Wind Chill and Standard Operative Temperature

13.3 Wind Chill and Standard Operative Temperature

Figure 13.2 shows the required conductance for the prevailing temper- ature and wind conditions. Wind has a small effect on this required conductance. However the effect of wind on clothing conductance can

be large and must be taken into account when choosing the amount of clothing necessary to provide the required conductance. Equation (12.18) gives the wind speed dependence of animal coat con- ductance and also works well for clothing conductance. The permeability factor, in the equation, however, depends strongly on the fabric which the clothing is made. Table 13.2 shows c values for a range of

Humans and their Environment

T ABLE

13.2. Wind permeability factor, c (eq. 12.18) for a range of fabrics

Fabric

c Very open weave shirt

1.1 Knit cotton undershirt or T shirt

0.86 Average of 13 civilian shirts (broadcloth or oxford weave) 0.61 Light worsteds, gabardines, Seersucker suiting

Uniform twill, 8.2 oz. Army Poplin, 6 oz. Army Byrd cloth, wind resistant JO cloth, special wind resistant

Data from (1968)

materials, measured by the rate of evaporation through the fabric. Since these data are for effects of wind on vapor transport, they are not ideal for computing effects of wind on heat transport, but lacking more direct information we use these values for both heat and vapor. According

to these figures, a 10 wind would double the conductance of JO cloth, and a1 wind would double the conductance of a very open weave shirt. The effect of wind on clothing and boundary conductance is addressed by another thermal index, the standard operative temperature,

The standard operative temperature, like the operative temperature, combines several environmental variables into a single environmental index which has dimensions of temperature. The operative temperature combined ra- diation and air temperature into a single equivalent temperature. The standard operative temperature adds wind effects. Standard operative temperature is the temperature of a uniform enclosure with still air which would result in the same heat loss from an animal or person as occurs in the windy, outdoor condition under investigation. The popular term for

is the wind chill factor. To derive an equation for

start with the energy budget equation (12.11)). By definition, M - is the same for the person in the standard enclosure and the person in the natural environment. Therefore the following can be written:

where the subscripts on the operative temperature and the conductances indicate the standard (still air enclosure) conditions. Solving for

gives:

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

so the standard operative temperature and the operative temperature are the same. The second is that

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

Equation 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 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 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 C, and the wind speed were 10

then the wind chill temperature would be 17" C. This would mean that even though the air temperature is only

C, the outdoors would feel as cold to you as a room with still air would at -

C.