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The comfort zone can be plotted on the psychrometric chart by the following procedure:

1. Find the annual mean temperature, Tav
   This is done by adding up, all the average dry-bulb temperature readings taken through the year (by the month) and dividing the total by the number of months (12).

   
   In our example, Tucson, we will take the 12 average values given in the climatic data under 'Monthly Average Hourly Dry Bulb Temperatures [øF]:' in the last page.
   

   
   Tav =        50.3 + 40.2 + 43.3 + 58.0 + 75.4 + 89.1 
               + 85.0 + 82.7 + 81.0 + 65.7 + 62.3 + 54.3
               __________________________________
   			 12

   Tav = 65.60 °F or 18.67°C

   
   

2. Find the thermal neutrality (Tn)
   Thermal neutrality is the temperature averaged for a large sample of people, when the individuals feel neither cold nor hot. It has been shown conclusively that this thermal neutrality is influenced by the climate, which the individuals are used to (a result of physiological acclimatisation, but also of habits) and correlates with the outdoor mean temperature (Tav) as

   Tn = 17.6 + [0.31 x Tav]
   provided 18.5 < Tn < 28.5°C

   In the case of Tucson,

   Tn = 17.6 + [0.31 x 18.67]
   Tn = 23.38°C (74.08°F)

   
   

3. Find the lower (L) and upper (U) limits of Tn
   

   Lower limit L = Tn - 2 = 23.38 - 2 L = 21.38

   Upper limit U = Tn + 2 = 23.38 + 2 U = 25.38

   
   

4. Find the SET slope expressions
   Over the last 50 years or so, numerous thermal indices have been constructed, to express in a single number the combined thermal effect of at least some of the four environmental variables. The latest one (as of 1987) is the standard effective temperature (SET), which combines the effect of DBT and humidity, when the MRT is the same as the DBT and there is no significant air movement. The SET lines are given as follows:
   

   * Up to 14°C, SET lines coincide with the DBT (vertical) lines.
   * Above 14°C, the SET coincides with DBT at the 50% RH curve, but the lines have a slope of 0.025 x (DBT - 14) for each g/kg (AH) vertical distance.

   The slope expressions become:
   

   For lower limit L 0.025 x (21.38 - 14) = 0.1845 degC/(g/kg)

   For upper limit U 0.025 x (25.38 - 14) = 0.2845 degC/(g/kg)

   
   

5. Find the base line intercepts for L and U from the SET slope expressions
   Base line intercept for L = L + [ AH(L) x 0.1845)]
   L_{base intercept} = 21.38 + [8 x 0.1845] = 22.85°C

   Base line intercept for U = U + [ AH(U) x 0.2845)]
   U_{base intercept} = 25.38 + [10.2 x 0.2845] = 28.28°C

Table of Contents

Background Preparation

Units & Dimensions

Psychrometric Chart

Collection of the Data
Plot the Comfort Zone

The comfort zone developed above means that if ambient conditions (outside shade-temperature and humidity) are within this zone, then, in effect, no building is required; that is; one would be comfortable under a shade tree or tent. (Althought these zones are shown by distinct lines, it should be remembered that these are generalized averages and should not be read to mean that a variation of one or two degrees Celsius or of a few percent relative humidity would place one either inside or outside the bounded condition.)

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