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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. It should be noted that humidex and heat indexes 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".

Contents 1 Meteorological considerations 2 Effects of the heat index (shade values) 3 Formula 4 References 5 External links

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 (71 °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).^[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.^{[original research?]}

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).

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%.

where

= Heat index (in degrees Fahrenheit)

= ambient dry-bulb temperature (in degrees Fahrenheit)

= relative humidity (in percent)

= -42.379

= 2.04901523

= 10.14333127

= -0.22475541

= -6.83783×10⁻³

= -5.481717×10⁻²

= 1.22874×10⁻³

= 8.5282×10⁻⁴

= -1.99×10⁻⁶

A more accurate formula is available, involving several more terms:

The formula for calculating the humidex using the air temperature in degrees Celsius and the relative humidity is:^[2]

where e is the Vapour pressure calculated as follows:

and

h = humidex

R = Relative humidity [%]

T = Temperature [^oC]

1. ^ Hourly Observations from Dhahran, Saudi Arabia. "History: Weather Underground". Accessed April 20, 2006.
2. ^ Definition of Humidex formula

• National Science Digital Library - Heat Index
• An Introduction to Heat Index - A lesson plan that deals with heat index
• NOAA heat index
• Heat Index Calculator, National Weather Service El Paso, NOAA
• Weather Images heat index chart
• Windows program for Heat Index, etc
• How dew point, heat index, Real Feel Temperature differ
• Hourly History from Dhahran, Saudi Arabia - Shows hourly observations from Dhahran, Saudi Arabia from July 8, 2003

v • d • e Meteorological data and variables
Atmospheric pressure · Baroclinity · Cloud · Convection · CAPE · CIN · Dew point · Heat index · Humidex · Humidity · Lifted index · Lightning · Pot T · Precipitation · Sea surface temperature · Surface solar radiation · Surface weather analysis · Temperature · Theta-e · Visibility · Vorticity · Wind chill · Water vapor · Wind