Meteorologists use two main indices to determine how air feels to human skin – the heat index, and the wind chill.  The heat index accounts for the decreasing ability of the body to dissipate heat as the relative humidity increases.  The wind chill accounts for the increasing ability of the body to dissipate heat as the wind speed increases. Various combinations of air temperature, relative humidity, and wind speed produce an “apparent” temperature which has an effect on the body’s rate of heat loss.

These indices imply that all humans feel the same level of discomfort under the same combinations of air temperature and relative humidity, or air temperature and wind speed.  However, this conclusion is relative.  Lifelong residents of the Gulf Coast states, for instance, will feel comfortable at considerably higher heat and humidity combinations than lifelong residents of Alaska.  Exercise and solar exposure, which add to the heat load on the body, are not considered by the indices; nor are the health of the individual, mental state, age, or body size.
 

NOAA Heat Index Chart

One of the natural processes that affect the apparent temperature to humans include the size of the individual human.  Body size regulates volume heat production within the body.  The more overweight the person, the greater the number of heat-generating cells versus the surface area from which heat is exchanged with its surroundings.  For example, overweight individuals have a large surface-to-volume ratio, and thus retain more heat than underweight individuals.

The type of body part affected also regulates volume heat production, although in lesser ways.  Body parts characterized by a large exposed surface-to-volume ratio (such as fingers, ears, the nose, and toes), will lose heat more quickly – especially with the loss being accelerated under cold air temperatures and high wind speeds.

Behavioral responses also assist in recovering a body back to isothermal equilibrium.  Examples of behavioral responses include a dip in the swimming pool, fanning the body, consuming beverages, and shedding or adding clothes.

The action processes of surface heat exchange include how the human body responds or reacts to the surrounding air temperatures.  The human body senses surrounding air temperatures through the processes of surface heat exchange, including radiational cooling and/or heating, sensible heat loss and/or gain, and evaporational cooling.  On a sunny day, exposed skin absorbs direct and diffuse insolation, converting the incoming solar radiation to heat.  This radiational heating of the body typically exceeds the radiational cooling of the body, and thus the human body heats up.  Just standing in the sun can cause the core temperature to warm as much as 8 degrees Celsius (46 degrees Fahrenheit).  On the other hand, at night, only radiational cooling of the body exists.  The warmer body emits more heat to the cooler surroundings than it absorbs, and thus the body cools off.

If the core temperature continues to warm, another physiological response may also be activated - sweating.  Evaporational cooling is more effective than accelerated sensible heat loss in cooling the body.  However, if the air is humid, evaporational cooling is decelerated.  In general, at air temperatures above 25 degrees Celsius (77 degrees Fahrenheit), people feel more comfortable when the air is dry than when the air is humid.

Contrary to popular belief, not all temperature-related fatalities are associated with extended periods of extreme weather (hot or cold).  Sometimes, people find themselves ill-prepared to cope with deteriorating, but not necessarily life-threatening, weather.  Public awareness of the health hazards will reduce the fatalities associated with this ill-preparedness.  Both heat and wind chill indices are a means of guiding the public, and possibly saving their lives.