Heat Index

With basic climate regions around the world, there are some places where the weather is considered hot. But just heat alone does not make high temperatures a threat. There is an old saying stating "It's not the heat, it's the humidity". Well, actually it's both.

Heat waves are not easily photographed, like the destruction of tornadoes, hurricanes and floods and therefore tend to not have the same visual impact as these other disasters. Yet, heat waves kill more people in the United States than all of the other weather related disasters combined. The 10-year average (1997-2006) for heat related deaths in the U.S. is 170 in a typical year.

Heat waves form when high pressure aloft (from 10,000 - 25,000 feet / 3,000 - 7,600 meters) strengthens and remains over a region for several days up to several weeks. This is common in summer (in both Northern and Southern Hemispheres) as the jet stream 'follows the sun'. On the equator side of the jet stream, in the middle layers of the atmosphere, is the high pressure area.

High pressure in the middle layers of the atmosphere acts as a dome or cap allowing heat to build up at the earth's surface.

Summertime weather patterns are generally slower to change than in winter. As a result, this mid-level high pressure also moves slowly. Under high pressure, the air subsides (sinks) toward the surface. This sinking air acts as a dome capping the atmosphere.

This cap helps to trap heat instead of allowing it to lift. Without the lift there is little or no convection and therefore little or no convective clouds (cumulus clouds) with minimal chances for rain. The end result is a continual build-up of heat at the surface that we experience as a heat wave.

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 dilate 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 95°F (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 32°F (0°C) can hold about 0.16 ounce of water. Air with a temperature of 80°F (27°C) can hold about an ounce 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
HIGH smaller lower less

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 15°F (8°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 Chart is below.

NWS Heat Index Chart

How to read the chart...Follow the temperature line until it intersects the relative humidity line. Then read the Heat Index on the curved line. For example, an air temperature of 100°F (38°C) and Relative Humidity of 40%. Follow the 100°F (38°C) temperature line until it intersects the 40% relative humidity line. Then curved line that also intersects is the Heat Index of 110°F (43°C), or Very Hot.

That is the temperature the body thinks it is and attempts to compensate for that level of heat. Remember, these values are in the SHADE. You can add up to 15°F (8°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.

Category Classification Heat Index/Apparent Temperature General Affect on People in High Risk Groups
I Extremely Hot 130°F or Higher
(54°C or Higher)
Heat/Sunstroke HIGHLY LIKELY with continued exposure
II Very Hot 105°F - 130°F
(41°C - 54°C)
Sunstroke, heat cramps, or heat exhaustion LIKELY, and heat stroke POSSIBLE with prolonged exposure and/or physical activity
III Hot 90°F - 105°F
(32°C - 41°C)
Sunstroke, heat cramps, or heat exhaustion POSSIBLE with prolonged exposure and/or physical activity
IV Very Warm 80°F - 90°F
(27°C - 32°C)
Fatigue POSSIBLE with prolonged exposure and/or physical activity

Print heat index charts/graphs...
Temperature / Humidity Temperature / Dew Point

8.5" x 14" | 12" x 20"

8.5" x 11" | 16" x 20"

Wheel (pdf)

8.5" x 14" | 12" x 20"

Wheel (pdf)

Next: Wind Chill