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These calculators calculate Humindex, the index used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. Humidex value is basically an equivalent or apparent temperature (one that the human body would feel). It's a standard for Canada, but variations are used around the world. Humidex differs from the heat index used in the United States in being derived from the dew point rather than the relative humidity.
J. M. Masterton and F. A Richardson of Canada's Atmospheric Environment Service developed the formula for determining the humidex in 1979.
Here, the dewpoint temperature should be given in Kelvin; the magic number 5417.7530 is a rounded constant based on the molecular weight of water, latent heat of vaporization, and the universal gas constant.
The calculator below uses this formula to calculate humindex by air temperature and dew point.
Also, there is another formula that relates humindex to temperature and relative humidity, which value easier to get than dew point value.
The calculator below uses this formula to calculate humindex by air temperature and relative humidity.
How to interpret humindex?
According to Environment and Climate Change Canada - Spring and Summer Weather Hazards humindex has the following meanings:
- 20 - 29: No discomfort
- 30 - 39: Some discomfort
- 40 - 45: Great discomfort; avoid exertion
*46 and over: Dangerous; possible heat stroke
Also, Canadian Centre for Occupational Health and Safety gives the following recommendations based on humindex reading for "Humindex 1 - Moderate physical work, unacclimatized worker, OR Heavy physical work, acclimatized worker":
- below 25: None
- 25-29: Supply water to workers on an "as needed" basis
- 30-33: Post Heat Stress Alert notice. Encourage workers to drink extra water. Start recording hourly temperature and relative humidity.
- 34-37: Post Heat Stress Warning notice. Notify workers that they need to drink extra water. Ensure workers are trained to recognize symptoms.
- 38-39: Work with 15 minutes of relief per hour can continue. Provide adequate cool (10-15C) water. At least 1 cup (240 mL) of water every 20 minutes. Workers with symptoms should seek medical attention.
- 40-41: Work with 30 minutes of relief per hour can continue. Provide adequate cool (10-15C) water. At least 1 cup (240 mL) of water every 20 minutes. Workers with symptoms should seek medical attention.
- 42-44: If feasible, work with 45 minutes relief per hour can continue. Provide adequate cool (10-15C) water. At least 1 cup (240 mL) of water every 20 minutes. Workers with symptoms should seek medical attention.
- above 45: Only medically supervised work can continue.
Source: Humidex Rating and Work