Whats "heat index" ?

Answer 1

The Heat index (HI) is an index that combines air temperature and relative humidity to determine an apparent temperature — how hot it actually feels. The human body normally cools itself by perspiration, or sweating, in which the water in the sweat evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate of water is reduced. This means 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 corresponds a temperature and humidity combination to a higher temperature in dry air.

In Canada, the similar humidex is used in place of the heat index.

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.

Sometimes the heat index and the wind chill factor are denoted collectively by the single terms "apparent temperature" or "relative outdoor temperature".
Contents
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* 1 Meteorological considerations
* 2 Effects of the heat index (shade values)
* 3 Formula
* 4 See also
* 5 Footnotes
* 6 External links

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Meteorological considerations

Outdoors in open conditions, as relative humidity increases, first haze and ultimately thicker cloud cover will 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. Because of this factor, it was once believed that the highest heat-index reading actually attainable anywhere on Earth is approximately 160 °F (70 °C). 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). This is a full 16 °F (10 °C) higher than what was believed to be the highest heat index possible on Earth.[1]

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 typically has the lower daytime temperatures in summer due to being closer to the Gulf of Mexico, yet the city has a higher heat index because of the usually high local humidity. Likewise, while Bakersfield usually has higher daytime temperatures than New Orleans, the humidity in Bakersfield is much less, so it doesn't feel as hot as New Orleans.

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). Of course in such extreme heat, the likelihood is that the humidity fell and the humidex was not quite that high.


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Effects of the heat index (shade values)
Fahrenheit Celsius Notes
80–90 °F 27–32 °C Caution — fatigue is possible with prolonged exposure and activity
90–105 °F 32–41 °C Extreme caution — sunstroke, heat cramps, and heat exhaustion are possible
105–130 °F 41–54 °C Danger — sunstroke, heat cramps, and heat exhaustion are likely; heat stroke is possible
over 130 °F over 54 °C Extreme danger — heat stroke or sunstroke are likely with continued exposure

Note that exposure to full sunshine can increase heat index values by up to 15 °F (8 °C).
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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+\frac{5}{9}(e-10)

where,

e=V (6.112x10^{\frac{7.5 T}{(237.7+T)}}\frac{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\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\\ RH\\ RH^2\\ RH^3 \end{bmatrix}

Answer 2

The heat index is a combination of the temperature and the relative humidity, giving us a reference to how it feels. The reason for this is the more moisture there is in the air(humidity) the less can evaporate. You feel cooler when sweat can evaporate from your skin.

Answer 3

It is taking the humidity into consideration as well as the temperature to determine how the ambient temperature feels to people...Higher humidity carries more heat and slows the evaporation of sweat so higher humidity gives a higher "heat index".

Answer 4

the definition in my meteorology book says that heat index combines air temperature and relative humidity to determine an apparent temperature- how hot it actually feels

Answer 5

The combination of temperature and humidity on the body. When you combine the two its what temperature it feel like. It can be 90 degrees temperature with a humidity of 70%. It would feel like it is hotter than 90 degrees...

Answer 6

It's the combination of the actual temperature and the humidity to determine how the domestic environment feels to you. I

Answer 7

the feeling of being "hotter" than it truly is due to humidity levels in the air.

Answer 8

Happiness is a state that exists only for a moment