Effect Measure

We’re having a heat wave

I’m at the beach and it’s hot. It’s supposed to be that way at the beach. When I get overheated I head back to the unit, which is air conditioned, and I cool off. Actually, I don’t. I stay the same temperature (body temperature), but that aside, it’s no problem. But not everyone is so lucky and recently a good section of the US south has been having a heat wave. The same thing happens every summer (although some are worse than others), and two years ago I wrote about this on the old site. Instead of just reposting it I decided to rewrite it. At the time I was struck by a headline in the Philadelphia Enquirer which said, “It’s not the heat . . . actually, yes, it is.” It bothered me because in fact the old adage, “it’s not the heat, it’s the humidity” is much more correct. Here’s why:

Heat and temperature are not the same thing. Without getting into thermodynamics, temperature measures the intensity of thermal energy while heat measures its quantity. They are related, of course. You can think of temperature as average thermal energy, the quantity of heat being the sum total of thermal energy over a volume of material. Thus you can have a thimble of water at 100 degrees Fahrenheit and a gallon of water at 100 degrees F. They both have the same temperature (100 degrees F.) but vastly different heat content. There is a lot more heat in the gallon of water than in the thimble of water at the same temperature. When you add heat energy to a body the average temperature rise will depend on how big a body it is (and what it’s made of).

You can think of the heat content of your body as being like the water in a bathtub. As you add heat energy through the faucet the level rises (i.e., the temperature rises). As you let water out the drain, the level decreases. Normally the level of the bath water stays almost constant at body temperature (98.6 degrees F.; I’m moving back and forth between temperature and bath level but you are smart enough to make the switch where needed, I am sure). Since the surrounding environment is usually cooler, heat continually drains out of us and to keep the level constant we need to keep adding heat, which we do by burning food or by exercising your muscles (which you do sometimes involuntarily when a cool environment tells your body to make more heat by shivering).

Besides losing heat by contact with a cooler environment there is another source source of thermal energy: other objects that radiate heat (like the sun or a furnace or just the surrounding environment). Heat radiation (mainly in the infrared) doesn’t depend on contact with the hot object. It can travel through empty space, which is how the sun is able to warm us. A body radiates heat in amounts proportional to its temperature, so you usually lose heat to the environment by radiation faster than you gain it by that means, but extra hot objects (like the sun or a fire) can actually produce a radiant heat load that heats you up (hence you feel cooler in the shade).

So it’s all a balancing act between heat inflow and heat outflow. But what happens if the drain starts to narrow, say because the surrounding environment got hotter?

Then the balance of heat entering and leaving the tub is changed. If more heat enters the tub than leaves it, the temperature will rise. Even on a day when the outside temperature is in the low nineties, your body naturally loses heat but the problem is, it loses it more slowly than when it is cooler outside. The drain is narrowing. And it can get worse. If the surrounding environment is actually warmer than body temperature, you can’t lose heat by contact with a cooler environment or by radiating your own heat to it (because it is radiating even more heat back to you). A fan won’t work because it is just blowing the warmer air past you and helping you gain heat faster. What’s a body to do?

Fortunately, there is one remaining way to lose heat, and that’s by turning body water into water vapor. This makes use of the “latent heat of vaporization,” the 540 calories of heat it takes to turn a gram of water into water vapor. This is pure heat loss. Hence sweating, even when it is “insensible” (doesn’t wet the skin), is an efficient way to lose heat and one of several reasons to stay hydrated in hot weather. The real problem comes when the air cannot hold any more water vapor (100% relative humidity or close to it) so your sweat cannot evaporate. Then this last way of losing heat is shut down. The faucet is still open (heat is coming into the body via metabolism and muscle action, a warm environment and radiant heat load) but the drain is now closed. Hence the tub begins to fill with heat. If your body core temperature rises too high (say above 106 degrees F.), it is a medical emergency (heat stroke) and high mortality is the rule.

So heat obviously plays a part. But it is the humidity that does the real job on most people. Air conditioning, interestingly, derives most of its effectiveness, not by dropping the temperature, but by drying out the air, allowing increased evaporative heat loss.

So now you know the real problem. No sweat.


  1. #1 6EQUJ5
    August 27, 2007

    The last place I worked used chilled water refrigeration for cooling. (No costly dehumidifying involved!) The ambient air was chilled to 60 F and trickled onto people from above. While this did cool the workplace overall, as evidenced by the wall thermometer, having cold clammy air trickled onto your head while the rest of you is slightly sweaty will quickly make you sick.

    The response? Block off the vents with cardboard or heavy paper, and get one or more personal fans to blow air on you.

  2. #2 Anne-Marie
    August 27, 2007

    Someone once told me that you burn more calories sweating to cool yourself off than you do when you metabolisms boosts to warm your body in the winter. Is there any truth in this? Any idea how many extra calories you burn when you sweat versus being in your thermal neutral zone?

  3. #3 Cathy
    August 27, 2007

    I’m reading this as I sit here near a southwestern beach dealing with unaccustomed humidity (and even rain! in the summer!) caused by the remnants of Hurricane Dean … and I am reminded of a T-Shirt I saw in Death Valley last winter. Picture two skeletons lying on lounge chairs and the caption “But it’s a dry heat.”

  4. #4 River
    August 27, 2009

    What a fascinating article! Thanks so much.

    Be well.

  5. #5 BostonERDoc
    August 27, 2009

    Heat and humidity are life saving when it comes to the H1N1 virus. In high humidity, the mode of transmission is contact. At low temp ( <68 F) and low humidity (< 35 %) airborne transmission becomes possible in animal studies. Finish resting up Revere and savor the humidity and heat—as someone in public health you are going to wish it back soon enough.