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Facts of the Matter
Richard Brill
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Cold enough for ya? Depends on who you ask
TEMPERATURE is a measure of hotness or coldness. My dictionary defines it thus: "the degree or intensity of heat present in a substance or object, esp. as expressed according to a comparative scale and shown by a thermometer or perceived by touch."
Our senses do not really perceive temperature per se and they can deceive us. We don't sense temperature the way a thermometer does. Our perception of hot and cold is actually sensing whether heat is being transferred to or away from our skin.
We say "it's cold in Hawaii when the temperature drops below 70 degrees. We are wearing sweatshirts and long pants while a visitor from Minnesota finds it balmy.
STAR-BULLETIN / 2006
When the temperature drops into the 70s here, folks bundle up.
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Is "it" warm or cold outside? It depends on what the individual is accustomed to. If one has become acclimated to winter temperatures well below freezing, 60 degrees is downright tropical!
The ocean may feel cold when you first get in, but you quickly acclimate.
If you get out of the water and then jump back in, it feels warm. The temperature of the water has not changed. Evaporation of water from skin to air has cooled the skin so the water is now relatively warm.
On a cool winter morning, the tile floor feels cold, but the carpet feels warm. A thermometer would show them to be at the same temperature. The tile conducts heat better than the carpet, your foot loses more heat to the tile than to the carpet, and your brain perceives the tile to be colder.
Two people may not agree on whether "it" is warm or cold, but they will always agree on which of two objects is warmer. This is the only absolute sense of temperature that we possess.
The perception of temperature depends on the rate of heat transfer, which depends on a number of factors. Heat always flows from warm to cool, so anything that is cooler than skin temperature will feel cool and anything warmer than skin temperature will feel warm. How warm or how cool depends on the rate of heat transfer.
Heat transfer takes place by conduction, convection, radiation, which are called "sensible" means of transfer. Changes of state, such as evaporation, are "latent" transfers.
Conduction is a contact phenomenon. Good conductors feel colder or warmer to the touch depending on their actual temperature and skin temperature.
The best insulator (the worst conductor) is dry air, so clothing that has small pores traps air and keeps it there. Goose down is such a good insulator because the feathers are made of fine filaments that trap air and keep it from moving.
Convection is another sensible method of heat transfer. A moving fluid (gas or liquid) transfers heat by physically moving warm energetic molecules and replacing them with cooler less energetic ones. To prevent heat loss by convection, a windbreaker made of impermeable fabric locks the air in the pores of clothing, whereas a porous shirt can allow air to circulate and cool us in a summer breeze.
The third method of sensible heat transfer is radiation. It is also the most complicated and most difficult to perceive. It is the method by which heat from the sun travels through space to make Earth habitable.
Every object radiates heat at a rate that is proportional to the fourth power of its absolute temperature. The "color" or wavelength of radiation also depends on the temperature, becoming shorter with higher temperatures.
Absolute temperature is measured on the Kelvin scale, with absolute zero at minus 273 degrees Celsius (-460 degrees Fahrenheit). Absolute zero is unattainable in principle, but is the theoretical temperature at which an object would contain no heat.
Because of this radiative effect, everything is continually exchanging heat, including our skin.
Skin temperature depends on air temperature and time spent in that environment. Such weather factors as wind chill and humidity cause changes in skin temperature. The normal temperature of skin is about 33 degrees Celsius (91 degrees Fahrenheit), which is 306 Kelvins.
At that temperature, the human body radiates about 140 watts in the infrared, the equivalent of a couple of ordinary light bulbs. This amounts to a little less than one watt per square foot of skin on a typical adult male.
A fire feels warm because your skin receives more radiation from the fire that it radiates. The freezer feels cold even from across the room because your skin receives less radiation from it than from the surroundings.
A static, inanimate object in still air loses heat at a rate that depends on the difference in temperature between it and the surroundings.
The human body is, of course, neither inanimate nor static. We produce metabolic heat at various rates depending on our level of activity. We lose heat by conduction, convection, radiation and evaporation. The bodies of all warm-blooded creatures like ourselves strive to maintain a constant body core temperature through a process known as homeostasis.
If you were perfectly motionless, nude, with no body hair your body would develop a "private climate" about 3 mm deep through which the temperature changes from skin temperature to the surrounding air temperature.
In perfectly still, dry air, perspiration, even an amount too small to be noticed, would evaporate and enrich the private climate in water vapor.
This would dissipate if wind were to carry the warm air away from the skin via convection as the water vapor diffused into the surrounding air.
The faster the air moves, the more cooling by convection and evaporation.
In a stiff wind you can be cool in air that would otherwise be stifling.
The lower the humidity the greater the cooling effect because of faster evaporation.
Therein lies the secret of Hawaii's natural air conditioning in the form of the trade winds, (whose absence we note with comments about the mugginess), and the concepts of "wind-chill" factor and the "humidity-comfort" index.
The next time someone says, "It's cold tonight," you might try explaining that "it" is not cold at all, it is only a matter of perception. My advice is to say, "It's not cold," and not "YOU are cold." That could get you into more trouble than it's worth.
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to
rickb@hcc.hawaii.edu