The heat index or humiture or humidex is an index that combines air temperature and relative
humidity in an attempt to determine the human-perceived equivalent temperature—how hot it feels.
The result is also known as the "felt air temperature" or "apparent temperature". For
example, when the temperature is 32 °C with very high humidity, the heat index can be
about 41 °C The human body normally cools itself by perspiration,
or sweating. Heat is removed from the body by evaporation of that sweat. However, relative
humidity reduces the evaporation rate because the higher vapor content of the surrounding
air does not allow the maximum amount of evaporation from the body to occur. This results in a
lower rate of heat removal from the body, hence the sensation of being overheated. This
effect is subjective; its measurement has been based on subjective descriptions of how
hot subjects feel for a given temperature and humidity. This results in a heat index
that relates one combination of temperature and humidity to another.
History The heat index was developed in 1978 by George
Winterling as the "humiture" and was adopted by the USA's 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. While both the humidex
and the heat index are calculated using dew point, the humidex uses a dew point of 45 °F
as a base, whereas the heat index uses a dew point base of 57 °F. Further, the heat index
uses heat balance equations which account for many variables other than vapor pressure,
which is used exclusively in the humidex calculation. A joint committee formed by the United States
and Canada to resolve differences has since been disbanded.
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,
this baseline corresponds to a dew point of 14 °C and a mixing ratio of 0.01. This corresponds
to an air temperature of 25 °C and relative humidity of 50% in the sea-level psychrometric
chart. A given value of relative humidity causes
larger increases in the heat index at higher temperatures. For example, at approximately
27 °C, the heat index will agree with the actual temperature if the relative humidity
is 45%, but at about 43 °C, any relative-humidity reading above 17% will make the heat index
higher than 43 °C. The formula described is valid only if the
temperature is 27 °C or more, and the relative humidity is 40% or more. 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, unless the air is above body temperature.
Sometimes the heat index and the wind chill are denoted collectively by the single term
"apparent temperature", "relative outdoor temperature", or "feels like".
Meteorological considerations Outdoors in open conditions, as the relative
humidity increases, first haze and ultimately a 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. However, in Dhahran, Saudi Arabia on July 8, 2003,
the dew point was 35 °C while the temperature was 42 °C, resulting in a heat index of
78 °C. This is comparable to the temperatures that are recommended to kill bacteria in many
meat products, and it is common in a sauna. Table of Heat Index values
This table is from the U.S. National Oceanic and Atmospheric Administration.
To find the Heat Index temperature, look at the Heat Index chart above. For example, if
the air temperature is 96°F and the relative humidity is 65%, the heat index—how hot
it feels—is 121°F. This table is an approximation of the Heat
Index, using the formula and first set of constants below, converted to Celsius.
Effects of the heat index Exposure to full sunshine can increase heat
index values by up to 8 °C. Formula
The formula below approximates the heat index in degrees Fahrenheit, to within ±1.3 °F.
It is the result of a multivariate fit to a model of the human body. This equation reproduces
the above NOAA National Weather Service table.
where = heat index
= ambient dry-bulb temperature = relative humidity
An alternative set of constants for this equation that 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: A further alternate is this:
where
For example, using this last formula, with temperature 90 °F and relative humidity
of 85%, the result would be: Heat index for 90 °F, RH 85% = 114.9.
See also Apparent temperature
Humidex Wind chill
References
External links Description of wind chill & apparent temperature
Formulae in metric units Heat Index Calculator Calculates both °F
and °C