Heat Index Chart

In an average year only the winter's cold -- not lightning, hurricanes, tornadoes, floods, or earthquakes -- takes a greater weather-related death toll than the summer's heat and humidity. In an effort to alert you to the hazards of prolonged heat/humidity episodes, the National Weather Service devised the "heat index." The heat index (HI) is an accurate measure of how hot it really feels when the affects of humidity are added to high temperature.

The human body contains several mechanisms to maintain its internal operating temperature at 98.6° F. When threatened with above "normal" temperatures, the body will try to dissipate excesss heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and, as a last resort, by panting. When weather conditions force the air temperature above 90° F and the relative humidity is high, the body is doing everything it can to maintain its normal temperature. Unfortunately, conditions can exceed the body's ability to cope with the combined affects of heat and humidity. At such times the body may sucumb to any of a number of heat disorders including sunstroke, heat cramps, heat exhaustion and heatstroke.

To use the heat index charts, find the appropriate temperature at the top of the chart. Read down until you are opposite the humidity/dewpoint. The number which appears at the intersection of the temperature and humidity/dewpoint is the heat index.

Heat Index Chart (Temperature & Dewpoint)
(° F)
Temperature (° F)
65 94 95 96 97 98100101102103104106107108109110112
66 94 95 97 98 99100101103104105106108109110111112
67 95 96 97 98100101102103105106107108110111112113
68 95 97 98 99100102103104105107108109110112113114
69 96 97 99100101103104105106108109110111113114115
70 97 98 99101102103105106107109110111112114115116
71 98 99100102103104106107108109111112113115116117
72 98100101103104105107108109111112113114116117118
73 99101102103105106108109110112113114116117118119
Note: Exposure to full sunshine can increase HI values by up to 15° F

Heat Index Chart (Temperature & Relative Humidity)
Temperature (° F)
55 98100103105107110113115118121124127131134137141
50 96 98100102104107109112114117119122125128131135
45 94 96 98100102104106108110113115118120123126129
40 92 94 96 97 99101103105107109111113116118121123
35 91 92 94 95 97 98100102104106107109112114116118
30 89 90 92 93 95 96 98 99101102104106108110112114
Note: Exposure to full sunshine can increase HI values by up to 15° F

For the more ambitious who would like to calculate the heat index, here is the formula:

[Heat Index formula]
where:HI = heat index
T = temperature (° F)
RH = relative humidity (%)

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Wind Chill Chart

How cold is it outside? Simply knowing the temperature doesn't tell you enough about the conditions to enable you to dress sensibly for all winter weather. Other factors including wind speed, relative humidity and sunshine play important roles in determining how cold you feel outside. A description of the character of weather known as "coldness" was proposed about 1940 by scientists working in the Antarctic. The "wind chill index" as developed to describe the relative discomfort/danger resulting from the combination of wind and temperature.

The wind chill index describes an equivalent temperature at which the heat loss from exposed flesh would be the same if the wind were near calm. For example, a wind chill index of -5 indicates that the affects of wind and temperature on exposed flesh are the same as if the air temperature were 5 degrees below zero eventhough the actual temperature is much higher.

The importance of the wind chill index is as an indicator of how to dress properly for winter weather. (Wind chill does not affect your car's antifreeze protection, freezing of water pipes, etc.) In dressing for cold weather an important factor to remember is that entrapped insulating air warmed by body heat is the best protection against the cold. Consequently, wear loose-fitting, lightweight, warm clothing in several layers. Outer garments should be tightly-woven, water-repellant and hooded. Mittens snug at the wrist are better protection than fingered gloves.

To use the chart, find the approximate temperature on the top of the chart. Read down until you are opposite the appropriate wind speed. The number which appears at the intersection of the temperature and wind speed is the wind chill index.

Wind Chill Chart
Temperature (° F)
Wind speeds above 40 mph have little additional chilling affect

In using the table above, values of wind chill below -10° F are considered bitterly cold. Values of wind chill below -20° F are extremely cold -- human flesh will begin to freeze within one minute!

If you would like to calculate the wind chill index for combinations of temperature and wind other than those given in the table above, you can use the formula:

WC = 91.4 - (0.474677 - 0.020425 * V + 0.303107 * SQRT(V)) * (91.4 - T)
where:WC = wind chill index
V = wind speed (mph)
T = temperature (° F)

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