The
heat index (
HI) 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
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 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, 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 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).
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 80
°F (approximately 27
°C),
the heat index will agree with the actual temperature if the
relative humidity is 45%, but at 110 °F (roughly 43 °C), any
relative-humidity reading above 17% will make the Heat Index higher
than 110 °F.
The heat index is calculated only if the actual temperature is
above 27 °C (80 °F), dew point temperatures greater than 16 °C (60
°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.
Sometimes the heat index and the
wind chill
factor 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 160 °F (71 °C).
However,
in Dhahran
, Saudi Arabia
on July 8, 2003, the dewpoint was 95 °F
while the temperature was 108 °F. The heat index at that
time was 172 °F (78 °C).
Effects of the heat index (shade values)
Note that exposure to full sunshine can increase heat index values
by up to 15 °F (8 °C).
Formula
Here is a formula for approximating the heat index in degrees
Fahrenheit, to within ±1.3 °F. It is
useful only when the temperature is at least 80 °F and the relative
humidity is at least 40%.HI = c_1 + c_2 T + c_3 R + c_4 T R + c_5
T^2 + c_6 R^2 + c_7 T^2R + c_8 T R^2 + c_9 T^2 R^2\ \,
where
- HI\,\! = Heat index (in degrees Fahrenheit)
- T\,\! = ambient dry-bulb
temperature (in degrees Fahrenheit)
- R\,\! = relative humidity (in percent)
- c_1\,\! = -42.379
- c_2\,\! = 2.04901523
- c_3\,\! = 10.14333127
- c_4\,\! = -0.22475541
- c_5\,\! = -6.83783
- c_6\,\! = -5.481717
- c_7\,\! = 1.22874
- c_8\,\! = 8.5282
- c_9\,\! = -1.99
A
more accurate formula is available, involving
several more terms:HI=
\begin{bmatrix}
1& T& T^2& T^3
\end{bmatrix}
\begin{bmatrix}
16.923 & 5.37941 & 7.28898\times 10^{-3}& 2.91583\times 10^{-5} \\
1.85212\times 10^{-1}& -1.00254\times 10^{-1}& -8.14971\times 10^{-4}& 1.97483\times 10^{-7} \\
9.41695\times 10^{-3}& 3.45372\times 10^{-4}& 1.02102\times 10^{-5}& 8.43296\times 10^{-10} \\
-3.8646\times 10^{-5}& 1.42721\times 10^{-6}& -2.18429\times 10^{-8}& -4.81975\times 10^{-11}
\end{bmatrix}
\begin{bmatrix}
1\\ R\\ R^2\\ R^3
\end{bmatrix} \,
References
- The Assessment of Sultriness. Part I: A Temperature-Humidity
Index Based on Human Physiology and Clothing Science, R. G.
Steadman, Journal of Applied Meteorology, July 1979, Vol 18 No7,
pp861-873
- The Assessment of Sultriness. Part II: Effects of Wind, Extra
Radiation and Barometric Pressure on Apparent Temperature Journal
of Applied Meteorology, R. G. Steadman, July 1979, Vol 18 No7,
pp874-885
- How do they figure the heat index? - By Daniel Engber -
Slate Magazine
-
[http://www.campbellsci.com/documents/technical-papers/heatindx.pdf
Heat Index Campbell Scientific Inc.
- Closely paraphrased from the public domain article Heat
Index on the website of the Pueblo, CO United States National
Weather Service.
External links