The Humid Operative Temperature
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the body temperature. Adding clothing therefore allows the person to
tolerate a hotter environment. In a desert, with low vapor pressure and
high solar loads, adding clothing (up to a point) decreases, rather than
increases heat load on a person. The inflection point of the graph occurs
when coat conductance becomes small enough to start controlling water
loss.
Keep in mind that Fig. 13.5 is for a low vapor pressure. It does not apply
at higher vapor pressure where any decrease in clothing conductance
would reduce latent heat loss. If the atmospheric vapor pressure is high
enough to keep the skin wet without clothing, then any addition of clothing
will decrease dissipation of heat. This brings out the point that clothing
must be matched to environment to be most useful in minimizing heat
stress. Proper clothing for one hot environment would not necessarily be
proper clothing for another.
13.5 The Humid Operative Temperature
Much effort has gone into deriving a single index that will indicate environmental heat stress for humans. The environmental variables that
affect heat stress are radiation, temperature, vapor pressure, and diffusion conductances to heat and vapor. For cold stress, where latent heat
loss is treated as a fixed value relatively independent of environment,
the standard operative temperature adequately combined radiation and
heat transfer characteristics of the environment into a single number. An
appropriate energy budget equation was then used to indicate the strain
imposed by a given stress, the stress being indicated by the operative
temperature. It would seem reasonable to attempt to extend the operative temperature concept to include atmospheric vapor pressure. If this
could be done, it would again enable the combination of all relevant environmental variables into a "stress index," and with an appropriately
derived energy budget equation, could indicate the resulting strain on the
individual.
The derivation proceeds in a way similar to the derivation of the operative temperature in Ch. 12: substitute Eq. (12.16) for hEs and Eq. (12.19)
for T, into Eq. (13.7) to get an energy budget equation in terms of physiologic and environmental variables. The vapor mole fraction difference
in Eq. (12.16) can be approximated using the Penman transformation
(discussed in detail in Ch. 14) to give:
e, - e, = es(T,) - e, (T,) + e, (T,) - e, 2 A(T, - T,) + D (13.8)
where A is the slope of the saturation vapor pressure versus temperature
function (Ch. 3) and D is the vapor deficit of the atmosphere. The slope
A has a fairly strong temperature dependence. If its value is taken at the
average of Ts and T, then Eq. (13.8) is almost exact. Taking A at the
average of Tb and T, gives adequate accuracy for our purposes here. With
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the body temperature. Adding clothing therefore allows the person to
tolerate a hotter environment. In a desert, with low vapor pressure and
high solar loads, adding clothing (up to a point) decreases, rather than
increases heat load on a person. The inflection point of the graph occurs
when coat conductance becomes small enough to start controlling water
loss.
Keep in mind that Fig. 13.5 is for a low vapor pressure. It does not apply
at higher vapor pressure where any decrease in clothing conductance
would reduce latent heat loss. If the atmospheric vapor pressure is high
enough to keep the skin wet without clothing, then any addition of clothing
will decrease dissipation of heat. This brings out the point that clothing
must be matched to environment to be most useful in minimizing heat
stress. Proper clothing for one hot environment would not necessarily be
proper clothing for another.
13.5 The Humid Operative Temperature
Much effort has gone into deriving a single index that will indicate environmental heat stress for humans. The environmental variables that
affect heat stress are radiation, temperature, vapor pressure, and diffusion conductances to heat and vapor. For cold stress, where latent heat
loss is treated as a fixed value relatively independent of environment,
the standard operative temperature adequately combined radiation and
heat transfer characteristics of the environment into a single number. An
appropriate energy budget equation was then used to indicate the strain
imposed by a given stress, the stress being indicated by the operative
temperature. It would seem reasonable to attempt to extend the operative temperature concept to include atmospheric vapor pressure. If this
could be done, it would again enable the combination of all relevant environmental variables into a "stress index," and with an appropriately
derived energy budget equation, could indicate the resulting strain on the
individual.
The derivation proceeds in a way similar to the derivation of the operative temperature in Ch. 12: substitute Eq. (12.16) for hEs and Eq. (12.19)
for T, into Eq. (13.7) to get an energy budget equation in terms of physiologic and environmental variables. The vapor mole fraction difference
in Eq. (12.16) can be approximated using the Penman transformation
(discussed in detail in Ch. 14) to give:
e, - e, = es(T,) - e, (T,) + e, (T,) - e, 2 A(T, - T,) + D (13.8)
where A is the slope of the saturation vapor pressure versus temperature
function (Ch. 3) and D is the vapor deficit of the atmosphere. The slope
A has a fairly strong temperature dependence. If its value is taken at the
average of Ts and T, then Eq. (13.8) is almost exact. Taking A at the
average of Tb and T, gives adequate accuracy for our purposes here. With