Survival in Cold Environments

13.2 Survival in Cold Environments

Equation (12.1 1) will be used as the basis for our examination of energy and thermal resistance requirements for humans. Consider

the lowest temperature at which a human can survive. This can be found by assuming extreme values for M,

E, and

If we assume d = m,

C then the lowest equivalent temperature for survival can be calculated for various resistances and metabolic rates. From Table A.3, with

. The boundary layer conductance is:

rnol

- - 1.4 x

= 0.79-.

m 2 s The convective-radiative conductance

0.17 m

= 0.16 + 0.79 = 0.95 rnol

These values are substituted into Eq. (12.1 along with the body temperature and latent heat loss, and the equation is solved for operative temperature to give:

This equation ignores a small temperature dependence of the radiative conductance and the metabolic rate and also assumes that skin latent

heat loss is independent of temperature. It does, however, show the main effects of

and on M. These are shown in Fig. 13.1 where M is plotted as a function of for three values of conductance. The highest value of M is for no clothing, the second is for a conduc- tance comparable to a heavy wool business suit, and the third is equivalent to a good quality winter sleeping bag. It can be seen that survival is pos-

sible at quite low temperatures, even without clothing, if metabolic rate can be kept high.

Humans and their Environment

-40 -30

0 10 20 30 40 Operative Temperature (C)

F I G U RE 13.1. Metabolic rate required to balance the energy budget at various operative temperatures, for three values of

Darwin (1832) made some interesting observations on survival among the natives of Tierra del Fuego under conditions which must have been near the limits of survival. He reports:

The climate is certainly wretched: the summer solstice was now passed, yet every day snow fell on the hills, and in the valleys there was rain accompanied by sleet. The thermometer generally stood about

but in the night fell to 38 or

40. ... while going one day on shore near Wallaston Island, we pulled alongside a canoe with six Fuegians. These were the most abject and miserable creatures I anywhere beheld. On the east coast the natives, as we have seen, have guanaco cloaks, and on the west, they possess sealskins. Amongst these central tribes the men generally have an otter skin, or some small scrap about as large as a pocket handkerchief, which is barely sufficient to cover their backs as low down as their loins. It is laced across the breast by strings, and according as the wind blows, it is

from side to side. But the Fuegians in the canoe were quite naked, and even one full-grown woman was absolutely so. It was raining heavily, and the fresh water, together with the spray, trickled down her body. In another harbor not far distant, a woman, who was suckling a recently-born child, came one day

alongside the vessel, and remained there out of mere curiosity, whilst the sleet

fell and thawed on her naked bosom, and on the skin of her naked baby! ... At night, five or six human beings, naked and scarcely protected from the wind and rain of this tempestuous climate, sleep on the wet ground coiled up like animals.

Perhaps a more useful way to plot Eq. (13.4) is to show the conductance required for different levels of activity and operative temperature. Using this graph the clothing thermal. conductance required for any given activity

Wind Chill and Standard Operative Temperature

10 20 Operative Temperature (C)

F I GURE 13.2. Thermal conductance required for survival in cold at various operative temperatures and activity levels.

and environment could be found. This is shown in Fig. 13.2. The following example illustrates the use of Fig. 13.2.

Example 13.1. Assume you are going outdoors when

C. Find the coat plus tissue conductance needed for standing, walking, and running.

Solution. The required conductances can be read directly from Fig. 13.2. If you intend standing for long periods of time at M

90 you would need

0.05 mol

For

you would need

0.1 mol

. And for

you would need 0.25 mol