Survival in Hot Environments
215
There are two important things to note about Eq. (13.5). First, when
the conductances are equal to the standard values, then Tes = Te, so
the standard operative temperature and the operative temperature are the
same. The second is that Tes = Te = Tb for an ectotherm which does not
control body temperature by internal heat production. People sometimes
assume that, since wind makes us cold, it also makes plants, snakes, and
spiders cold. It can be seen from Eq. (13.5) that this is not true (except to
the extent that wind reduces T,).
Equation (1 3.5) can be used to derive a wind chill chart similar to those
used by the weather service. To simplify this we assume that the clothing
conductance is low enough so that the boundary layer, tissue, and radiative
conductances can be ignored. We also assume that the wind dependence
of clothing conductance is given by Eq. (12.18). Equation (13.5) then
becomes:
Comparison of this equation with the wind chill chart given by Landsberg
(1969) indicates that the value of c used for wind speeds below about
10 m/s is 0.046. This is lower than even the most wind resistant fabrics
given in Table 13.2, so it apparently assumes a best case. Wind chill with
more permeable clothing would be more serious than the standard chart
indicates. Landsberg's chart shows little change in wind chill at wind
speeds above 10 d s . This may be the result of his basing the wind chill
relation on boundary-layer conductance, which increases in proportion to
the square root of wind speed, rather than on the permeability of clothing,
which increases more nearly linearly with wind speed. Figure 13.3 shows
wind chill temperature (standard operative temperature) as a function of
operative temperature (near air temperature for these wind conditions)
for three values of wind speed.
To use Fig. 13.3, enter the chart at the air temperature, go to the wind
speed, and read off the wind chill temperature. For example, if the air
temperature were 0" C, and the wind speed were 10 d s , then the wind
chill temperature would be - 17" C. This would mean that even though the
air temperature is only O" C, the outdoors would feel as cold to you as a
room with still air would at - 17" C.
13.4 Survival in Hot Environments
The same considerations apply to survival in hot environments as do for
determining the upper lethal limit of animals. However, one additional
factor needs considere&-that of sweating. The rate of sweat evaporation
may be either environmentally or physiologically controlled. If the skin
surface is wet, the rate of water loss from sweating is given by Eq. (12.16)
with g,, + oo. If the skin surface is not wet, latent heat loss is controlled
by sweat rate. Control of sweat rate is still not entirely understood, but
apparently it involves sensing of surface heat flux (Kerslake 1972). Thus,
changes in metabolic rate or external environment can cause changes in
215
There are two important things to note about Eq. (13.5). First, when
the conductances are equal to the standard values, then Tes = Te, so
the standard operative temperature and the operative temperature are the
same. The second is that Tes = Te = Tb for an ectotherm which does not
control body temperature by internal heat production. People sometimes
assume that, since wind makes us cold, it also makes plants, snakes, and
spiders cold. It can be seen from Eq. (13.5) that this is not true (except to
the extent that wind reduces T,).
Equation (1 3.5) can be used to derive a wind chill chart similar to those
used by the weather service. To simplify this we assume that the clothing
conductance is low enough so that the boundary layer, tissue, and radiative
conductances can be ignored. We also assume that the wind dependence
of clothing conductance is given by Eq. (12.18). Equation (13.5) then
becomes:
Comparison of this equation with the wind chill chart given by Landsberg
(1969) indicates that the value of c used for wind speeds below about
10 m/s is 0.046. This is lower than even the most wind resistant fabrics
given in Table 13.2, so it apparently assumes a best case. Wind chill with
more permeable clothing would be more serious than the standard chart
indicates. Landsberg's chart shows little change in wind chill at wind
speeds above 10 d s . This may be the result of his basing the wind chill
relation on boundary-layer conductance, which increases in proportion to
the square root of wind speed, rather than on the permeability of clothing,
which increases more nearly linearly with wind speed. Figure 13.3 shows
wind chill temperature (standard operative temperature) as a function of
operative temperature (near air temperature for these wind conditions)
for three values of wind speed.
To use Fig. 13.3, enter the chart at the air temperature, go to the wind
speed, and read off the wind chill temperature. For example, if the air
temperature were 0" C, and the wind speed were 10 d s , then the wind
chill temperature would be - 17" C. This would mean that even though the
air temperature is only O" C, the outdoors would feel as cold to you as a
room with still air would at - 17" C.
13.4 Survival in Hot Environments
The same considerations apply to survival in hot environments as do for
determining the upper lethal limit of animals. However, one additional
factor needs considere&-that of sweating. The rate of sweat evaporation
may be either environmentally or physiologically controlled. If the skin
surface is wet, the rate of water loss from sweating is given by Eq. (12.16)
with g,, + oo. If the skin surface is not wet, latent heat loss is controlled
by sweat rate. Control of sweat rate is still not entirely understood, but
apparently it involves sensing of surface heat flux (Kerslake 1972). Thus,
changes in metabolic rate or external environment can cause changes in