The heat index, also known as humidex, is a measurement that seeks to show how hot a day is, and it is derived from a combination of the air temperature and the relative humidity, along with a few other variables. In short, it is an attempt to determine what temperature a human being feels.
When a person is hot, the body cools itself by sweating, and the excess body heat is removed by the evaporation of the sweat. However, the rate at which the sweat which a person produces is evaporated is dependent upon the amount of water vapor that is in the surrounding air, i.e., the relative humidity. The higher the relative humidity is, the slower the process of sweat evaporation is, so that the process by which the body cools itself is inversely dependent upon the concentration of water vapor in the air. Thus, notwithstanding that the air temperature may be low, where there is a high concentration of water vapor in the air a person may yet feel overheated because the heat stored in the body is been removed at a slower rate than where the concentration of water vapor in the air is less. The effect is, of course, a subjective one and its measurement is likewise subjective, based, as it is, upon the subjective descriptions of how hot subjects feel at a given temperature/humidity combination.
The index was developed by George Winterling, the retired television weatherman, in 1978 based on the work that had earlier been done by Robert G. Steadman of La Trobe University, Bundoora, Victoria, Australia and it was adopted for use in 1979 by America's National Weather Service. In arriving at the index, the Weather Service employs heat balance equations that include many other variables than just the amount of water vapor in the air.
As defined, the heat index is equal to the actual air temperature when the partial pressure of water vapor in the air has a baseline value of 1.6 kilopascals (kPa). At standard atmospheric pressure (101.325 kPa), this baseline is equivalent to a dew point (i.e., the temperature below which the water vapor in a volume of humid air at a given constant barometric pressure will condense into liquid water) of 14º Celsius (57º Fahrenheit) and a ratio of 0.01, i.e., 10 grams of water vapor per kilogram of dry air. As temperature increases, the amount of water vapor in the air that is required to raise the index to a level higher than the actual temperature falls. Thus where the actual temperature is 27º Celsius (80º Fahrenheit), the heat index and actual temperature will coincide when relative humidity is 45 percent; however, if the actual temperature rises to about 43º Celsius (110º Fahrenheit), any humidity reading that exceeds 17 percent will raise the index to above the actual temperature.
The figures derived from the formula used to calculate the index are considered to be valid only where certain criteria are met, to wit: The actual temperature is above 27º Celsius (80º Fahrenheit); the dew point temperature is greater than 12º Celsius (54º Fahrenheit); and relative humidity is greater than 40%. in utilizing the temperature figures derived from the index, it should be kept in mind that they are based on shade temperatures and not those taken under the open sun and also that wind effects are not factored, which can reduce apparent temperatures. Exposure to full sunlight can increase heat index values by up to 1/3º Celsius (15º Fahrenheit).
Because there is an inverse relationship between the maximum potential temperature and the maximum potential relative humidity (in open conditions, haze and then thicker cloud cover develops as relative humidity increases, thereby reducing the amount of direct sunlight that reaches the Earth's surface), it used to be thought that the actual heat index reading possible anywhere on Earth was about 71º Celsius (160º Fahrenheit). It is now known that this is not so. On 8 July 2003, in Dhahran, Saudi Arabia, the dew point was 35º Celsius (95º Fahrenheit), while actual temperature was 42º Celsius (108º Fahrenheit), giving a heat index reading of 78º Celsius (172º Fahrenheit), a temperature that is comparable to that which is recommended for killing bacteria in many meat products!
As noted above, the body's ability to cool itself is directly related to the hotness at any given time. When the body is unable to cool itself properly or when too much fluid or salt is lost through excessive sweating, body temperature rises and this may lead to heat-related illnesses such as heat cramps, heat exhaustion or even heat stroke, a serious condition that requires immediate medical attention. Additionally, a high index value may be an indication of approaching intense thunderstorms. The Environmental Protection Agency of the United States, in conjunction with some other American government agencies, has developed a guidebook which provides best practices in case of heat waves.
The heat index should not be confused with the Canadian humidex which, though similar, differs on a number of points, to wit: The Canadian humidex uses a different dew point value, 7º Celsius (45º Fahrenheit), rather than the 14º Celsius (57º Fahrenheit) used for the heat index and, also, the Canadian humidex does not use the many other variables that are used in the measurement of the heat index, relying exclusively on water vapor pressure. A joint committee set up to resolve the differences between the Canadian and American system did not achieve anything and has since been disbanded.