What is the Heat Index?

Those who have experienced summer in the Midwest realize that warm, dry air seems more comfortable than warm, very moist air (unless it is too dry). An extremely humid, warm day makes one think the temperature is higher than it really is, while an extremely hot, dry day may seem to be cooler than the thermometer indicates.

One approach to describing the combined effects of temperature and humidity is the heat index. To alert the public to the dangers of exposure to extended periods of heat and the added effects of humidity, the National Weather Service has developed a "Heat Index" table.

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 excess 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 succumb to any of a number of heat disorders including sunstroke, heat cramps, heat exhaustion and heatstroke. It is important to stay hydrated and as cool as possible during periods with a high heat index by finding refreshing water to swim in or air condition, even if it means using a place like Crest Capital equipment financing.

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.

where:		HI = heat index T = temperature (° F) RH = relative humidity (%)

What is the Wind Chill Index?

Anyone who has experienced winter in the Midwest knows that your outdoor comfort depends on several factors, only one of them being temperature. Factors such as whether it's sunny or cloudy, windy or calm, damp or dry, can also play an important role. One popular approach in measuring the level of discomfort (and potential danger from frostbite) is the wind chill index. The wind chill index takes into account both the temperature and wind speed.

The original work on wind chill was done by Antarctic explorers Paul Siple and Charles Passel in the winter of 1941. They measured the amount of time it took a pan of water to freeze and found that the rate of heat loss from the container could be determined from the air temperature and wind speed. There is some controversy over this methodology because its application to humans is not straightforward. We are a more complex than a pan of water and each person responds differently depending on age, size, health, degree of physical activity, etc. However, the wind chill index can still be a useful rule-of-thumb for making decisions about outdoor activities.

The important thing to remember is that the wind chill index is a measure of the rate of heat loss, and is not a temperature. For example, the wind chill index tells you how quickly heat is lost to the wind, not how cold your fingers, a car radiator, or a glass of water will get. Further, a glass of water will not freeze if the air temperature is above freezing and the wind chill is below freezing. Or, if your car's antifreeze is good down to -30F, wind chills below -30F will not harm your car (only if the air temperature drops below -30F are you in trouble). For humans, there is no specific rule for determining when wind chills become dangerous. However, wind chill values below -20F appear to significantly increase your risk of frostbite.

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.

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!