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Heat Index

Summer Weather Safety and Survival

The Heat Index

Your local weather experts at the National Weather Service, as part of our mission to protect life and property, have developed 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".

Here’s a more detailed explanation of the Heat Index.

We use a table like this 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. 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 and Relative Humidity of 40%. Follow the 100°F temperature line until it intersects the 40% relative humidity line. Then curved line that also intersects is the Heat Index of 110°F, 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 to these values if you are in direct sunlight.

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

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

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