|
E. Linacre and B. Geerts |
10/'97 |
The standard effective temperature was introduced in Note 6.G. This temperature considers all environmental and bodily conditions that affect human thermoregulation. Using similar arguments, Steadman (1984) contrived a scale of ‘apparent temperatures’ experienced by people exposed to different conditions (1). The apparent temperature is defined as the dry-bulb temperature for thermal equilibrium of an adult walking, assuming moderate humidity and neither wind nor solar radiation, and with the same thermal resistance between the skin and the atmosphere as in the given circumstances.
The temperature is based on a consideration of the effects of the following on the thermal resistance: exercise, the respective effects of clothing on moisture loss and heat loss, ambient temperatures of -40 to + 50°C, atmospheric vapour pressure, direct solar radiation (Qd), diffuse solar radiation (regarded as 14% of Qd), the upwards terrestrial radiation, the fraction of the body covered by clothing, the degree of wind penetration into the clothing, the effect of temperature on the thermal conductivity of the clothing, the advective transfer of heat from the surface, etc. The ground’s albedo was assumed to be 20%. An elaborate series of expressions takes all these factors into account in calculating the ‘apparent temperature’ experienced by the individual.
In the second part of Steadman’s paper, values of this apparent temperature were derived in 120 sets of circumstances, and the values were then compared in a multiple regression analysis with the concurrent ambient shade temperature (T in ° C), vapour pressure (P in kPa), windspeed (v, in m/s, at 10 m height) and solar radiation (Qs W/m2). This provided much simpler empirical expressions for calculating the apparent temperature, either indoors (Tp), in the shade outdoors (Tpv) or exposed to the Sun (Tpvg). In each case, 95% of the errors involved in using the simpler expressions below were less than 1 K.
|
indoors |
Tp = - 1.3 + 0.92 T + 2.2 P |
° C |
|
outdoors, in the shade |
Tpv = - 2.7 + 1.04 T + 2.0 P - 0.65 v |
° C |
|
outdoors, in the sun |
Tpvg = - 1.8 + 1.07 T + 2.4 P - 0.92 v + 0.044 Qs |
° C |
Thus, for example, the apparent temperature is 24.5° C in the shade outdoors, in a breeze of 3 m/s, with air at 25° C and 50% relative humidity (i.e. P is 15.8 hPa or 1.6 kPa). However the apparent temperature increases to 26.4° C when the relative humidity increases to 80%, ceteris paribus.
References
(1) Steadman, R.G. 1984. A universal expression of apparent temperature. J. Appl. Meteor. 23, 1674-87.