Heat index

The heat index (HI ) is a given in units of the temperature variable which describes the perceived temperature based on the measured air temperature and relative humidity. Background is their joint effect on the human organism and thereby certain sensitivity to heat, so the heat index is an expression of how these factors influence their combination on the actual temperature and thus well-being of a person. This influence effect on the impairment of thermoregulation, in particular of perspiration and has a major impact on an individual's quality of life. High humidity hinders the ability of the transpiration through the skin and is therefore perceived in combination with a high temperature as heat and humidity. This loads the circuit much more than a dry heat, so for example deserts with temperatures can be tolerated by the organism beyond 40 ° C much easier than rain forests at a much higher humidity, but only 30 ° C.

History

The development of heat index goes back to the works of Robert G. Steadman in 1979. The highest ever recorded heat index was determined on 8 July 2003 in Dhahran, Saudi Arabia. He was 78 degrees Celsius, which on the assumption that on earth the highest possible heat index of 71 degrees would be refuted.

Calculation

The lower formula for the heat index in degrees Celsius or degrees Fahrenheit has an accuracy of ± 0.7 ° C or ± 1.3 ° F. It is applicable at temperatures of at least 26.7 ° C or 80 ° F and a relative humidity of at least 40%. When variables are to be used, the relative humidity in% for T is the temperature in ° C or ° F and φ.

Properties

At high temperatures, the heat required to increase the index increase of the relative humidity is less than at low temperatures. To be seen at around 27 ° C equivalent heat index when the humidity is below 45 %. At 43 ° C, however, already enough a humidity of about 17 % in order to push up the heat index above this temperature. At temperatures below 20 ° C, no influence of humidity on the heat index shows more. At temperatures near or below 0 ° C is mostly used Windchill to replace the heat index here.

With increasing humidity to precipitate and a increasing clouds, making the direct sunlight is reduced and the temperatures drop developed. A drop in temperature is due to the lowering of the saturation amount of water vapor in turn is equivalent to an increase of the relative humidity ( and vice versa). Both are therefore a negative feedback linked together, which is why one can roughly estimate that a temperature of 50 ° C does not coincide with a relative humidity of 90 % in the atmosphere can be realized. The world therefore hereby maximum heat index is around 70 ° C.

Effect on the organism

In full sunshine the actual heat index can be up to 8 ° C above the calculated value.