Humidex
Encyclopedia
The humidex is an index number used by Canadian
meteorologists
to describe how hot the weather feels to the average person, by combining the effect of heat and humidity
. The humidex is a unit-less number based on the dew point
, but it is equivalent to dry temperature in degrees Celsius. For example, if the temperature is 30 °C (86 °F), and the calculated humidex is 40, then it indicates the humid heat feels approximately like a dry temperature of 40 °C (104 °F). The index is widely used in Canadian weather reports during summer.
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 (see below: Humidex formula) was developed by J.M. Masterton and F.A. Richardson of Canada's Atmospheric Environment Service in 1979. Humidex differs from the heat index
used in the United States
in being derived from the dew point
rather than the relative humidity
.
The record humidex in Canada occurred on July 25, 2007, when Carman, Manitoba
hit 53.0. This breaks the previous record of 52.1 set in 1953 in Windsor, Ontario
.
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 42. The humidex tends to be higher than the U.S. heat index at equal temperature and relative humidity.
The humidex formula is as follows:
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
meteorologists
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
to describe how hot the weather feels to the average person, by combining the effect of heat and humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
. The humidex is a unit-less number based on the dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
, but it is equivalent to dry temperature in degrees Celsius. For example, if the temperature is 30 °C (86 °F), and the calculated humidex is 40, then it indicates the humid heat feels approximately like a dry temperature of 40 °C (104 °F). The index is widely used in Canadian weather reports during summer.
According to the Meteorological Service of Canada
Meteorological Service of Canada
The Meteorological Service of Canada , also known as "The Canadian Weather Service", is a division of Environment Canada, which primarily provides public meteorological information and weather forecasts and warnings of severe weather and other environmental hazards...
, 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 (see below: Humidex formula) was developed by J.M. Masterton and F.A. Richardson of Canada's Atmospheric Environment Service in 1979. Humidex differs from the heat index
Heat index
The heat index is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature. The human body normally cools itself by perspiration, or sweating, which evaporates and carries...
used in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
in being derived from the dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
rather than the relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
.
The record humidex in Canada occurred on July 25, 2007, when Carman, Manitoba
Carman, Manitoba
Carman is a small Agricultural town of about 2,900 people that lies in the Pembina Valley Region of southern Manitoba. Carman is located at the junction of Highways 3 and 13, just 40 minutes southwest of Winnipeg . It is surrounded by the Rural Municipality of Dufferin...
hit 53.0. This breaks the previous record of 52.1 set in 1953 in Windsor, Ontario
Windsor, Ontario
Windsor is the southernmost city in Canada and is located in Southwestern Ontario at the western end of the heavily populated Quebec City – Windsor Corridor. It is within Essex County, Ontario, although administratively separated from the county government. Separated by the Detroit River, Windsor...
.
Humidex formula
When the temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
is 30 °C (86 °F) and the dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
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 42. The humidex tends to be higher than the U.S. heat index at equal temperature and relative humidity.
The humidex formula is as follows:
External links
- "Frequently Asked Questions", MSC, 2004.
- Wind Chill and Humidex Criticism about the use of Wind chill and humidex
- More Humidex info