Heat WaveA National Problem, Heat kills by taxing the human body beyond its abilities. In a normal year, about 175 Americans succumb to the demands of summer heat. Among the large continental family of natural hazards, only the cold of winter not lightning, hurricanes, tornadoes, floods or earthquakes takes a greater toll. In the 40-year period from 1936 through 1975, nearly 20,000 people were killed in the United States by the effects of heat and solar radiation. In the disasterous heat wave of 1980, more than 1,250 people died.
And these are the direct casualties. No one can know how many more deaths are advanced by heat wave weather - how many diseased or aging hearts surrender that under better conditions would have continued functioning.
North American summers are hot; most summers see heat waves in one section or another of the United States. East of the Rockies, they tend to combine both high temperature and high humidity although some of the worst have been catastrophically dry.
NOAA'S National Weather Service Heat Index Program
Considering this tragic death toll, the National Weather Service (NWS) has stepped up its efforts to alert more effectively the general public and appropriate authorities to the hazards of heat waves - those prolonged excessive heat/humidity episodes. Based on the latest research findings, the NWS has devised the "Heat Index" (HI), (sometimes referred to as the "apparent temperature"). The HI, given in degrees F, is an accurate measure of how hot it really feels when relative humidity (RH) is added to the actual air temperature. To find the HI, look at the Heat Index Chart. As an example, if the air temperature is 95°F (found on the left side of the table) and the RH is 55% (found at the top of the table), the HI - or how hot it really feels - is 110°F. This is at the intersection of the 95° row and the 55% column.
IMPORTANT: Since HI values were devised for shady, light wind conditions, EXPOSURE TO FULL SUNSHINE CAN INCREASE HI VALUES BY UP TO 15°F. Also, STRONG WINDS, PARTICULARLY WITH VERY HOT, DRY AIR, CAN BE EXTREMELY HAZARDOUS.
Note on the HI chart the shaded zone above 105°F. This corresponds to a level of HI that may cause increasingly severe heat disorders with continued exposure and/or physical activity.
The "Heat Index vs. Heat Disorder" table (next to the HI chart) relates ranges of HI with specific disorders, particularly for people in higher risk groups.
Possible Heat Disorders for
People in Higher Risk Groups
|130° or Higher||Heatstroke/Sunstroke Highly Likely with Continued Exposure.|
|105° to 130°||Sunstroke, Heat Cramps or Heat Exhaustion Likely, and Heatstroke possible with Prolonged Exposure and/or Physical Activity.|
|90° to 105°||Sunstroke, Heat Cramps or Heat Exhaustion Likely, and Heatstroke possible with Prolonged Exposure and/or Physical Activity.|
|80° to 90°||Fatigue Possible With Prolonged Exposure and/or Physical Activity.|
The NWS will initiate alert procedures when the HI is expected to exceed 105°-110°F (depending on local climate) for at least two consecutive days. The procedures are:
- Include HI values in zone and city forecasts.
- Issue Special Weather Statements and/or Public Information Statements presenting a detailed discussion of (1) the extent of the hazard including HI values, (2) who is most at risk, (3) safety rules for reducing the risk.
- Assist state/local health officials in preparing Civil Emergency Messages in severe heat waves. Meteorological information from Special Weather Statements will be included as well as more detailed medical information, advice, and names and telephone numbers of health officials.
- Release to the media and over NOAA's own Weather Radio all of the above information.
How Heat Affects the Body
Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and - as the last extremity is reached - by panting, when blood is heated above 98.6 degrees. The heart begins to pump more blood, blood vessels dilate to accommodate the increased flow, and the bundles of tiny capillaries are threading through the upper layers of skin are put into operation. The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler atmosphere. At the same time, water diffuses through the skin as perspiration. The skin handles about 90 percent of the body's heat dissipating function.
Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation and high relative humidity retards evaporation. The evaporation process itself works this way: the heat energy required to evaporate the sweat is extracted from the body, thereby cooling it. Under conditions of high temperature (above 90 degrees) and high relative humidity, the body is doing everything it can to maintain 98.6 degrees inside. The heart is pumping a torrent of blood through dilated circulatory vessels; the sweat glands are pouring liquid including essential dissolved chemicals, like sodium and chloride onto the surface of the skin.
Too Much Heat
Heat disorders generally have to do with a reduction or collapse of the body's ability to shed heat by circulatory changes and sweating, or a chemical (salt) imbalance caused by too much sweating. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body's inner core begins to rise, and heat-related illness may develop.
Ranging in severity, heat disorders share one common feature: the individual has overexposed or overexercised for his age and physical condition in the existing thermal environment. Sunburn, with its ultraviolet radiation burns, can significantly retard the skin's ability to shed excess heat. Studies indicate that, other things being equal, the severity of heat disorders tend to increase with age heat cramps in a 17-year old may be heat exhaustion in someone 40, and heat stroke in a person over 60. Acclimatization has to do with adjusting sweat-salt concentrations, among other things. The idea is to lose enough water to regulate body temperature, with the least possible chemical disturbance.
Reprinted with permission of NOAA - National Weather Service.Heat Index/Heat Disorders Heat
POSSIBLE HEAT DISORDERS FOR PEOPLE IN HIGH RISK GROUPS
130° or HEATSTROKE/SUNSTROKE HIGHLY Higher LIKELY WITH CONTINUED EXPOSUREHEATSTROKE POSSIBLE WITH PROLONGED EXPOSURE AND/OR PHYSICAL ACTIVITY
105° to SUNSTROKE, HEAT CRAMPS OR 130° HEAT EXHAUSTION LIKELY, AND
90° to SUNSTROKE, HEAT CRAMPS ANDPHYSICAL ACTIVITY
105° HEAT EXHAUSTION POSSIBLE WITH PROLONGED EXPOSURE AND/OR physical activity.
80° to FATIGUE POSSIBLE WITH
90° PROLONGED EXPOSURE AND/OR PHYSICAL ACTIVITY