We have had a streak of very uncomfortable weather lately. My thermometer outside is recording 90F (26C). The psychrometer records relative humidity at 99%. On the “how it feels’ chart —I feel terrible. My skin is damp and clammy. I’m tired. The only relief is a dip in the back-garden pool, or a trip down to the beach. One senses relief and revival immediately after plunging into the cooling liquid. When that is not possible, one must retreat into a refrigerated room, or a climatically controlled building, or a big box store where simply entering offers immediate relief.
My daughter, who happens to be in Rome, Italy on a school education tour called in the early morning to report that the temperature there at mid day was 93F or 34C). Her text stated, “Don’t worry Daddy, It’s not too bad here. We had lunch at an outdoor cafe, where we sat under a shady arcade. A faint breeze made it very pleasant.”
How could that be? After closing the text app, I looked up the weather in Rome for that day. The Italian meteorological society reported that the temperature maxed out at 35 C (95 F). and relative humidity was 32%! That is the difference.
Humidity in the air. That is the culprit.
Why does atmospheric humidity make such a difference?
It was about two million years ago in East Africa where the human species (Homo sapiens) arose, at a time when the East African climate was moderately warm and very dry. Humans are adapted to a near desert climate, to a dry air and higher temperatures.
One of the more significant evolutionary advantages of humans is their means of maintaining a constant body temperature. Some organisms (most) only respond to ambient (outside) temperatures, being more active when the air is warmer and less active when cooler or cold air replaces the warm. These animals (poikilotherms) respond with reduced activity and lower rates of metabolism in cold air . For some (i.e. reptiles) such variations in body temperature and changes in metabolism actually helped to improve their survivability. When low temperatures coincide with food scarcity, a common occurrence in nature, reptiles simply go into a low metabolic state conserving energy until temperatures rise and they are energized to search for their next meal..
Humans evolved with large brains which needed a constant blood flow, and energy supply. Along with other mammals and birds humans evolved mechanisms to maintain body temperature at a constant. Birds and mammals are termed “homeothermic” organisms. Homeotherms can continue active hunting both day and night. They can exploit both warm and even very cold climates or environments, and can more effectively metabolize foods using enzymes to improve and speed up chemical reactions and cellular metabolism. Enzymes work best at 20C to 35C (68F to 95F) temperatures which approximate those of most homeotherms.
One disadvantage for homeotherms is that they need more energy than poikilotherms to survive, since they must use a great deal of food energy to simply maintain a constant body temperature. But the physical activity advantages and enzymatic activity of homeothermy far outweighed any disadvantages.
It is noteworthy to remind the reader here that humans evolved and spread out over the Earth during the Pleistocene Era or last Ice Age. This was a relatively unusual episode in Earth history, a time when ice sheets thousands of feet thick covered vast areas (33%) of existing continents. Humans evolved during the last (Pleistocene Era) Ice Age, a climactic period they were well adapted to.
Human skin functions to modulate temperature and help keep the human body close to the near 100F (98.6F or 37 C) range. The deeper layer of the human skin (dermis) has a vast array of capillaries which can relax or constrict and control the flow of blood close to the surface where it can release body heat to the exterior air and thus modulate body temperature. The reddening of the skin during such events—the blush.
Then too the skin is supplied with a vast number (2-4 million) of sweat glands. These are tiny coiled, tube-like organelles embedded in the dermal skin with a pore that opens onto the skin surface. Each gland is surrounded by a reticulum of capillaries which carry blood and which transfer water and salt from the blood by means osmosis into the gland which then moves the salty water to the skin surface. On the skin surface the water evaporates into the air and as it does, it carries away heat.
In order to change from a liquid (water) to a gas (water vapor) water in the liquid state must absorb heat energy. Gases are in a higher state of energy than liquids. Each cubic centimeter (ml) of water (about a thimble full) requires about 570 calories to make this change from liquid to gas. This energy (heat of vaporization) is absorbed from the skin surface and used to change liquid to gas as sweat evaporates.
You may all recall the chilling effect of wearing wet clothes. Evaporation is a very effective means of removing heat. And the human ability to pour sweat onto the skin surface to cool itself was an essential element of human survival in the savannas of East Africa where humans evolved.
The Kung! Bushmen of the Kalahari Desert in Botswana and South Africa use very primitive slow acting poison darts to kill game. A Lesser Kudu shot with a poison arrow will wander off and take days to die. Kung Bushmen hunters use a method of tracking and chasing game to speed up the process and dispatch it when it falls. or more often force the kudu to flee and continue to follow closely at it attempts escape. The human hunters have an advantage..kudu have no skin sweat glands, they cool their bodies by relatively ineffective panting and lolling their tongues as the run. During the hunting chase in the hot Kalahari kudu soon become overheated and collapse from heat exhaustion while the human hunters are unaffected.
It is noteworthy to remind the reader that the water of sweat on the human skin surface evaporates fully (removing the heat) but the salts of sweat, such as sodium chloride, and acids (acetic, uric, lactic,) as well as urea are left behind. These accumulate on the skin and when left for long periods can be altered by skin bacteria to generate other compounds which can cause offense to others near-by (and also to the producer). This is one good reason why soap was invented. Young first term male college students often forget this and are then shunned as pleasant company. Alternately depending on the level of offense these freshmen may be overpowered at night by his/her colleagues and forced into the nearest shower stall with water on full.
But this “evaporation of water method of cooling” only works well in dry air into which water vapor can readily evaporate. If the air around the body is saturated (full to capacity) with water vapor, evaporation is limited. At saturation (100%RH) no water can evaporate from the skin and no change of sweat into water vapor takes place’ No body heat is lost, but metabolism must continue and heat continues to be generated..so body temperature rises.
There is no loss of nearly 600 calories of heat energy for each thimble full of evaporated sweat! Body temperature can rise above 98.6F/ 37C and hyperthermia ensues. When body temperature climbs above 39C-40C (@ 104F) confusion, nausea, vomiting, rapid breathing, may occur. The
Sitting in one place in quiet air, the human body naturally evaporates water into the immediate surroundings. As sweat evaporation proceeds a layer of saturated air tends to accumulate in a more saturated gaseous film around the body. This close body layer of saturated air retards evaporation of sweat and the cooling process. One ancient method of relief is to employ a hand fan. The well known Spanish decorative fan used in Spain’s relatively dry and hot climate is well known. But any way to move the saturated air away and replace it with fresh (less saturated) air will increase cooling such as the regular swing of a cowboy hat, the tiny battery operated fans one often sees in hot climes. O one can move to a more open area where currents of air will move the saturated air layer away from the skin.
So keep cool…by encouraging evaporation of body sweat…avoid tight clothes, and most of all remember that air humidity is of almost equal importance as temperature.
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