Humidex Calculator
What this calculator does
This humidex calculator estimates how hot the air feels by combining air temperature with moisture using the Environment and Climate Change Canada humidex framework. It is especially useful for warm-season communication where humidity materially changes comfort and health risk.
Humidex is similar in purpose to heat index, but it is built from a different equation and is commonly used in Canadian weather communication. Because the index ties the moisture burden to vapor pressure through dew point, it offers a helpful way to express why a humid day can feel much hotter than the thermometer suggests.
Inputs explained
- Air temperature: Enter the current dry-bulb air temperature.
- Dew point temperature: Enter the dew point so the calculator can estimate the moisture contribution directly.
- Unit handling: The page calculates in Celsius internally and can present the interpreted result cleanly for users.
How it works / method
The page first converts dew point to vapor pressure using the standard Environment Canada style relationship. It then computes a humidex adjustment term and adds that term to the air temperature. The final value is not a direct air temperature replacement; it is a comfort and risk index for hot and humid weather.
Formula used
e = 6.11 x exp(5417.7530 x ((1 / 273.16) - (1 / (Td + 273.15)))); Humidex = T + 0.5555 x (e - 10)
T and Td are handled in C. The dew point drives the vapor pressure term, which then raises the apparent heat burden above the dry-bulb air temperature.
Practical note: Humidex is a comfort and risk indicator, not a replacement for air temperature. Sun, wind, clothing, work rate, and health status can change the real heat burden substantially.
Canadian Heat Discomfort Index
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Comfort: --
Step-by-step example
Suppose the air temperature is 30 C and the dew point is 24 C. The humidex value helps communicate how heavy and oppressive the air may feel.
- Enter 30 for air temperature.
- Enter 24 for dew point.
- The calculator converts dew point to vapor pressure and computes a humidex in the upper 30s or low 40s.
- That range points to noticeable discomfort and a stronger need for hydration and reduced exertion.
- If dew point drops while temperature stays the same, humidex usually falls quickly.
Use cases
- Explaining Canadian-style warm-weather comfort messaging in forecasts or preparedness content.
- Comparing hot days where the thermometer value is similar but the dew point is very different.
- Helping users move from raw moisture metrics such as dew point to a more intuitive heat-burden scale.
- Supporting outdoor planning when humidity is the main reason the weather feels oppressive.
Assumptions and limitations
- Humidex is built for warm and humid weather and should not be interpreted as a physical air temperature measurement.
- The result depends on the dew point estimate or measurement, so poor humidity inputs will propagate into the final value.
- It does not directly account for solar radiation, terrain, personal heat generation, or formal occupational exposure standards.
- Different regions may communicate heat stress with different indices, so humidex is not the only valid framework.
If you need a sun-sensitive screening metric, compare humidex with WBGT. If you want a direct moisture indicator, look at the dew point page instead.
Frequently Asked Questions
The Canadian formula is H = T + (5/9)(e − 10), where T is air temperature in °C and e is vapour pressure in hPa. Vapour pressure comes from dew point: e = 6.11 × exp(5417.7530 × (1/273.16 − 1/(273.15 + Td))). So at 30°C with a 20°C dew point, e ≈ 23.4 hPa, giving H = 30 + (5/9)(13.4) ≈ 37.4. Environment Canada treats this as the perceived temperature — Canadians read it the way Americans read heat index.
H = Tair + 0.5555 × (e − 10), with Tair in degrees Celsius and vapour pressure e in hPa. Vapour pressure is computed from dew point using the Clausius–Clapeyron relation. The 0.5555 factor is just 5/9, a rough conversion of vapour-pressure excess into "felt" degrees. The result is unitless by convention — Canadians say "the humidex is 38" rather than "38°C." It collapses humidity and temperature into one number a layperson can read on the morning forecast.
Environment Canada uses these bands: below 30 little discomfort, 30–39 some discomfort, 40–45 great discomfort and avoid exertion, above 45 dangerous, above 54 heat-stroke imminent. Above 40 the elderly, infants, and outdoor workers are at real risk. The 1995 Chicago heatwave reached humidex values above 50 for several days running and killed over 700 people. Treat the upper bands as medical emergencies, not just uncomfortable weather. Local public-health alerts in Canada start at humidex 40.
A humidex reading of 35 means a healthy adult will feel some discomfort and start sweating more than usual, but it is not a danger zone for most people. It corresponds roughly to an air temperature near 30°C with a dew point of about 21°C. Outdoor work is fine with regular water breaks. Strenuous sport is okay but you will fatigue faster. It is the typical mid-July afternoon in southern Ontario — uncomfortable, not dangerous, and a clear hint to move shade-side.
Below 29 is generally comfortable for most people in shade with light activity. Some sources mark 20–29 as "no discomfort," 30–39 as "some discomfort," and onwards. Personal tolerance varies — fitness, acclimatisation, age, and clothing all shift the line. For office HVAC design and outdoor event planning, anything under 30 is the working target. If you are scheduling an outdoor meeting in summer, look at the humidex forecast, not just the air temperature, to predict how the crowd will hold up.
Canada developed humidex in 1965 to communicate summer heat-and-humidity stress in a single number meaningful to the public. The country gets short, intense humid summers in the south — Toronto, Montreal, Windsor — and the humidex captures the discomfort better than air temperature alone. The US uses heat index for the same purpose; Australia uses apparent temperature. They are different formulas built around the same idea: a layperson should not need three numbers to understand whether to stay indoors.
Two routes. If you have RH directly, first convert RH to dew point using the Magnus formula, then dew point to vapour pressure, then plug into H = T + (5/9)(e − 10). If you already have dew point, skip the first step. Most online calculators just take T and RH as inputs and chain the conversions internally. Watch units: vapour pressure must be in hPa, not Pa or kPa, or your humidex will be wildly off.
Because humid air sabotages your body's main cooling mechanism — sweat evaporation. When vapour pressure is high, sweat sits on the skin instead of evaporating, so heat builds up. The humidex equation effectively adds the "extra felt heat" caused by elevated water vapour. At 30°C with very dry air, humidex equals air temperature. At 30°C with a 22°C dew point, humidex jumps to 39 — same air, but your physiology is in a much harder place. Hence the higher number.
Sources & references
- NOAA National Weather Service - Heat index guidance
- NOAA National Weather Service - Wind chill chart and formula
- NOAA National Weather Service - Wet Bulb Globe Temperature overview
- Bureau of Meteorology - Apparent temperature background
- Bureau of Meteorology - Apparent temperature glossary
- Environment and Climate Change Canada - Humidex guidance
- Schema.org - FAQPage and WebApplication vocabulary