It's not the heat

It's not news the nation is having a heat wave although it is making the news, as temperatures in the nineties and above were reported in city after city, from one coast to the other and in between. Good opportunity to talk physics and physiology.

Your body is a prodigious producer of heat, which you can see just by thinking about the food you consume. To be charitable to you and your lack of gluttony, let's say you consume 2500 Calories a day. A Calorie is a measure of the chemical energy released in the form of heat when the organic material in the food is burned. These are large calories (aka kilocalories) which is why we use a capital "C". One large Calorie is the amount of heat it takes to raise a kilogram of liquid water (2.2 pounds) one degree centigrade. So 100 Calories will take a kilogram of water from just above the freezing point to just up to the boiling point, so the amount of heat you burn daily could heat from zero to 100 degrees centigrade (i.e., from 32 degrees F. to 212 degrees F.) 25 kilos of water, over 6 gallons worth. To work efficiently your body needs to stay within a very narrow range of temperatures (close to "body temperature," or 37 degrees C., 98.6 degrees F.). It does this by getting rid of the extra heat to the environment by four mechanisms of heat transfer: conduction, convection, radiation and evaporation.

The first three of these depend on a temperature difference between you and your surroundings. Conduction occurs when two bodies with different temperatures are in contact. With time the temperatures equalize (First Law of Thermodynamics). So that's one way heat is transfered from one place to another, in this case the heat of your body to the layer of air at the surface of your skin. Convection is another way. Instead of "diffusing" from one place to another, heat can be carried when a hot object or hot body of gas is moved from one place to another. That's the way we move heat from our basement furnaces to the upper floors of our house by moving hot air through ducts or hot water in the radiators and pipes. When there is a breeze or we fan ourselves we are moving the warmed air at the skin surface away and replacing it with cooler air that has yet to be warmed by conduction. Radiation is the third way of losing (or gaining heat). Heat on the macroscopic level is a reflection of the kinetic energy in the form of tiny movements of the molecules that make up a body. The higher the temperature the more energetically the molecules in our body dart back and forth, although the range of each darting motion is extremely tiny. Since the molecules contain electric charges they also act like small antennas and radiate electromagnetic waves just like a radio transmitter, but at infrared frequencies (you can "see" your radiation if you have a special instrument, a thermograph). Hotter objects radiate away more energy than cooler ones, so you are also radiating away your heat. You can also absorb radiant heat, which is why we are heated by the sun. If your surroundings are cooler than you are they radiate heat to you, but you radiate more back so the net balance is loss of body heat, once again.

Thus as long as your surroundings are below body temperature you will lose heat, but you may not lose it fast enough to keep up with the amount produced as you burn food for essential (and not so essential) activities. Things get worse if the environment is at the same temperature as you are, 95 - 100 degrees F., or even hotter. Now you aren't losing any heat to the environment. You might even be gaining heat. And you continue to produce heat. All is not lost, because there is one other way to lose heat, evaporation. For every gram of water (sweat) that evaporates from your body it takes 540 (small) calories with it, i.e., going from 100 degrees centigrade water to 100 degree centigrade water vapor takes 540 calories (.54 Calories). This doesn't depend on the surrounding air temperature, either, so you can continue to lose heat this way as long as you can sweat and the sweat can evaporate. If you don't drink enough or you take certain drugs that shut down your sweat glands you can't sweat. If the air can't hold any more water vapor your sweat can't evaporate, either. It just runs off you in liquid streams. This happens when the relative humidity reaches 100%.

In other words, when it is very hot and it is very humid, you are screwed. Blowing more hot, humid air over your body won't help. The old saying, "It's not the heat, it's the humidity" is largely correct. People can withstand fairly hot and dry environments (like Tucson or Phoenix), but will quickly succumb to very hot, humid ones. One normal physiologic reaction to heat is to generate as little of it as possible. Stop moving your muscles unnecessarily. Take a siesta.

Sounds like a good idea. See you in a couple of hours.


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Or do what we used to do, get an air hose, jump in the pool, and stay at the bottom until your skin looks like a raisin.
That said, I looked at the back page of the main section USAToday and I can't remember when I ever saw the whole country orange/red (except for the Pacific northwest which was yellow.)

By G in INdiana (not verified) on 19 Jul 2006 #permalink

This of course is directly relevant to the foreseeable future situation w/r/t energy supplies, i.e. peak oil.

To the extent that large regions of the US are hot and humid, they depend on air conditioning, which in turn is a huge consumer of energy. When oil production goes flat, while demand continues to rise, the cost of all forms of energy will rise, and air conditioning will become a far more serious cost issue than at present.

Long term forecast: present population levels in the south east will not sustain; people will start moving to drier climates. Between now & then, expect increasing numbers of deaths in heat waves. Oh and with the climate crisis, expect extremes of temperature to increase, thus more and hotter heat waves.

Looks like Ma Nature is cleaning house....

Now the really interesting question is, what's the relationship between ambient temperature and male fertility? That is, can we look forward to further declines in sperm counts as a function of heat waves? One can hope so anyway, since male infertility is one of nature's kindest ways to reduce overpopulation.

I was looking on the heat index for the first time this year. It's nice to have some objective values to express subjective heat stress. Employers should take measures when temperatures are high (more than 28 degrees celcius).
Steadman concluded 15 variables are of influence on subjective experience of heat. Body weight and clothing are among them.
Heat is measured by a humid globethermometer and usually is measuring 5 degrees less than the actual celcius thermometer. But the table with measures is speaking for itself about risky behaviour.
I'll improvise its translation here, hoping it will come at EM a bit orderly.

light work medium work heavy work exposure time rest
30.1 C 26.8 C 25.1 C 110 min. 10min
30.4 27.5 25.5 100 20
30.6 28.0 25.9 45 15
30.9 28.5 26.6 40 20
31.2 29.0 27.3 35 25
31.5 29.5 28.0 30 30
31.8 29.8 28.7 25 35
32.1 31.1 29.4 20 40
32.4 31.4 30.1 15 45
32.7 31.7 30.8 10 50
33.0 32.0 31.5 5 55

Source: general regulations labour protection (ARAB) Belgium.

In Dubai they have many thousands of men captured and let them work in 42 degrees C for 12 to 15 hours constructing luxury appartment buildings for about 3 dollars a person a day. The emirs will not release them and give them their passports back, simply because they don't want to do that.

Sorry, you have to spread the numbers in the table a bit more in order to read the amount of exposure (working) time and the length of the pause after that exposure (rest), combined with type of work (light, medium, heavy).

People in humid environments never truely appreciate the wonders of dry air. Coming from Colorado, I find the "hotness" of Boston truely unbearable, despite the actual air temp. not being any higher. I lived the first 21 years of my life without an air conditioner, and was not the least bit uncomfortable.

Now, the poor thing is running full blast from almost the minute I get home.

Conduction is why to set your waterbed temp to about 96-7°F in the summer. Any cooler than that will result in too much heat loss and you wake up shivering and/or achey. But at a degree or two below body temp, it will cool you enough for a good night's sleep on the hottest day, and can also cool you during the day (see: si, si siesta)Here in Misery, we have those lovely 95/95 days. 95° and 95%. As I speak, it is 94° and clear with the humidity at only 52%. Gives us a heat index up around 104. Good day for blogging and sudoku.

By Man of Misery (not verified) on 19 Jul 2006 #permalink

BTW, any comments on adding some alcohol to the water in a spritzer when sitting in front of a fan? How much alcohol does it take to increase the evaporation quotient to overcome high RH? Years ago, we encouraged poor folks living in brick buildings in the city (which become brick ovens after a few days) to get a fan and spritzer if they couldn't afford A/C. Without the spritzer, a fan blowing 100° air is counterproductive. It heats you up instead of cooling you off. For some reason (like it didn't work that well), we gave it up and got them free public transport to a mall or other cooling site. But that only works if they are willing to go out. I always wondered about filling the spritzers with half & half rubbing alcohol & water, but could never get an answer.Chicago came & looked at our plans after they lost several hundred (mostly poor and/or elderly) folks a few years ago while KC & St. Louis didn't during the same heat wave and conditions. We had lost ours a few years prior and learned from our mistakes, most of which was a lack of preparedness.

By Man of Misery (not verified) on 19 Jul 2006 #permalink

MoM: Don't know what the enthalpy ("latent heat of vaporization") of alcohol is, although it probably is no where near that of water so it's coolilng effect wouldn't be as high, although it would work I should think because it isn't limited by the relative humidity (it has its own vapor pressure and there is no alcohol in the air). However rubbing alcohol is poisonous when ingested or inhaled so that is another hazard. Air conditioning is the best answer, in the mall, the supermarket or a shelter.

Well, ethanol is also toxic when ingested, but that doesn't seem to stop people doing it. A particularly bad idea when under heat stress.

By Man of Misery (not verified) on 19 Jul 2006 #permalink

Latent heat of evaporation for methanol & ethanol are about 10% lower than water.

Evaporating alcohol around electrical fans is a very bad idea.

By Ground Zero Homeboy (not verified) on 19 Jul 2006 #permalink