Heat Index

Image You have heard it said that “It’s not the heat, it’s the humidity” Well, actually it’s both. Our bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and, as the last extremity is reached, by panting. As the body heats up, the heart begins to pump more blood, blood vessels to accommodate the increased flow, and the tiny capillaries in the upper layers of skin are put into operation.


The body’s blood is circulated closer to the skin’s surface, and excess heat drains off into the cooler atmosphere by one or a combination of three ways…

  • radiation,
  • convection, and
  • evaporation.

At lower temperatures, radiation and convection are efficient methods of removing heat. However, once the air temperature reaches 35°C, heat loss by radiation and convection ceases. It is at this point that heat loss by sweating becomes all-important. But sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation (sweat changing to water vapor). The downside of this method of cooling is that high relative humidity retards evaporation.


Relative humidity is a measure of the amount of water vapor contained in the air, divided by the maximum amount the air can hold, expressed as a percent. A relative humidity of 50% means the air contains ½ of the water vapor it can actually hold. The maximum amount of water vapor the air can hold is dependent upon the temperature (the “relative” in relative humidity). The higher the temperature, the more water (actually water vapor) the air can hold. For example, air with a temperature of 0°C can hold about 4.5ml of water. Air with a temperature of 26°C can hold about an 30ml of water.


So, what does this all mean? Sweat is evaporated (changes from a liquid to a gas, i.e. water vapor) when heat is added. The heat is supplied by your body. The results are summed up in the table below…




Capacity for air

to hold water

Amount of


HEAT removed

from the body










We, at the National Weather Service, as part of our mission for protecting life and property, have a measure of how the hot weather “feels” to the body. The Heat Index is based on work by R.G. Steadman and published in 1979 under the title “The Assessment of Sultriness, Parts 1 and 2.” In this work, Steadman constructed a table which uses relative humidity and dry bulb temperature to produce the “apparent temperature” or the temperature the body “feels”.


We use this table to provide you with Heat Index values. These values are for shady locations only. Exposure to full sunshine can increase heat index values by up to 10°C. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous as the wind adds heat to the body. The Heat Index Table is below.


      Relative Humidity %    



Remember, these values are in the SHADE. You can add up to 10°C to these values if you are in direct sunlight.


The chart below tells you the risk to the body from continued exposure to the excessive heat.




Heat Index/Apparent Temp (°C)

General Affect on People in High Risk Groups


Extremely Hot

54°C or Higher

Heat/Sunstroke HIGHLY LIKELY with continued exposure


Very Hot

40°C – 54°C

Sunstroke, heat cramps, or heat exhaustion LIKELY, and heatstroke POSSIBLE with prolonged exposure and/or physical activity



32°C – 40°C

Sunstroke, heat cramps, or heat exhaustion POSSIBLE with prolonged exposure and/or physical activity


Very Warm

26°C – 32°C

Fatigue POSSIBLE with prolonged exposure and/or physical activity