Humidex

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The humidex is a measurement 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.

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 current formula for determining the humidex was developed by J.M. Masterton and F.A. Richardson of Canada's Atmospheric Environment Service in 1979.

According to the Meteorological Service of Canada, a humidex of at least 40 causes "great discomfort" and above 45 is "dangerous." When the humidex hits 54, heat stroke is imminent.

The record humidex in Canada occurred on June 20, 1953, when Windsor, Ontario hit 52.1. (The residents of Windsor would not have known this at the time, since the humidex had yet to be invented.) More recently, the humidex reached 50 on July 14, 1995 in both Windsor and Toronto.

The humidex formula is as follows:

humidex = (air temperature) + h

h = (0.5555)*(e - 10.0)

e = 6.11 * exp [5417.7530 * ((1/273.16) - (1/dewpoint in kelvins))]

Complete:

[edit] External links

• "Humidity", Meteorological Service of Canada, 2002.
• "Frequently Asked Questions", MSC, 2004.

Meteorological data and variables
Atmospheric pressure | Baroclinity | Cloud | Convection | CAPE | CIN | Dew point | Heat index | Humidex | Humidity | Lightning | Pot T | Precipitation | Sea surface temperature | Surface solar radiation | Surface weather analysis | Temperature | Theta-e | Visibility | Vorticity | Wind chill | Water vapor | Wind