- Biometeorology

7/29/2019 - Biometeorology

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Biometeorology of Humans

in Desert Environments

Chapter 19


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The Heat Balance

of the Human Body - Components

Longwave radiation gain or loss

Conductive and convective exchange with the

atmosphere gain or loss Metabolic source - gain

Direct, diffuse, and reflected solar radiation -

gain

Evaporation loss

Conduction with the ground gain or loss


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The Energy Conservation

Equation

QM + QLR + QSR + QH + QE + QG = QS

QS is the net rate of heat gain or loss asa result of an imbalance in the terms.

QS not equal to zero means that the

body temperature will increase or

decrease.

QSmust remain small, or be large for

only a short time, in humans because of

the small range in tolerable body

temperatures.


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Long-wave (Infrared) Radiation

Emitted by the skin surface- IR (lost) intensity = Tskin

4, with = Tskin~

92 F, 33.3 C

- clothing absorbs some of the emitted IR and re-

emits it back to the skin and to the environment. Absorbed by the skin surface

- IR (gained) intensity = Tenviron4, where Tenviron

applies to the atmosphere, the ground, etc.

- clothing absorbs some of the IR from theenvironment, and emits it to the skin and back tothe environment

Whether there is a net gain or loss by the skin

depends on Tskin versus the average Tenviron


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Metabolic Source of Heat

Body at rest generates about 80 Cal of heatper hour enough to heat 1 liter of ice waterto boiling point.

The greater the level of exercise, the moreheat generated normal to fast walkgenerates ~280 Cal per hour.

Over a typical work day, the metabolicenergy generated in a day would be about 3times that of a person at rest.


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The greater the

activity level, and

the greater the body

weight, the greaterthe perspiration rate

required to maintain

a heat balance.

Surrogate for

rate of heat gain


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Conductive and Convective Exchange of

Heat Between the Skin and the Atmosphere

In a very shallow layer above the skinsurface, conduction is responsible for theheat exchange the direction of heat

transfer is from hot to cold. Outside this layer, convection transfers the

heat.

This is analogous to how heat is transferredbetween the ground surface and theatmosphere


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Thus, heat is gained by the body if the airtemperature is greater than the skin surface

temperature. A shallow boundary layer forms over the skin

surface, with this thin boundary-layer air having atemperature near the skin temperature reducesheat transfer.

Wind strips the boundary layer away from the skin,increases the temperature contrast, and increasesthe heat transfer.

Thus, heat is gained most rapidly on windy days

with air T over ~92 F, when no clothes protect theboundary layer from the wind.

Note parallel with the concept of wind-chill factor.


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Rate

ofHeat

Gain


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How might perspiration rate be

estimated?


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Evaporation

Takes place in the respiratory system and from theskin.

Respiratory evaporative cooling is responsible forloss of only ~25% of metabolic heat generated inresting person. In a cool environment, the rest ofthe heat is lost through conduction and radiation.

In the hot desert where conduction/convection, IRradiation, and solar radiation all represent heat

gains, or when metabolic heat generation is high,perspiration from the skin is the main/only way thatthe body loses heat.


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Rate of Evaporation From the Skin

Depends On

Humidity

Wind speed

Temperature

In the desert, these conditions are favorable enough for

evaporation that we often dont see any moisture on the skin.


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Fluid

balance


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Example of the Efficiency of Evaporation

For Heat Regulation - Sauna

Dry air that is heated to ~110 C (230 F)

Pan of water taken into this environment will boil,and a steak will cook in the amount of time that aperson is in the sauna.

Why doesnt the body overheat? rapidevaporation into the hot, dry environment.

Evidence of evaporative loss of heat? Blow on the skin to remove the boundary layer blister

forms immediately Nostrils become raw

Water thrown on hot rocks in sauna makes it feel hotterbecause it reduces evaporation rate.


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Solar Radiation

Direct, Diffuse, Reflected

Amount absorbed by the body depends

Sun angle

Cloud cover Dust content of atmosphere

Albedo of surface, and slope

Position of the body

Clothing


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Maintaining the Heat Balance

QM + QLR + QSR + QH + QE + QG = QS


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What the Body Does To Maintain The

Heat Balance in Hot Environments

Increase perspiration

Dilation of arteries to increase capillary blood flow

at the skin surface (if air T is less than blood T)

But, even with unlimited water consumption,

strenuous physical activity in the desert can cause

dangerously high body temperatures

heat is not transported fast enough from the interiorof the body to the surface where the heat is lost

not enough perspiration is generated to produce the required

cooling


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High work rate

Low work rate


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Deaths per summer day in Shanghai

Threshold

A li ti ti t D t H t


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Acclimatization to Desert Heat

- Worked 100 min at 120 F -


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Physical Effects of Heat Stress

and Dehydration


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Survival daily water requirement,

July, acclimatized person at rest


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Number of Days Without Water For Which

It is Possible to Walk At Night

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