Heat index

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The heat index ( HI ) is a quantity given in units of temperature to describe the perceived temperature based on the measured air temperature and, above all, the relative humidity . It goes back to the work of the American Robert G. Steadman from 1979. The Humidex (humidity index) , which was developed in Canada and is more common, is calculated in a similar but simpler manner.

A high level of air humidity has a negative effect on the organism of humans and all mammals , especially at high temperatures, i.e. when it is humid . It hinders the body's thermoregulation through perspiration from the skin, thus impairing its subjective well-being and objectively increasing the risk of heat damage . As a result, a stay in the desert is usually easier for people to cope with than in the tropical rainforest with its lower temperature, but much higher humidity.

The heat index and humidex are intended to express this relative load as a measure. They become important at air temperatures above about 20 to 25 ° C. For air temperatures below about 10 ° C, the wind speed is more important for the perceived temperature than the air humidity, which is expressed in the degree of wind chill .

The highest heat index recorded to date was recorded on July 8, 2003 in Dhahran , Saudi Arabia , and was 78 ° C, refuting the original assumption that the highest possible heat index on earth was 71 ° C.

calculation

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

Parameter description
parameter ϑ in ° C ϑ in ° F
c 1 −8.784695 −42.379
c 2 1.61139411 2.04901523
c 3 2,338549 10.1433127
c 4 −0.14611605 −0.22475541
c 5 −1.2308094 · 10 −2 −6.83783 · 10 −3
c 6 −1.6424828 · 10 −2 −5.481717 · 10 −2
c 7 2.211732 · 10 -3 1.22874 · 10 −3
c 8 7.2546 · 10 −4 8.5282 · 10 −4
c 9 −3.582 · 10 −6 −1.99 · 10 −6

properties

At high temperatures, the increase in relative humidity required to increase the heat index is less than at low temperatures. An equivalent heat index is found at around 27  ° C if the humidity is below 45%. At 43 ° C, however, an air humidity of over 17% is sufficient to let the heat index rise above this temperature. At temperatures below 20 ° C there is no longer any influence of air humidity on the heat index. From temperatures close to or below 0 ° C, wind chill is usually used to replace the heat index here.

As the air humidity rises, precipitation and increasing cloud cover develop , which reduces direct sunlight and lowers the temperature. The saturation amount of the water vapor decreases with the temperature , so that the relative humidity increases (and vice versa). Air humidity and temperature are thus linked to one another via a negative feedback, which is why it was initially roughly estimated that a temperature of 50 ° C together with a humidity of 90% in the atmosphere is not feasible and that the world's highest possible heat index is around 70 ° C. This assumption was later refuted in reality. (See introduction of the article above.)

Sun and shade are not taken into account: In full sunshine, the actual heat perception can still be far above the calculated value.

Warning table

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US Heat Index (Metric Version)
  Temperature (° C)
27 28 29 30th 31 32 33 34 35 36 37 38 39 40 41 42 43
Relative humidity (%)
40 27 28 29 30th 31 32 34 35 37 39 41 43 46 48 51 54 57
45 27 28 29 30th 32 33 35 37 39 41 43 46 49 51 54 57
50 27 28 30th 31 33 34 36 38 41 43 46 49 52 55 58
55 28 29 30th 32 34 36 38 40 43 46 48 52 55 59
60 28 29 31 33 35 37 40 42 45 48 51 55 59
65 28 30th 32 34 36 39 41 44 48 51 55 59
70 29 31 33 35 38 40 43 47 50 54 58
75 29 31 34 36 39 42 46 49 53 58
80 30th 32 35 38 41 44 48 52 57
85 30th 33 36 39 43 47 51 55
90 31 34 37 41 45 49 54
95 31 35 38 42 47 51 57
100 32 36 40 44 49 54

Caution Increased caution Danger Increased risk  

The simplified warning level table is:

Heat index Hints
(27 ... 32) ° C Caution - Long periods of time and physical activity can lead to symptoms of exhaustion.
(32 ... 40) ° C Increased Caution - There is a possibility of heat damage such as sunstroke, heat convulsion, and heat collapse.
(40 ... 54) ° C Danger - sunstroke, heat convulsion, and heat collapse are likely; Heat stroke is possible.
above 54 ° C Increased risk - heat stroke and sunstroke are likely.

literature

  • J. Masterson, FA Richardson: Humidex, A Method of Quantifying Human Discomfort Due to Excessive Heat and Humidity. Environment Canada, Downsview, Ontario 1979.
  • LP Rothfusz: The heat index equation. NWS Southern Region Technical Attachment, SR / SSD 90-23, Fort Worth / Texas 1990.
  • RG Steadman, 1979: The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. In: J. Appl. Meteor. 18, pp. 861–873 ( abstract with link to PDF, ametsoc.org)

Web links

Individual evidence

  1. ↑ The Assessment of sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science, RG Steadman, Journal of Applied Meteorology, July 1979, Vol 18 No7, pp861-873 doi : 10.1175 / 1520-0450 (1979) 018 <0861: TAOSPI> 2.0.CO; 2