Heat Index

The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, as an attempt to determine the human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The heat index was developed in 1978 by George Winterling as the "humiture" and was adopted by the USA's National Weather Service a year later. It is derived from work carried out by Robert G. Steadman. Like the wind chill index, the heat index contains assumptions about the human body mass and height, clothing, amount of physical activity, thickness of blood, sunlight and ultraviolet radiation exposure, and the wind speed. Significant deviations from these will result in heat index values which do not accurately reflect the perceived temperature.

The heat index is defined so as to equal the actual air temperature when the partial pressure of water vapor is equal to a baseline value of 1.6 kilopascals [kPa] (0.23 psi). At standard atmospheric pressure (101.325 kPa), this baseline corresponds to a dew point of 14 °C (57 °F) and a mixing ratio of 0.01 (10 g of water vapor per kilogram of dry air). This corresponds to an air temperature of 25 °C (77 °F) and relative humidity of 50% in the sea-level psychrometric chart.

27-32°C 80-91°F Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
32-41°C 90-105°F Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
41-54°C 105-130°F Danger: heat cramps and heat exhaustion are likely, heat stroke is probable with continued activity.
Over 54°C Over 130°F Extreme danger: heat stroke is imminent

Summer Simmer Index

Summer Simmer Index provides a proven indicator of heat stress concerns and discomfort using meaningful equivalent temperature values for general public acceptance and awareness. Based on sound, scientific principles, it is confirmed by independent physiological models and backed up by hundreds of tests on human individual subjects. It is derived from studies by the American Society of Heating and Refrigeration Engineers (ASHRAE) and confirmed by tests and analyses done at Kansas State University. Unlike the index derived from those studies, however, it meets all subjective and objective requirements while relating to a dry environment. As such, the new SSI is the only temperature-humidity index that uses the results of proven physiological models and human tests and can be related to a dry environment for acceptance by the general public. By doing so the index, like the wind chill factor used during the winter, provides a meaningful and realistic temperature equivalent that can be used not only as an indication as to how hot it feels, but also as a readily identifiable warning for individuals subject to the physiological dangers of heat exposure.

Zone 0
< 70
Cool Cool conditions
Zone 1
70 <= SSI < 77
Slightly cool Most people feel quite comfortable, even though it is slightly cooler
Zone 2
77 <= SSI < 83
Comfortable Very comfortable for majority of people
Zone 3
83 <= SSI < 91
Slightly warm Most people feel comfortable, but slightly warm
Zone 4
91 <= SSI < 100
Warm Increased discomfort
Zone 5
100 <= SSI < 112
Very warm Risk of sunstroke and heat exhaustion, significant discomfort
Zone 6
112 <= SSI < 125
Extremely warm Severe discomfort, danger of heatstroke, heat exhaustion and dehydration
Zone 7
125 <= SSI < 150
Dangerously warm Extreme danger of heatstroke and dehydration
Zone 8
> 150
Life threatening Imminent danger of circulatory collapse after prolonged exposure

Scharlau Winter Index

Valid for: temperatures -5 to +6°C (23 to 43°F) and humidity >= 40%

The Scharlau Winter Index has been established experimentally by K.Scharlau to asses cold exposure and measure the intensity of the human overcooling stress. The index is only valid if temperature is between -5 and +6°C (23 to 43°F) and humidity is above or equal to 40%. It reflects the feeling of discomfort due to radiative and evaporation heat losses from the exposed skin.

SWI >= 0 Comfortable
-1 < SWI < 0 Slightly uncomfortable
-3 < SWI <= -1 Moderatly uncomfortable
SWI <= -3 Highly uncomfortable

Scharlau Summer Index

Valid for: temperatures 17 to 39°C (63 to 102°F) and humidity >= 30%

The Scharlau Summer Index indicates the degree of thermal comfort or discomfort during the summer and higher temperatures. It is only defined in the temperature range between 17 and 39°C (63 to 102°F) and relative humidity above 30% (in the absence of wind).

SWI >= 0 Comfortable
-1 < SWI < 0 Slightly uncomfortable
-3 < SWI <= -1 Moderatly uncomfortable
SWI <= -3 Highly uncomfortable


The humidex (humidity index) is an index number used by the Canadian meteorologists to describe how hot the weather feels to an average person as a result of the combined effect of heat and humidity. It is a dimensionless number calculated from the dew point.

The formula used to calculate it was developed by J.M.Masterton and F.A.Richardson of Canada's Atmospheric Environment Service in 1979. Unlike the heat index used in the United States, it is derived from dew point, rather than relative humidity.

Humidex above 40 is considered very high and in such conditions all unnecessary exposure and outdoor activity should be limited. If the reading is in the mid to high 30s, certain types of outdoor exercise should be reduced or modified, depending on the age and health of the individual, physical shape, type of clothes etc.

HX < 29 Comfortable
30 <= HX < 34 Noticeable discomfort
35 <= HX < 39 Evident discomfort
40 <= HX < 45 Great discomfort, avoid exertion
45 <= HX < 54 Dangerous discomfort
HX > 54 Heat stroke possible

Relative Strain Index

Valid for: temperatures 26 to 35°C (79 to 95°F)

The relative strain index (RSI) was developed by Lee and Henschel. It is also a measure of discomfort resulting from the combined effect of temperature and humidity. It assumes a person dressed in a light business suit, walking at a moderate pace in a very light air motion. It is applicable to assess heat stress of manual workers under shelter at various metabolic rates.

RSI < 0.15 Comfortable
0.15 < RSI <= 0.25 Slight discomfort
0.25 < RSI <= 0.35 Discomfort
0.35 <= RSI < 0.45 Significant discomfort
RSI < 0.45 Extreme discomfort

Thom's Discomfort Index (TDI)

Thom's Discomfort Index (TDI) indicates the level of discomfort as a result of high temperature and the combined effect with relative humidity. It was developed by E.C.Thom a researcher working at the U.S. Weather Bureau.

TDI < 21 No discomfort
21 < TDI <= 24 Less than half of the population feels discomfort
24 < TDI <= 27 More than half of the population feels discomfort
27 < TDI <= 29 Most individuals feel discomfort and deterioration of psychophysical conditions
29 < TDI <= 32 Everyone feels significant discomfort
32 < TDI Dangerous, very strong discomfort which may cause heat strokes