat index — also called apparent temperature or "feels like" temperature — combines air temperature and relative humidity to calculate how hot conditions actually feel to the human body. When the air is humid, your sweat cannot evaporate efficiently, limiting your body's primary cooling mechanism. Understanding heat index is vital for personal safety, outdoor work planning, athletic training, and emergency management across both the UK and United States.
The US National Weather Service uses the Rothfusz regression equation as its primary heat index formula:
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T2 - 0.05481717R2 + 0.00122874T2R + 0.00085282TR2 - 0.00000199T2R2
Where T is temperature in degrees Fahrenheit and R is relative humidity as a percentage. This formula is accurate to within +/- 1.3 degrees Fahrenheit. The NWS applies adjustment factors for low humidity conditions (below 13% RH with temperatures 80–112 F) and high humidity conditions (above 80% RH with temperatures above 80 F).
For simpler approximation, the Steadman simplified formula gives a useful result: HI = 0.5 x (T + 61.0 + ((T - 68.0) x 1.2) + (RH x 0.094)). This simpler version is used for initial screening — if the result exceeds 80 F, the full Rothfusz equation is applied.
The terms "heat index" and "apparent temperature" are often used interchangeably, but there are distinctions:
Direct sunlight can increase the heat index by as much as 10–15 degrees Fahrenheit (5–8 degrees Celsius) above the shaded value, which is why outdoor workers and athletes need to use WBGT or add a solar correction to the standard heat index.
| Heat Index | Category | Possible Heat Disorders |
|---|---|---|
| 80–90 F (27–32 C) | Caution | Fatigue possible with prolonged exposure and physical activity |
| 91–103 F (33–39 C) | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103–124 F (39–51 C) | Danger | Heat cramps and exhaustion likely; heat stroke possible |
| 125 F+ (52 C+) | Extreme Danger | Heat stroke highly likely — life threatening emergency |
The National Weather Service issues official heat products based on heat index thresholds. These vary slightly by region (acclimatisation differences mean what is dangerous in Minnesota may be tolerable in Florida):
US regions particularly affected: the Southwest desert (extreme dry heat), Gulf Coast (extreme humid heat — the most physiologically dangerous combination due to sustained high dew points overnight preventing body recovery), Southeast, and the Ohio Valley during summer heat waves.
The United Kingdom has a temperate maritime climate and was historically sheltered from extreme heat events. However, climate change has dramatically altered the UK's summer heat profile:
The UK Met Office issues Heat-Health Alerts through a 4-tier system (Green/Yellow/Amber/Red) in partnership with the UK Health Security Agency (UKHSA). A Red Heat-Health Alert — the highest tier, requiring systemic response across health and social care — was first issued in July 2022.
Painful muscle spasms occurring during or after exercise in the heat. Caused by salt and fluid loss through sweating. Treatment: rest in a cool place, drink water or electrolyte drinks, gently stretch affected muscles. Usually the least serious heat illness.
More serious than heat cramps. Symptoms: heavy sweating, cold/pale/clammy skin, fast but weak pulse, nausea, muscle cramps, tiredness, weakness, dizziness, headache, fainting. Core body temperature typically 37–40 C (98.6–104 F). Treatment: move to cool environment, apply cool wet cloths, sip cool water. If no improvement within 15 minutes, seek medical attention.
A life-threatening medical emergency. Two types: Classic heat stroke (non-exertional, affects elderly and infants in hot environments) and Exertional heat stroke (during intense physical activity). Symptoms: temperature above 40 C (104 F), hot/red/dry or damp skin, rapid/strong pulse, confusion, loss of consciousness. Call 999 (UK) or 911 (US) immediately. Cool the person immediately using any available means — ice baths, ice packs to neck/armpits/groin, cool water misting with fanning.
The Wet Bulb Globe Temperature (WBGT) is a composite index that accounts for temperature, humidity, wind speed, and solar radiation. It is the gold standard for assessing heat stress in occupational and sports settings. Key thresholds:
| WBGT | Athletic Guidance (US) | UK Construction / Outdoor Work |
|---|---|---|
| Below 18 C | Normal activity | Normal work |
| 18–22 C | Caution; hydration essential | Introduce controls, additional water breaks |
| 23–27 C | Limit strenuous activity; rest breaks | Significant controls required; consider schedule changes |
| Above 28 C | Cancel or postpone strenuous outdoor activity | Only essential work; maximum controls |
Certain populations are at significantly higher risk from heat: the elderly (reduced ability to regulate body temperature, more likely to take medications that impair heat adaptation), infants and young children (high body surface area to mass ratio, inability to communicate distress), and those with chronic conditions (cardiovascular disease, diabetes, obesity). In the UK, NHS heat guidance specifically advises checking on elderly neighbours and relatives during heatwaves. In the US, the CDC reports that from 1999–2023, excessive heat was the leading cause of weather-related mortality, with the elderly accounting for the majority of deaths.
The heat index combines air temperature and relative humidity to estimate how hot conditions actually feel. The NWS uses the Rothfusz regression equation. As a simple guide: at 90 F (32 C) and 70% humidity, the heat index is approximately 105 F (41 C) — significantly hotter than the actual air temperature because high humidity prevents sweat evaporation.
The NWS classifies 80–90 F (27–32 C) as Caution, 91–103 F (33–39 C) as Extreme Caution with cramps and exhaustion possible, 103–124 F (39–51 C) as Danger with heat stroke possible, and above 125 F (52 C) as Extreme Danger with heat stroke highly likely.
Heat exhaustion (core temperature up to 40 C) causes heavy sweating, weakness, nausea, and dizziness — serious but manageable with rest and cooling. Heat stroke (core temperature above 40 C) is a life-threatening emergency causing confusion, loss of consciousness, and organ damage. Call 999 (UK) or 911 (US) immediately and cool the person by any means available.
The UK's temperate climate means fewer than 5% of homes have air conditioning, compared to over 90% in the US South. UK homes, offices, and public transport are not designed for sustained high temperatures. The population is less acclimatised to extreme heat. When temperatures exceed 35 C (95 F), as they did in July 2022, the lack of cooling infrastructure creates significant public health risks.
The UK Met Office and UKHSA jointly operate a 4-tier Heat-Health Alert system: Green (no action needed), Yellow (social/community impacts possible), Amber (significant health impacts across the population, especially vulnerable groups), and Red (health emergency with severe impacts across the whole population). A Red alert was first issued in July 2022.
Yes — the standard NWS heat index assumes shade. Direct sunlight can increase the heat index by 10–15 F (5–8 C) above the shaded value. Outdoor workers and athletes should add this solar factor or use the Wet Bulb Globe Temperature (WBGT) index, which accounts for radiant heat from the sun.
During a Heat Advisory or Excessive Heat Warning: stay in air-conditioned buildings during peak heat hours (10 am–4 pm), drink water consistently even without thirst (avoid alcohol), wear loose light-coloured clothing, never leave children or pets in parked cars, check on elderly neighbours, and delay outdoor exercise to early morning or evening.
Humidity is the primary driver of the gap between air temperature and heat index. At 95 F (35 C) with 20% humidity, the heat index is about 92 F — actually lower than air temperature. At 95 F with 80% humidity, the heat index exceeds 128 F (53 C) — dangerously higher than air temperature. High humidity prevents sweat evaporation, eliminating the body's most important cooling mechanism.