|
Know the Heat Index
Dec 29, 2003
Understanding how to read the Heat
Index Chart, Find your local heat Index on the USA Heat
Index Map, How to calculate your Heat Index, What is heat
stress?
The heat index combines the effects of heat and humidity.
When heat and humidity combine to reduce the amount
of evaporation of sweat from the body, outdoor exercise
becomes dangerous even for those in good shape. Key
rules for coping with heat are to drink plenty of water
to avoid dehydration and to slow down and cool off when
feeling fatigued, a headache, a high pulse rate or shallow
breathing. Overheating can cause serious, even life-threatening
conditions such as heat stroke. The apparent temperature,
which combines the temperature and relative humidity,
is a guide to the danger.
Below is the heat stress index based on the apparent
temperature.
Apparent temperature
heat stress index
| Category |
Apparent temperature |
Dangers |
| Caution |
80-90°F |
Exercise more fatiguing than usual |
| Extreme caution |
90-105°F |
Heat cramps, exhaustion possible |
| Danger |
105-130°F |
Heat exhaustion likely |
| Extreme danger |
Greater than 130°F |
Heat stroke imminent |
Note: The heat index includes
certain assumptions, including-- that the body
is:
5' 7" tall, 147 pounds, in shade, , walking
at a mild pace of 3 mph, not dripping in sweat
If any of these factors change, e.g., more exertion,
and/or more weight, the heat index willchange
for that individual.
|
Effects of heat illnesses:
Heat cramps: Exercising
in hot weather can lead to muscle cramps, especially
in the legs, because of brief imbalances in body salts.
Cramps become less frequent as a person becomes used
to the heat. Heat syncope or fainting: Anyone not used
to exercising in the heat can experience a quick drop
in blood pressure that can lead to fainting. As with
heat cramps, the cure is to take it easy.
Heat exhaustion:
Losing fluid and salt through perspiration or replacing
them in an imbalanced way can lead to dizziness and
weakness. Body temperature might rise, but not above
102 degrees. In some cases victims, especially the elderly,
should be hospitalized. Heat exhaustion is more likely
after a few days of a heat wave than when one is just
beginning. The best defense is to take it easy and drink
plenty of water. Don't take salt tablets without consulting
a physician.
Heatstroke: In
some cases extreme heat can upset the body's thermostat,
causing body temperature to rise to 105 degrees or higher.
Symptoms are lethargy, confusion and unconsciousness.
Even a suspicion that someone might be suffering from
heatstroke requires immediate medical aid. Heatstroke
can kill.
As the heat index rises, so do health risks. When
the heat index is 90°-105°F, heat exhaustion
is possible. When it is above 105°F, it is probable.
Heatstroke is possible when the heat index is above
105°F, and very likely when it is 130°F and
above. Physical activity and prolonged exposure to the
heat increase the risks.
Formula to calculate Heat
Index
If you know the relative humidity and the dry air
temperature, then you can use the following equation
to calculate the heat index.
Heat index (HI), or apparent temperature (AI) = -42.379
+ 2.04901523(Tf) + 10.14333127(RH) - 0.22475541(Tf)(RH)
- ((6.83783 x 10-3)(Tf2) - ((5.481717
x 10-2)(RH2) + ((1.22874 x 10-3)(Tf2)(RH))
+ ((8.5282 x 10-4)(Tf)(RH2)) -
((1.99 x 10-6)(Tf2)(RH2))
Note: In order for the Heat Index formula to
work correctly, you must use the relative humidity in
percentform. In other words, if the relative humidity
is 65%, use 65 for RH in the formula, not .65.
Summer Simmer Index
If you know the relative humidity and the dry air
temperature, then you can use the following equation
to calculate the summer simmer index.
Summer simmer index (SSI) = 1.98(Tf - (0.55 - 0.0055(RH))(Tf-58))
- 56.83
Tf= air temperature in degrees Fahrenheit, RH= relative
humidity expressed as a whole number
|
|
