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The humidex is a number used by Canadian meteorologists to reflect the combined effect of heat and humidity. It differs from the heat index used in the United States in using dew point rather than relative humidity. According to the Meteorological Service of Canada, a humidex of at least 30 causes "some discomfort", at least 40 causes "great discomfort" and above 45 is "dangerous." When the humidex hits 54, heat stroke is imminent.

The current formula for determining the humidex was developed by J.M. Masterton and F.A. Richardson of Canada's Atmospheric Environment Service in 1979. The term is widely used in Canada during the summer months in weather reports.

The record humidex in Canada occurred on July 25, 2007, when Carman, Manitoba hit 53.0.[1] This breaks the previous record of 52.1 set in 1953 in Windsor, Ontario (The residents of Windsor would not have known this at the time, since the humidex had yet to be invented).


Humidex formula

When the temperature is 30 °C (86 °F) and the dew point is 15 °C (59 °F), the humidex is 34 (note that humidex is a dimensionless number, but that the number indicates an approximate temperature in °C). If the temperature remains 30 °C and the dew point rises to 25 °C (77 °F), the humidex rises to 41. The humidex tends to be higher than the U.S. heat index at equal temperature and relative humidity.

The humidex formula is as follows:

humidex = (air temperature in Celsius) + h
h = (0.5555)*(e - 10.0)
e = 6.11 * exp [5417.7530 * ((1/273.16) - (1/dewpoint in kelvins))]


\text{Humidex} = \text{Air temperature}\ +\ 0.5555 \times (6.11 \times e^{5417.7530 \times \left(\frac{1}{273.16} - \frac{1}{\text{dewpoint in kelvins}}\right)} - 10)

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