Short takes on science, business, health, agriculture and possibly the kitchen sink

Robert Wise   email  
Living with Heat: The Challenge and the Body's Response
How the human body copes with heat stress and learns to do it better

Photo from "Riding and Training in the Heat",, permission requested.

To feel comfortable, even to survive, your body must continually shed heat. It generates heat by metabolizing food and working its muscles. In sunlight, it gains heat constantly from the sun's beam. Walking in full sun in moderate temperatures, it must continually lose the heat equivalent to seven 100 watt light bulbs, or a small space heater. It's easier if you are acclimated to your outdoor environment.

The body sheds heat by convection*, radiation and perspiration. But as temperatures rise toward the normal skin temperature of 91 Degrees Fahrenheit, radiation and convection become less and less cooling. Above 91F, they begin to warm the body. As I described in my last post, you feel warm even in shade, and a breeze feels hot. In these temperatures, only perspiration keeps you from heat stroke.

"Sweat- ewwh!" you might be thinking. Folks raised in cool climates may see it as an extreme condition, to be avoided- "Don't sweat yourself." Growing up with bright sunlight and heat, you learn to tolerate it and even appreciate it to a degree. If you're exercising in this heat and not sweating, then you have something to worry about.

I constructed the chart below to show how the body cools in various temperatures, from a comfortable 70F to the 105F often used in hot-room (Bikram) yoga. It assumes you are walking, bareheaded, in full sun with a light breeze. The dashed lines show the body's heat load under the above assumptions, also walking in full shade, or resting in full shade. Even a hat will reduce your heat load significantly.

Chart of the cooling and heat load on an average human body in a 5 mph breeze, mostly following the calculations in the UBC Physics Department reference given below. In the lower 70's, cooling exceeds the heat load on the body; a light windbreaker might be needed.

Cooling by convection and radiant heat is almost proportional to air temperature in these conditions- proportional, that is, to the difference in temperature between your skin and the surrounding air. Above skin temperature, these processes begin to warm the body, adding to its heat load.

Convection works much better in moving air. With no wind, it cools only half as much as radiation. In the light wind shown on the graph, it cools about four times as much. Ceiling fans need only stir the air, at their lowest setting, to make the room feel cooler.

Radiative cooling is harder to estimate. It's the balance between the heat radiated by your body and the heat radiated to it by the surfaces around you. The graph assumes that the surfaces around you are about the same temperature as the air- an assumption often used in studies of indoor climate. But it won't be true if you are walking on a hot asphalt pavement or riding in an air-conditioned car.

Perspiration and humidity
The graph assumes that your body will start sweating as soon as it needs to, and will sweat enough to take care of any heat load not dissipated by convection or radiation. This will happen if you are acclimated to your outdoor environment (discussed below), if you're drinking plenty of water and if the air is dry enough for sweat to evaporate easily. The wind helps; evaporation goes slower in calm air.

Perspiration works better in dry climates- sweat evaporates so quickly you hardly notice it. Down home, in heat and high humidity, you are easily drenched in sweat. I didn't find a good way to represent this on the diagram, but the Heat Index provides an idea of the additional heat load caused by high humidity (table in notes below).

Near 105F, you need to sweat enough to dissipate 1000 watts or more- about 1-1/2 quarts of sweat per hour. This is about the maximum sweat output considered "normal." But with acclimation to extreme heat, some people can sweat as much as 2-4 quarts per hour.

Acclimation: learning to live in your climate
I'm sometimes tempted to ask certain neighbors, "Do you really live in Florida, or just a set of air conditioned boxes with the thermostat set to Upper Michigan?"

Not a fair question, because I spend most of my day in air conditioning too. But I like to know I can tolerate the climate I live in. I get out in it regularly, whether bicycling in Florida summer or walking in the snow in Wisconsin winter.

However you feel about it, acclimation will improve your health, especially if you work or exercise outdoors. As the climate warms, it's going to be more important for more people, more often.

The best way to acclimate to a warm climate is to work or exercise in the heat, preferably two hours a day for 2-3 weeks. About 75% of the benefits are gained in the first five days. An hour every other day will do, but the process takes longer.

Working out in an air-conditioned gym won't do it, though acclimation goes quicker if you're already in good shape. Hot-room yoga won't accomplish it, either: there's plenty of heat stress, but little exercise.

How does the body change with acclimation? It sweats faster and starts sweating at lower temperatures. Blood volume increases, and heart function, blood pressure regulation and blood distribution improve. Less salt is lost in sweat and urine. Your overall ability to perform work or exercise improves.

It works differently for hot-dry climates than hot-humid, where the body must learn to sweat even more. Humans seem better adapted to the hot-dry environment.

Acclimation isn't permanent: we lose it within a few weeks if not regularly exposed to heat. So we probably all need to get re-acclimated each spring. But if you're able to keep up regular program outdoor exercise or work year-round, you'll re-acclimate as a matter of course.

So one way to build resilience in the face of global warming is to make sure you stay acclimated to your local climate- provided your health permits it. For most of us, our climate is already hotter than the climatic normal, and will probably get a little hotter each year.

And it may help to keep in mind the basic heating and cooling processes acting on your body. At least you won't make the mistake of seeking your "place in the sun."

I'll add some footnotes to this post in a day or two, with the full derivation of the numbers in the diagram, and some background on thermal radiation. The next post will discuss clothing and accessories that help in hot weather.

* Convection has two scientific definitions. Here I'm using the more general one: any transfer of heat through a liquid or gas. Convection also denotes the motion generated by such heat transfer, such as the growth of cumulus clouds or the bubbling in a lava lamp.

National Weather Service Heat Index Chart

From NOAA web page

Computation of chart data
Numbers for the chart were computed using equations presented in the article "Heat Balance in the Human Body", a web page on the University of British Columbia's website. Assuming that the human body is in thermal balance with its surroundings, the cooling processes must balance the heat gains:
M + S = C + R + P
Where M is the metabolic rate needed to support minimal body processes plus muscle movement,
S is the heat energy absorbed from sunlight,
C is the body's cooling by convection,
R is cooling by radiant heat, and
P is cooling by perspiration.
As I've described, C and R can turn negative, heating the body rather than cooling it. My chart doesn't show "negative cooling", just the additional perspiration needed to balance it. There are some much smaller heat gains or losses by conduction through the soles of the feet, which are ignored here.

Metabolic rate: The heat generated by basic metabolic processes for an average human body, awake and at rest, is about 100 watts; sleeping, about 70. When walking, about 325 watts is generated;; when running or bicycling fast, between 450 and 800 watts.

Sunlight: The full power of the sun's beam is about 1000 watts per square meter. Assuming that only half a square meter of the body's total 1.8 square meter area is exposed to direct sun, and that half of that is covered with clothing, the power of the heat and light received from the sun's beam is about 250 watts.
S = (0.5)(1000 W/m**2)(0.5 m**2) = 250 W
We assume no heat gain from sunlight falling on clothing, which may not be correct. I'll revisit this point in a later post.

Convection: Cooling or heating by convection is proportional to the temperature difference between the skin and the surrounding air, and to the total area of skin. An empirical factor relates their product to the wind speed.
C = (K)(Area)(skin T - air T)
where T is in degrees Kelvin. Normal skin temperature is about 33 deg C (91.4 deg F) = 306 deg K.
K is 3 W/m**2/deg K in calm air, or 26 W/m**2/deg K with a 2 m/sec wind (4.47 mph)
Surface area of an average human body is about 1.8 square meters.

Radiation: Cooling or heating by thermal radiation is proportional to the difference in the fourth power of temperature, in deg K, between the skin and the surface(s) radiating heat towards it. This implies an average temperature for the surfaces surrounding the body, called the Mean Radiant Temperature. For indoor measurements, this is often assumed to be the same as air temperature, and this assumption is used in the chart.(Some more notes on radiant hear below.) We also assume that the clothed and unclothed regions of the body have same skin temperature.
R = (Area)(e)(S)((skin T)**4 - (air T)**4)
e is the emissivity of human skin, about 0.98
S is the Stefan-Boltzmann constant, 5.67 x 10**-8 W/m**2/deg K**4
(Although the temperature differences work out to huge numbers, the resulting curve of R is almost a straight line in the range of temperatures shown on the chart; you can see a very slight curvature.)

Perspiration: The heat lost by perspiration can be calculated, as shown in the UBC article. But rather than calculate it, for the chart I assume that any excess of heat load over cooling by convection and thermal radiation is made up for by perspiration.

Note on Light vs Radiant Heat:
It's all radiation, of course, but the wavelengths are very different. Imagine an electric stove burner which you have just switched on. At first it's not too warm to touch, then a little too warm, and holding your hand a few inches above you can feel the radiant heat. It's in long wavelengths, the ones typically given off by objects and creatures on earth. As the burner warms, you feel the heat become more intense. Just before it begins to glow, you feel intense heat, from shorter wavelengths like the ones in the sun's beam. About half the energy of the beam is in these wavelengths. When the burner begins to glow, it is starting to radiate in visible wavelengths, emitting light rather than heat. About forty per cent of the sun's energy comes in these wavelengths, most of them much shorter than the stove burner emits, sensed as very bright light. A small percentage comes as still shorter wavelenghts, not visible as light: ultraviolet, the wavelengths that cause sunburn.


Post a comment...
All comments are moderated. I may answer flame mail directly, but will not post it unless it makes a good point.

University of British Columbia, Physics Department, "Heat Balance in the Human Body,"
..estimates heat load and cooling effects on the human body, with full calculations and an example..

Cornell University, student downloads, "DEA 3500 Notes: Thermal Regulation"

Korey Stringer Institute, U. of Connecticut,"Heat Acclimatization",

Human Performance Resource Center, "How can heat acclimatization prevent heat illness?",

Tan, C.L, Wong, N.H. and Jusuf, S.K., "Outdoor mean radiant temperature estimation in the tropical urban environment", Building and Environment · June 2013

..diagrams in this paper show Mean Radiant Temperature approximating air temperature at night and in shade..

Shashua-Bar, L, Perlmutter, D. and Erell, E., "The influence of trees and grass on outdoor thermal comfort in a hot-arid environment" INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 31: 1498–1506 (2011)
..infrared photography of test site shows radiant temperature of various surfaces..

ACE Study Examines Effects of Bikram Yoga on Core Body Temps April 21, 2015

..mentions "standard" of 105 deg F, 40% humidity, limited water breaks..,"New Study Finds Bikram Yoga Poses Potential Threat of Causing Heat Related Illness,"

Revisions to this post:
Aug 16: Revised cooling chart, minor rewording.
Aug 21: Added notes on chart calculations. Recomputed chart, corrected 100 watt error in heat load used on earlier version.

Heat: the Feeling and the Science
I've been griping about this unusually hot Florida summer, but after three days in Las Vegas I will complain no more. Temperatures were above normal there, too, in this unusually hot summer of an unusually hot decade.

It's a dry heat, they say, and the Heat Index confirms it. When the air temperature reached 113 Thursday afternoon, it only felt like 106. Whoopee.

Shade gave no relief from the heat. Our hotel entry yard, permanently shaded by a roof, felt oppressively hot. Twenty feet underground, on the lowest level of the parking garage where the sun never shines, it was hot.

The hotel's pools were crowded with guests trying to cool off. Everyone wore shoes to the edge of the pool to avoid putting bare feet on the scalding pavement. Attendants offered everyone two towels- one to dry off with, the other to spread on your lounger, which was too hot to touch. The water felt just a shade cooler than a heated pool.

The misters around the roof of the poolside bar were highly effective, unlike those at Florida theme parks. Droplets never reached table level, evaporating instantly into a jet of cool air.

Even the wind felt hot. At dinner on the pool patio- 108 degrees two hours after sunset- the occasional gust from dry thunderstorms nearby felt like a hot blast from a space heater.

Walking around the strip at night- about 100 degrees at 11 PM- I was surprised to see beggars sitting along the walkway with their signs and cups. How could homeless people survive in this heat?

Some of them live underground. Las Vegas has a network of storm drains, many big enough to walk in, where it can be 20 degrees cooler than the surface. Some homeless live in the drains; some duck in during the day when the heat in their tents and shanties is too much. They enjoy a cooler environment than the tourists, though even moderate rain can make a storm drain into a death trap.

Storm drain entrances, Las Vegas. From, permission requested.

What I actually sensed as "heat" was not the temperature, but the my body's interaction with the air and nearby surfaces. The body needs to lose heat constantly to maintain its 98.6 degree temperature. At moderate air temperatures, it cools by conduction to the surrounding air and even by convection- as if little cumulus clouds floated up from the skin. But when the air temperature exceeds body temperature, convection stops, and conduction begins to warm the body.

Another part of the sensation is radiant heat. The body radiates heat, and if it's warmer than most of its surroundings, it loses more radiant heat than it gains. But walk on a hot asphalt pavement, and you feel the difference: you're gaining more radiant heat than you lose.

The sun's beam is a strong source of radiant heat as well as light, and at moderate temperatures you feel cooler in the shade. It can be quite comfortable at 90 degrees in shade with a light breeze, if you haven't trained your body to expect air conditioning.

But in full sun you feel the radiant heat, as well as heat from the sunlight- absorbed by your skin and clothing and transformed into heat.

With all earth's climates growing hotter, and air conditioning becoming more expensive as energy costs rise, we'll all need to deal with more heat in the future. It's something worth thinking about. My next post will review some techniques for living in hot weather without air conditioning.

August 7, 2016
Mr. Wise:
Several years ago, I came close to moving to Las Vegas. A close friend was an editor at the Las Vegas Sun and there was a window for me to become a local columnist, with my friend's support. It turned out the office politics didn't unfold our way.

During the time I was considering it, though, I spent some time in Vegas, and I discovered the heat was dramatically different from what I experienced in Florida. The lack of humidity made human sensitivity to heat quite tricky. I could never quite tell how to stay properly hydrated outdoors.Also, the aridity of the area made for shocking differences from Florida. It was very clear that without constant engineering intervention in the water supply nothing would flourish for a large population. Returning to the area about 10 years later, I was a little frightened -- more people and cars; less available water; rising heat levels.

Since I've stayed in Florida, I'm becoming alarmed about the sure and certain rising sea levels. In about 50 years, we're going to have serious flooding from I-4 south. Miami Beach is already having serious flood control difficulties. My home city of Tampa is making some plans -- changing regulations for new housing; reinforcing some coastal areas -- but we're still building roads that will certainly go underwater in heavy storms within a few years.

Given the huge changes that are sure to come, we'll need to invest heavily in how to deal with a new landscape. I know that will create investment opportunities. I wish I could see investors getting organized in ways that will make the changes solvable and profitable, especially since the public appetite for taxation for public improvements is at a low point.

I can't help but feel that many of the solutions will be applicable to both Nevada and Florida, despite their differences. Some young entrepreneur will do well if he or she steps up with a strong voice and a plausible plan.

George Meyer
Tampa, Florida

Thanks for your thoughts. I don't see how Las Vegas can cope, in the long term, with rising energy costs and a prolonged drought in the southwest. The water level in Lake Meade is already near its minimum. If blackouts or brownouts start to happen, those folks in the storm drains may have a lot of company.

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Revisions to this post:

A Whiff of Sulphur
A correction, with some background on the biggest corporate tax break of all

Many of the shopping centers, office parks and industrial parks around us were built during a multi-decade surge in commercial construction starting in 1954, when Congress revised the tax code to permit “accelerated depreciation”. In effect, any new commercial building became a tax shelter. Owners and investors could take huge tax deductions in the first few years of a building’s life, making income from the building tax-free and often sheltering income from other sources. Accelerated depreciation fed a boom in shopping center construction, to the detriment of retailers in urban business districts. It was one of the main drivers of the century-long decline of independent retail.

A non-expert observer might think that accelerated depreciation was still fueling this boom. I did. Developers continue to build suburban shopping centers. Uptown business districts decay unless deliberately redeveloped or “gentrified”. Stores sit vacant in older shopping centers while new centers go up nearby. Accelerated depreciation still provides the biggest corporate tax break, in terms of total dollars saved. And in my personal experience, entering tax data for my rental property always leads me to the Modified Accelerated Depreciation System (MACRS). “Accelerated” is its middle name.

Yet MACRS is so strongly Modified it negates its Accelerated tag, at least for real estate. It depreciates commercial real estate on a straight-line basis over 39 years, a hairs-breadth faster than the 40-year straight-line depreciation used prior to 1954. Enacted in 1986, MACRS removed the strong tax incentive that biased developers and investors toward new construction.

In Standing Alone: The Independent Retailer in America , I wrote that the tax code ought to be revised “ put renovations and major repairs on an equal footing with new construction.” I stand by that opinion, but I must thank the very prescient members of the 99th U.S. Congress for addressing my concern long before I was aware of it. Which is a little embarrassing for me, but encouraging for the future of independent retail.

Still more encouraging is an evolving set of tax rules introduced in 2010 with incentives for improvements to existing stores and restaurants. These expenses can be claimed as depreciation on a 15-year schedule, yielding deductions even more generous than those allowed on new construction during the 1960s-70s.

The same rules apply to new restaurant construction and to improvements in leased property (such as the build-out required to open a store in a shopping mall.) An unrelated rule allows theme parks to be depreciated over 12.5 years.

When improving an existing store or restaurant, part or all of the expense can be claimed in the first year, like an ordinary repair, under section 179. The line between repair and improvement blurs; IRS advises that if a repair is “a betterment to the property” or “to restore the property”, it can be depreciated as an “improvement”. The property owner can tune these options depending on how fast his business income will repay the expenses.

So today we have strong tax incentives to repair and improve existing buildings. The bias toward new construction has been reversed, except for new restaurant buildings and theme parks.

(WARNING: Please don’t take any part of the this discussion as tax advice—talk to your tax advisor instead. If “the devil is in the details,” U.S. tax law is an earthly hell teeming with multifarious detail. This post can convey no more than a mild whiff of sulphur.)

And a Note: Minor changes were made to the book text on June 21, 2016 to reflect the above information. Any copies printed after that date will be correct on these points. Since this was not a full revision, the ISBN remains the same.

June 20
The rules on depreciation are complex and confusing - so much so that Senator Ron Wyden (D-OR), top Democrat on the Senate Finance Committee, has vowed to simplify and reform this issue.
- Rose Page, Tax Preparer
Merritt Island, FL

Post a comment...
All comments are moderated. I may answer flame mail directly, but will not post it unless it makes a good point.

“U.S. Tax Policy and the Shopping-Center Boom of the 1950s and 1960s”, Thomas W. Hanchett, The American Historical Review, Vol. 101, No. 4 (Oct., 1996), pp. 1082-1110 (partial paywall)
Abbreviated versions of Hanchett's analysis can be found here and here, but only the journal article details the tax code revisions of 1986.

“Tax Breakdown: Accelerated Depreciation,” Committee for a Responsible Federal Budget, 2013

IRS Publication 946: How to Depreciate Property

Turbotax 2015 (I used the program to check various depreciation situations in a what-if tax return.)

Revisions to this post:
June 16: minor rewording
Aug 16: added note on manuscript changes

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