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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. Humidity is deemed not to raise the apparent temperature at all if the actual temperature is below approximately 68 °F (20 °C) — essentially the same temperature colder than which Wind Chill is thought to commence.

As relative humidity increases, however, first haze and ultimately thicker cloud cover must develop, reducing the amount of direct sunlight reaching the surface; thus there is an inverse relationship between maximum potential temperature and maximum potential relative humidity, therefore making, say, a simultaneous temperature of 120 °F (50 °C) and 90% relative humidity physically impossible. Because of this factor, it was believed that the highest heat-index reading actually attainable anywhere on Earth is approximately 160 °F (70 °C) in the shade that is 172 °F (78 °C) in the sun. However, in Dhahran , Saudi Arabia on July 8, 2003, the dewpoint was 95 °F (35 °C) while the actual temperature was 108 °F (42 °C) giving it a relative humidity of 67%. The heat index at that time was 176 °F (80° C) in the shade and 191 °F (88 °C) in the sun. This is a full 16 °F (10 °C) higher than what was believed to be the highest heat index possible on Earth. Hourly Observations from Dhahran, Saudi Arabia . "History: Weather Underground". Accessed April 20, 2006.

A good example of the difference between heat index and true temperature would be comparing the climates of New Orleans, Louisiana and Bakersfield, California . New Orleans has lower heat temperatures due to being closer to the Gulf Of Mexico , yet the city has a higher heat index because it is a humid heat. Likewise, while Bakersfield actually has hotter daytime temperatures, Bakersfield has a dry heat, so it doesn't feel as hot.

Sometimes the heat index and the Wind Chill Factor are denoted collectively by the single terms "apparent temperature" or "relative outdoor temperature".

A typical measurement in the United Kingdom in the hottest areas in the hottest type of weather generally has temperatures that can get above about 90 °F (32 °C) in the shade with 50% humidity which on the humidex chart is 104 °F (40 °C) in the shade and 119 °F (48 °C) in the sun. You should note that humidex and heat indexes are taken in the shade and not the sun; so extra care must be taken while in the sun! For the record, temperatures reached 101.3 °F (38.5 °C) in Kent in mid August 2003 which is the national record; assuming that this was 50% humidity; it would have a humidex of 126 °F (52 °C) in the shade and 140 °F (60 °C) in the sun. Of course in such extreme heat, the likelihood is that the humidity fell and the humidex was not quite that high.

WARNING - Add approximately 15 °F (8 °C) to the heat index for measurements in direct sunshine.


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 calculating 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\,\! = 10.1433127
:c_4\,\! = -0.22475541
:c_5\,\! = -6.83783 × 10-3
:c_6\,\! = -5.481717 × 10-2
:c_7\,\! = 1.22874 × 10-3
:c_8\,\! = 8.5282 × 10-4
:c_9\,\! = -1.99 × 10-6.

The formula for calculating the heat index using the air temperature in degrees Celsius and the Relative Humidity is:

:HI = T+ rac{5}{9}(e-10)
where,
:e=V (6.112x10^{ rac{7.5 T}{(237.7+T)}} rac{R}{100})
and
:V = Vapour Pressure
:R = Relative Humidity (%)
:T = Temperature (oC)

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 imes 10^{-3}& 2.91583 imes 10^{-5} \
1.85212 imes 10^{-1}& -1.00254 imes 10^{-1}& -8.14971 imes 10^{-4}& 1.97483 imes 10^{-7} \
9.41695 imes 10^{-3}& 3.45372 imes 10^{-4}& 1.02102 imes 10^{-5}& 8.43296 imes 10^{-10} \
-3.8646 imes 10^{-5}& 1.42721 imes 10^{-6}& -2.18429 imes 10^{-8}& -4.81975 imes 10^{-11}
\end{bmatrix}
\begin{bmatrix}
1\ RH\ RH^2\ RH^3
\end{bmatrix}



DEVELOPMENT OF HUMIDEX/HEAT INDEX


The humidex was first developed in 1965 by Canadian meteorologists, who are no strangers to heat and humidity in the summer. The highest humidex on record in Canada was observed at Windsor, Ontario on June 20, 1953 , when it reached 52.1 °C (126 °F).


SEE ALSO



FOOTNOTES



EXTERNAL LINKS


CAUTION - Some heat index charts may be wrong because of the base measurement not starting at 0% because of people comparing it to their environment feel factor; example 32 °C may be said to feel like 32 °C at 30% humidity (their average humidity) and feels less when it is below, when really it is at 0% that it feels like 32 °C. Ensure that the windchill chart is the new windchill chart for up to date calculations.





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