When the surrounding temperature approaches the normal human body temperature of about 99 F our bodies must regulate that internal temperature and get rid of the excess heat our bodies generate. To do that, human beings sweat and this sweat then evaporates causing a cooling effect. Other animals, like dogs and cats do not sweat, but they pant expelling the excess heat in their breath. Most animals with fur or feathers, pant. Most animals with hair, sweat. Other animals like reptiles must go underground to cooler surroundings, or die.
The Heat Index is an attempt to quantify how well the average human can function in hot weather when the humidity is high. The Heat Index is only practical above about 70 F and, at very high temperatures and high relative humidity does not give realistic figures either. Ideally, the Heat Index supplies a temperature reading at which we would feel the same discomfort were we in a dry environment. The temperature that our body is maintained at is regulated by the combination of the total amount of internal heat we generate minus the amount of heat we get rid of by sweating. When the humidity is high, we begin to feel uncomfortable because we sweat more and feel lethargic. This lethargy is helpful because it prevents us from creating more internal heat. When the surrounding relative humidity is such that sweat no longer evaporates, our bodies can sometimes overheat and we can go into heat stroke. The controversy about this measurement is that it does not take into account things like wind, shade, sunlight, and exertion. If it is 90 F and 65% relative humidity on a sunny day we are likely to be less comfortable and more endangered than if we had 90 F and 65% relative humidity on a cloudy day. The heat load on our bodies caused by the sun is not considered in the heat index. Also, even a light breeze causes considerable benefit even though that is not considered in the heat index.
So, the Heat Index is only a partial answer to how hot it is outside. Of much more significance to avoiding heat stroke is not staying in the direct sunlight too long and do not get dehydrated.
That values of the Heat Index converter are generated by the following formula:
Heat index(HI), or apparent temperature(AI)= -42.379 + 2.04901523(Tf)
+ 10.14333127(RH) - 0.22475541(Tf)(RH) - 6.83783x10**(-3)*(Tf**(2))
- 5.481717x10**(-2)*(RH**(2)) + 1.22874x10**(-3)*(Tf**(2))*(RH)
+ 8.5282x10**(-4)*(Tf)*(RH**(2)) - 1.99x10**(-6)*(Tf**(2))*(RH**(2))
Tf = Temperature in degrees Farenheit
RH = % Relative Humidity
Note: In all the formulas here, / means to divide, * means
to multiply, ** means the following term is an exponent(i.e. 10**(4)
means 10 to the 4th power), - means to subtract, + means to add.
A number followed by a "x10" to some exponent is in
scientific notation to conserve space. The standard rules of algebra
apply to all the formulas.
Wind Chill Index
The Wind Chill Index is intended to represent the combined effective results of cold temperature and wind. The human body produces internal heat that acts to offset the cold environment and produces a blanket of warm air surrounding the body that shields it from the cold air. When this blanket of warm air is disturbed by wind or drafts, the skin will get colder. If the skin temperature is cold enough, the body temperature will begin to decline. If it declines too far, sickness or death may result. Before that happens, the extremities, like fingers, toes, ears, the nose, and lips may actually freeze and the cells rupture. When this happens, these parts are essentially dead. So even if a the body does not get cold enough to kill the person, severe damage can occur.
The effect of moisture on the body will make Wind Chill even worse. When the moisture evaporates the skin gets even colder quicker. When a person is bundled up against the cold and walks around, they will sweat. If they then get blasted by a cold wind, that sweat will evaporate. Moisture in the nose and on the lips makes those prime candidates for freezing.
You might notice that at 4 mph wind speed and less the wind chill temperature is actually higher than the air temperature. With the blanket of warm air around the body intact, you are warmer than the surrounding air. The Wind Chill Index does not really have much use above about 35 degrees F. The equation used to find the Wind Chill Index is the same as is used by the National Weather Bureau:
T(wc) = 0.0817(3.71V**0.5 + 5.81 -0.25V)(T - 91.4) + 91.4
T(wc) is the wind chill, V is in the wind speed in statute miles
per hour and T is the temperature in degrees Fahrenheit.
The formula to calculate a Celsius wind chill using V as the wind
speed in kilometers per hour and T in degrees Celsius is:
T(wc) = 0.045(5.27V**0.5 + 10.45 - 0.28V) (T - 33) + 33
There are some objections to the use of the Wind Chill Index as a simple guide to how cold is cold. It takes no account of whether the sun is shining or not. Bright sunlight greatly relieves the effects of cold and wind. Many people ski all day long in wind chill far below 0 F. And then there is the question of how the Index was created. It was made by observing the speed at which water freezes under different wind speeds. This may not be truly representative of how the cold and wind affect humans and animals. So while it is a guide to tell you when to take precautions, it should not be construed to be the last word in when you can go outside or not.