The atmosphere and the Weather

Temperature, Humidity, and Human Comfort

Most Deadly of the Natural Disasters: The Heat Wave article published by the New York Times on August 13, 2002.

Most French Heat Wave of August 2003 article published by the Los Angeles Times on August 22, 2003.

Biological Control Systems

Living things, like mechanical engines, need regulators for effective operation. The regulators of living things are called biological control systems.

Among the many control systems are the ones that maintain a balance in the biophysical and biochemical functioning of the body, keep internal body temperature at a stable level in all kinds of weather.

Many homeostatic biological control systems are at work in the body. Homeostasis is the stable operation of physiological activities.

Nearly all energy from food and other sources leaves the body as heat. Thus, the body needs a well-functioning homeostatic control system for thermal regulation.

Enzyme-controlled biochemical reactions in the body are usually most efficient at around 98°F (37°C). This temperature is the average set point for the inner body temperature of mammals.

In thermoregulation, the "flow rate" of body heat is related to the "heights" of such "compartments" as the body core, its surrounding muscle layers, the skin-fat layers around them, and the blood.

This heat stress, or load, is the major disturbance of the thermoregulatory system. The body compensates for it in two ways:

In heat conservation, muscles shiver, vasoconstriction (the narrowing of blood vessels) occurs, and cell metabolism increases.

In heat dissipation, sweating and vasodilation (the widening of blood vessels) occur. Sweating is the only means by which humans can survive for long in a place where air temperature exceeds body temperature.

These reactions are programmed by the brain's hypothalamus from information fed back from its own thermoreceptors and from thermoreceptors in the skin. Thermoregulation has a high set point, about 98° F, an indication that it is easier for the body to heat itself than to cool itself.

Thermoregulation and other forms of homeostasis do not maintain a condition at an unvarying level, but within an acceptable range. For example, most people experience a significant daily temperature variation during which body temperature dips nearly two degrees Fahrenheit (one degree Celsius) from an early evening peak to an early morning low.

Heat exchange and body temperature --> body temperature is governed by the difference between the rates of heat gain and heat dissipation. The body has both internal and external sources of heat.

The following figure illustrates the external processes of energy exchange between the body and its surroundings.

Thermoregulation --> the body regulates heat flow (between the body and the environment) so that its internal (core) temperature varies by no more than about 2 degrees Celsius. The Control systems by which the brain directs the body's automatic responses to elevated or lowered core temperature are illustrated in this figure.

"It is not so much the heat, it's the humidity" -- somebody will claim in a hot, muggy day when the relative humidity is high. Is this a correct statement?

Yes, in warm weather the main source of body cooling is through evaporation of perspiration.

• high air temperature, low humidity -> perspiration on the skin evaporates quickly, often making us feel that the air temperature is lower than it really is.
• high air temperature and humidity -> body moisture does not readily evaporates -> less cooling

Hypothermia and Hyperthermia

Sometimes thermoregulatory processes fail to maintain the body's core temperature within its normal range. As the core temperature departs more and more from its optimal range, thermoregulatory processes may fail, resulting in rapidly deteriorating and potentially lethal conditions.

Hypothermia refers to conditions that develop when human core temperature drops below 35°C (95°F). Initially, shivering becomes more violent and uncontrollable. In addition, the victim has difficulty speaking and becomes apathetic and lethargic. If core temperature falls below 32°C (90°F), enzyme activity further slows. Eventually shivering ceases, muscles become rigid, and coordination deteriorates. Mental abilities also are impaired seriously and the victim is generally unable to help himself or herself. At a core temperature of 30°C (86°F), a person may drift into unconsciousness. Typically, a person becomes totally unresponsive, even to pain, at a core temperature under 26°C (79°F). Death may ensue at core temperatures below 24°C (75°F), because the heart rhythm becomes uncontrollably irregular (ventricular fibrillation) or uncontrollably halted (cardiac arrest). The lowest core temperature measured in adults who subsequently recovered from hypothermia is about 16°C (61°).

Hypothermia rapidly can become a serious threat to survival. Only a 3°C (5.4°F) drop in core temperature greatly impairs the body's ability to regulate its core temperature. Thermoregulation essentially is ineffective when core temperature declines to 29°C (84°F). Hence, the first signs of hypothermia should never be ignored; action should be taken immediately.

Treatment of hypothermia victims takes two forms: prevention of further heat loss and addition of heat. Depending upon circumstances, further heat loss can be prevented by replacing wet clothing with dry clothing (reducing heat loss via evaporation), finding shelter from the wind (reducing heat loss via convection and evaporation), and insulating the person from the ground (reducing heat loss via conduction). The body can be heated by an external source, such as a space heater or other human bodies. If the victim is conscious, administering a warm (not hot), non-alcoholic beverage, helps warm the core from the inside. Efforts to warm the victim should never be discontinued in favor of moving the person or going for help. Medical attention, however, should be sought as soon as possible.

In some situations, thermoregulation is unable to prevent a rise in core temperature. Hyperthermia refers to those conditions that take place when core temperature climbs to 39°C (102°F) or higher. Hyperthermia may be divided into two stages. Symptoms of the first stage, known as heat exhaustion, include profuse sweating, nausea, vomiting, and general weakness leading to an inability to continue normal activities. A person suffering from heat exhaustion should be taken immediately to a cool environment. Removal of excess clothing and sponging with water speeds the lowering of the core temperature. If necessary, evaporative and convective cooling can be accelerated by placing the victim in front of fans. The person should be given fluids orally, if tolerated, because dehydration typically is the primary cause of heat exhaustion. Most victims of heat exhaustion recover without any complications.

Failure to treat a person with heat exhaustion usually leads to a further rise in core temperature. If the core temperature reaches 41°C (106°F), enzymes begin to fail and thermoregulatory mechanisms breakdown. The victim has a rapid and strong pulse, exhibits psychotic behavior, and may slip into unconsciousness. Symptoms at a core temperature of 41°C (106°F) or higher constitute heat stroke (or sunstroke), a life-threatening emergency.

A heat stroke victim must be treated promptly because, once thermoregulation fails, core temperature rises rapidly and death may occur within a few hours. Bringing about a rapid drop in core temperature is the top priority. Although initial field treatment can be similar to that for heat exhaustion, medical attention and hospitalization should be sought as soon as possible.

Heat cramps may occur as a concurrent symptom of heat exhaustion or by themselves. Heat cramps are extremely painful contractions of the large muscles of the calf, thigh, abdomen, or shoulder and result from the excessive loss of sodium and potassium salts (electrolytes) in sweat. Heat cramps differ from exertion-induced cramps which involve the entire muscle. Rather, an individual bundle of muscles will contract for a few minutes and then relax while an adjacent bundle contracts and so on. Hence, victims of heat cramps experience the sensation of a wandering cramp throughout the entire muscle.

Victims of heat cramps should rest in a cool environment. If not nauseated, the person should be fed a commercially available electrolyte solution such as Gatorade or Spirit. If such a solution is not available, heat cramps can be relieved by administering a salt solution consisting of a one-fourth teaspoon of table salt in a liter (approximately a quart) of water. If a person complaining of severe muscle cramps is also nauseated, he or she is probably also suffering from heat exhaustion and medical attention is needed as soon as possible.

Summary of Human Responses to Thermal Stress

To Cold	To Heat
Thermoregulatory responses Constriction of skin blood vessels Concentration of blood Flexion to reduced exposed body surface Increased muscle tone Shivering Inclination to increased activity	Thermoregulatory responses Dilation of skin blood vessels Dilution of blood Extension to increase exposed body surface Decreased muscle tone Sweating Inclination to reduced activity
Consequential disturbances Increased urine volume Danger of inadequate blood supply to skin of fingers, toes, and exposed parts leading to frostbite Increased hunger	Consequential disturbances Decreased urine volume. Thirst and dehydration Difficulty in maintaining blood supply to brain leading to dizziness, nausea, and heat exhaustion. Difficulty in maintaining chloride balance, leading to heat cramps. Decreased appetite
Failure of regulation Falling body temperature Drowsiness Cessation of heartbeat and respiration	Failure of regulation Rising body temperature Heat regulating center impaired Failure of nervous regulation terminating in cessation of breathing.

Indexes of Heat and Humidity

Scientists have developed a variety of indexes that attempt to gauge the combined effect of temperature and humidity on humans and advise people of the potential danger of heat stress. Since the summer of 1984, the National Weather Service has regularly reported the heat index (sometimes called the apparent temperature index) which was developed by R.G. Steadman in 1979. The heat index calculation is described in this web page.

When air is hot and muggy, a number of heat-related problems may occur. Among these are heat cramps, heat exhaustion, and heat stroke. For more information on heat-related problems and how heat affects the body, check out the National Weather Service Heat Wave web page.

In an effort to draw attention to this serious weather-related health hazard -> creation of an index called the heat index (HI).

The heat index combines air temperature and relative humidity to determine an apparent temperature -- what the temperature "feels" like to the average person.

For example, an air temperature of 100°F and a relative humidity of 60% produce an apparent temperature of 130°F. This following link contains US Maps of Heat Index values.

The Role of Wind

Because of the danger of frostbite, weather reports and forecasts during winter in northern localities and in mountainous regions include the windchill equivalent temperature (WET), sometimes referred to simply as the windchill index or windchill factor.

The WET takes into account the increasing rate of heat loss due to a combination of low ambient air temperature and high wind speeds.

US Maps of windchill equivalent temperature values.