7/29/2019 - Biometeorology
1/26
Biometeorology of Humans
in Desert Environments
Chapter 19
7/29/2019 - Biometeorology
2/26
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
7/29/2019 - Biometeorology
3/26
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.
7/29/2019 - Biometeorology
4/26
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
7/29/2019 - Biometeorology
5/26
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.
7/29/2019 - Biometeorology
6/26
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
7/29/2019 - Biometeorology
7/26
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
7/29/2019 - Biometeorology
8/26
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.
7/29/2019 - Biometeorology
9/26
Rate
ofHeat
Gain
7/29/2019 - Biometeorology
10/26
How might perspiration rate be
estimated?
7/29/2019 - Biometeorology
11/26
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.
7/29/2019 - Biometeorology
12/26
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.
7/29/2019 - Biometeorology
13/26
7/29/2019 - Biometeorology
14/26
Fluid
balance
7/29/2019 - Biometeorology
15/26
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.
7/29/2019 - Biometeorology
16/26
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
7/29/2019 - Biometeorology
17/26
7/29/2019 - Biometeorology
18/26
Maintaining the Heat Balance
QM + QLR + QSR + QH + QE + QG = QS
7/29/2019 - Biometeorology
19/26
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
7/29/2019 - Biometeorology
20/26
7/29/2019 - Biometeorology
21/26
High work rate
Low work rate
7/29/2019 - Biometeorology
22/26
Deaths per summer day in Shanghai
Threshold
A li ti ti t D t H t
7/29/2019 - Biometeorology
23/26
Acclimatization to Desert Heat
- Worked 100 min at 120 F -
7/29/2019 - Biometeorology
24/26
Physical Effects of Heat Stress
and Dehydration
7/29/2019 - Biometeorology
25/26
Survival daily water requirement,
July, acclimatized person at rest
7/29/2019 - Biometeorology
26/26
Number of Days Without Water For Which
It is Possible to Walk At Night
Top Related