Heat index
Encyclopedia
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 sweat
ing, which evaporates
and carries heat
away from the body. However, when the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which relates one temperature and humidity combination to another at a higher temperature in drier air.
The heat index was developed in 1978 by George Winterling
as the "humiture," and was adopted by the National Weather Service
a year later. It is derived from work carried out by Robert G. Steadman. Like the wind chill
index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, thickness of blood, sunlight and ultraviolet radiation exposure, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature.
In Canada
, the similar humidex
is used in place of the heat index. The humidex
differs from the heat index in using the relative humidity
rather than the dew point
.
The heat index is defined so as to equal the actual air temperature when the partial pressure
of water vapor
is equal to a baseline value of 1.6 kPa
. At standard atmospheric pressure (101.325 kPa), this baseline corresponds to a dew point
of 14 °C (57 °F) and a mixing ratio
of 0.01 (10 g of water vapor per kilogram of dry air). This corresponds to an air temperature of 25°C and relative humidity of 50% in the sea-level psychrometric chart.
At high temperatures, the level of relative humidity needed to make the heat index higher than the actual temperature is lower than at cooler temperatures. For example, at approximately 27 °C (80 °F), the heat index will agree with the actual temperature if the relative humidity is 45%, but at about 43°C (110°F), any relative-humidity reading above 17% will make the Heat Index higher than 43°C (110 °F).
The formula described is considered valid only if the actual temperature is above 27 °C (80 °F), dew point temperatures greater than 12 °C (54 °F), and relative humidities higher than 40%. The heat index and humidex figures are based on temperature measurements taken in the shade and not the sun, so extra care must be taken while in the sun. The heat index also does not factor in the effects of wind, which lowers the apparent temperature.
Sometimes the heat index and the wind chill
are denoted collectively by the single terms "apparent temperature" or "relative outdoor temperature".
, Saudi Arabia
on July 8, 2003, the dewpoint was 35 °C (95 °F) while the temperature was 42 °C (107.6 °F), resulting in a heat index of 78 °C (172.4 °F).
. Percent figures are relative humidity.
Note that exposure to full sunshine can increase heat index values by up to 8 °C (14 °F).
where
= heat index (in degrees Fahrenheit)
= ambient dry-bulb temperature
(in degrees Fahrenheit)
= relative humidity (in percent)
An alternative set of constants for this equation which is within 3 degrees of the NWS master table for all humidities from 0 to 80% and all temperatures between 70 and 115 °F and all heat indexes < 150 °F is
C1 = 0.363445176,
C2= 0.988622465,
C3=4.777114035,
C4=-0.114037667,
C5=-0.000850208,
C6=-0.020716198,
C7=0.000687678,
C8=0.000274954,
C9=0 (unused).
Temperature
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...
and 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...
in an attempt to determine the human-perceived equivalent temperature — how hot it feels, termed the felt air temperature
Felt air temperature
Felt air temperature is a quantitative measure that indicates the amount of heat the human body loses outdoors in a given time and place....
. The human body normally cools itself by perspiration, or sweat
SWEAT
SWEAT is an OLN/TSN show hosted by Julie Zwillich that aired in 2003-2004.Each of the 13 half-hour episodes of SWEAT features a different outdoor sport: kayaking, mountain biking, ice hockey, beach volleyball, soccer, windsurfing, rowing, Ultimate, triathlon, wakeboarding, snowboarding, telemark...
ing, which evaporates
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
and carries heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
away from the body. However, when the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate causing it to retain more heat than it would in dry air. Measurements have been taken based on subjective descriptions of how hot subjects feel for a given temperature and humidity, allowing an index to be made which relates one temperature and humidity combination to another at a higher temperature in drier air.
The heat index was developed in 1978 by George Winterling
George Winterling
George Alfred Winterling was an early television weather broadcaster, Chief Meteorologist for television station WJXT in Jacksonville, Florida for almost fifty years, and helped develop television weather forecasting.-Education:...
as the "humiture," and was adopted by the National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
a year later. It is derived from work carried out by Robert G. Steadman. Like the wind chill
Wind chill
Wind chill is the felt air temperature on exposed skin due to wind. The wind chill temperature is always lower than the air temperature, and the windchill is undefined at the higher temps...
index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, thickness of blood, sunlight and ultraviolet radiation exposure, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature.
In Canada
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...
, the similar humidex
Humidex
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...
is used in place of the heat index. The humidex
Humidex
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...
differs from the heat index in using 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...
rather than 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...
.
The heat index is defined so as to equal the actual air temperature when the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
is equal to a baseline value of 1.6 kPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
. At standard atmospheric pressure (101.325 kPa), this baseline corresponds to a 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...
of 14 °C (57 °F) and a mixing ratio
Mixing ratio
In chemistry and physics, the dimensionless mixing ratio is defined as the abundance of one component of a mixture relative to that of all other components...
of 0.01 (10 g of water vapor per kilogram of dry air). This corresponds to an air temperature of 25°C and relative humidity of 50% in the sea-level psychrometric chart.
At high temperatures, the level of relative humidity needed to make the heat index higher than the actual temperature is lower than at cooler temperatures. For example, at approximately 27 °C (80 °F), the heat index will agree with the actual temperature if the relative humidity is 45%, but at about 43°C (110°F), any relative-humidity reading above 17% will make the Heat Index higher than 43°C (110 °F).
The formula described is considered valid only if the actual temperature is above 27 °C (80 °F), dew point temperatures greater than 12 °C (54 °F), and relative humidities higher than 40%. The heat index and humidex figures are based on temperature measurements taken in the shade and not the sun, so extra care must be taken while in the sun. The heat index also does not factor in the effects of wind, which lowers the apparent temperature.
Sometimes the heat index and the wind chill
Wind chill
Wind chill is the felt air temperature on exposed skin due to wind. The wind chill temperature is always lower than the air temperature, and the windchill is undefined at the higher temps...
are denoted collectively by the single terms "apparent temperature" or "relative outdoor temperature".
Meteorological considerations
Outdoors in open conditions, as relative humidity increases, first haze and ultimately thicker cloud cover develops, reducing the amount of direct sunlight reaching the surface; thus there is an inverse relationship between maximum potential temperature and maximum potential relative humidity. Because of this factor, it was once believed that the highest heat index reading actually attainable anywhere on Earth is approximately 71 °C (159.8 °F). However, in DhahranDhahran
Dhahran is a city located in Saudi Arabia's Eastern Province, and is a major administrative center for the Saudi oil industry. Large oil reserves were first identified in the Dhahran area in 1931, and in 1935 Standard Oil of California drilled the first commercially viable oil well...
, Saudi Arabia
Saudi Arabia
The Kingdom of Saudi Arabia , commonly known in British English as Saudi Arabia and in Arabic as as-Sa‘ūdiyyah , is the largest state in Western Asia by land area, constituting the bulk of the Arabian Peninsula, and the second-largest in the Arab World...
on July 8, 2003, the dewpoint was 35 °C (95 °F) while the temperature was 42 °C (107.6 °F), resulting in a heat index of 78 °C (172.4 °F).
Table of Fahrenheit heat index values
This table is from the U.S. National Weather ServiceNational Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
. Percent figures are relative humidity.
| °F | 90% | 80% | 70% | 60% | 50% | 40% | °C |
|---|---|---|---|---|---|---|---|
| 80 | 85 | 84 | 82 | 81 | 80 | 79 | 27 |
| 85 | 101 | 96 | 92 | 90 | 86 | 84 | 30 |
| 90 | 121 | 113 | 105 | 99 | 94 | 90 | 32 |
| 95 | 133 | 122 | 113 | 105 | 98 | 35 | |
| 100 | 142 | 129 | 118 | 109 | 38 | ||
| 105 | 148 | 133 | 121 | 41 | |||
| 110 | 135 | 43 |
Effects of the heat index (shade values)
| Celsius | Fahrenheit | Notes |
|---|---|---|
| 27–32 °C | 80–90 °F | Caution — fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps |
| 32–41 °C | 90–105 °F | Extreme caution — heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke |
| 41–54 °C | 105–130 °F | Danger — heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity |
| over 54 °C | over 130 °F | Extreme danger — heat stroke is imminent |
Note that exposure to full sunshine can increase heat index values by up to 8 °C (14 °F).
Formula
Here is a formula for approximating the heat index in degrees Fahrenheit, to within ±1.3 °F. It is the result of a multivariate fit (over temperatures at least 80°F and relative humidity at least 40%) to a model of the human body.where
= heat index (in degrees Fahrenheit) = ambient dry-bulb temperatureDry-bulb temperature
The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. Dry bulb temperature is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It is the...
(in degrees Fahrenheit)
= relative humidity (in percent) An alternative set of constants for this equation which is within 3 degrees of the NWS master table for all humidities from 0 to 80% and all temperatures between 70 and 115 °F and all heat indexes < 150 °F is
C1 = 0.363445176,
C2= 0.988622465,
C3=4.777114035,
C4=-0.114037667,
C5=-0.000850208,
C6=-0.020716198,
C7=0.000687678,
C8=0.000274954,
C9=0 (unused).
External links
- Description of wind chill & apparent temperature Formulae in metric units
- Heat Index Calculator Calculates both °F and °C