humidex calculator

What is humidex?

Humidex is a weather index used in Canada to describe how hot the air feels to the human body, not just what a thermometer reads. It combines air temperature with moisture in the air (humidity) to estimate perceived heat stress.

When humidity is high, sweat evaporates more slowly. Your body then cools less efficiently, so the same temperature can feel much hotter. That is exactly what humidex captures.

How this humidex calculator works

This calculator takes:

• Air Temperature (°C)
• Relative Humidity (%)

It first estimates dew point from temperature and humidity, then uses the standard humidex equation:

Humidex = T + 0.5555 × (e − 10)

where T is temperature in °C and e is vapour pressure derived from dew point. This is the same approach used in many weather and climate tools.

Humidex interpretation guide

Humidex	How it generally feels	Suggested action
Below 20	Comfortable	Normal activity is fine.
20–29	Little or no discomfort	Stay hydrated during outdoor activity.
30–39	Some discomfort	Take breaks in shade or cool areas.
40–45	Great discomfort	Reduce exertion and drink water regularly.
46–53	Dangerous heat stress possible	Avoid heavy outdoor work; monitor symptoms.
54+	Very dangerous	Heat stroke risk is high; seek cooling immediately.

Humidex vs. heat index

Humidex and the U.S. heat index aim to answer the same question: “How hot does it feel?” The difference is mostly in methodology and regional usage:

• Humidex: widely used in Canada, based on vapour pressure/dew point.
• Heat Index: widely used in the U.S., based on temperature and relative humidity regression.

Both are useful. Just avoid comparing their numbers as if they are the exact same scale.

Practical heat safety tips

1) Hydrate early, not just when thirsty

Thirst can lag behind fluid loss. During high humidex days, drink water steadily and replace electrolytes if sweating heavily.

2) Adjust your workout or work schedule

Try early morning or late evening when humidex is lower. If you must be outside, use a work-rest cycle.

3) Dress for evaporation

Lightweight, breathable fabrics help sweat evaporate more effectively than heavy or non-breathable clothing.

4) Watch for warning signs

• Dizziness
• Nausea
• Headache
• Rapid pulse
• Confusion

If symptoms escalate, move to a cool place and get medical help quickly.

Limitations of humidex

Humidex is a great quick indicator, but it does not include every factor affecting thermal stress:

• Direct sun exposure
• Wind speed
• Clothing and physical activity level
• Individual differences (age, health conditions, acclimatization)

So treat humidex as a practical guide, not a perfect physiological model.

Final thoughts

On hot and humid days, temperature alone can be misleading. Use this humidex calculator to understand perceived heat, plan safer outdoor activity, and reduce the risk of heat-related illness. A quick check can make a big difference in comfort and safety.