Extremes of heat and cold environmental conditions can be very detrimental to the health and comfort of humans as our bodies attempt to maintain a state of homeothermy at a core temperature of about 37 oC (98.6 oF). When we are exposed to extremes of heat and cold, our bodies have natural reactions to the extreme conditions.
If the body cools to dangerously low core temperatures, we call the condition hypothermia. On the other side of the coin, we have the condition called hyperthermia, when the body heats to dangerously high internal temperatures. (A hint to remember the differences. Hypodermic needles get you under the skin ==> hypothermia is under safe body temperature. Hyperactive kids display an above normal level of active.)
Two of the main reactions to extreme temperatures that the body takes involve changing the metabolic rate and the perspiration rate. In cold conditions, the body “shuts down” loss of water through perspiration and this decreases the heat loss due to evaporative cooling of the skin. The body will also try to increase the metabolic heat input to warm the body through involuntary muscle activity such as shivering. In too warm conditions, the body reduces metabolism and then “opens up” the perspiration mechanisms so that the excess heat is lost through evaporative cooling.
For many years, scientists have studied how humans retain or lose heat under a wide variety of environmental conditions, not only for health, but also to maximize comfort levels. It followed from these studies that some form of comfort index should be devised for use by biometeorologists and weather forecast services for informing the public at large or specific segments of the population (outdoor workers, athletes, etc.) when conditions posed a hazard.
One of the two prime ways of losing heat is through thermal radiation from the body, but this is not directly affected by the weather (although the total radiation balance can be).We can see that under cold conditions, the other main loss of heat from the body is through convective heat loss, with air temperature and wind speed being the most important parameters. In contrast, under hot conditions, evaporative heat loss is the prime mechanism for cooling the body with convective loss secondary; here air temperature and moisture content of the air are the most important parameters.
The first attempts at developing a weather-related comfort index were to incorporate the chilling properties of the wind with the temperature. The result was the wind chill index. Later, indices relating the high air temperature and humidity were devised to attempt to quantify the discomfort of hot, humid conditions.
Recently, the work of Steadman and Kalkstein have attempted to include additional environmental factors and unify indices of heat and cold. Steadman’s work resulted in a complex apparent temperature concept where a series of equations define a model for heat gain/loss of the human body that included clothing factors as well as environmental ones. Kalkstein and his co-workers developed the weather stress index which incorporated Steadman’s apparent temperature but related its values to relative stress for a location through climate normals in order to account for acclimatization of local residents and their adapted responses to extreme events.
While absolute temperatures and other environmental factors are more important for injury or physical health determination, thermal comfort is more a reaction to the rate of heat loss or gain over a short time period. Thus, we can feel cold in summer and warm in winter under the appropriate combination of conditions and the past history of conditions. For example, after several days of temperatures exceeding 35 oC (95 oF) at high relative humidity, a cold frontal passage which drops the humidity and temperature to, say 25 oC (77 oF) will feel cool. But during the winter, temperatures of 25 oC in cold climates would feel extremely hot.
When we develop weather comfort indices for public use, we need to produce a general measure of comfort that is 1) easily interpreted and 2) easily calculated from basic weather measurements. As a result the various stress indices we hear in the media reports are usually based on calculations using the three basic measurements of wind speed, air temperature, and moisture content usually using relative humidity.
The difficulty with all these indices is that they try to tell us how we will feel under certain conditions, but there are wide variations in how each of us respond to our environment including our sex, age, racial characteristics, state of health and mind, and level of acclimatization. Reactions to the reports of these indices, according to a survey by Driscoll at Texas A&M University range from the very positive:
They help me determine how I will dress, eat and exercise
to the very negative:
They make me feel more uncomfortable, at times border on scare tactics.
And, of course, there are many misconceptions of what the indices mean because media people and even scientists do not fully understand the indices they are reporting. For example, I had thought for years that windchill temperature was the equivalent temperature for zero wind speed that the particular combination of ambient wind and air temperature produced. In fact, the windchill temperature calculated by the Siple equation refers to conditions with a wind speed of 5 mph (8 km/h).