Psychrometric Tables
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Computation of Psychrometry Tables

The psychrometric equation can be written in the form:

e_w - e_D = a p (T - T_w)

where e_w and e_D represent the saturation vapour pressures of liquid water at the wet-bulb and dew-point temperatures respectively, p represents the pressure, T and T_w represent the dry-bulb and wet-bulb temperatures respectively, and a represents a quantity generally determined experimentally and having different values for a ventilated and for a non-ventilated psychrometer. With temperatures in degrees Celsius, a becomes 6.4309 * 10^-4 (°C)^-1 for a ventilated and 7.7170 * 10^-4 (°C)^-1 for a non-ventilated psychrometer.

The values of a in the above paragraph apply when the temperature of the wet-bulb thermometer is 0°C or higher. If the wet-bulb is frozen (i.e., has become an ice bulb), the psychrometric equation becomes:

e_i - e_D = a_i p (T - T_i)

where e_i represents the saturation vapour pressure with respect to ice at the ice bulb temperature T_i and a_i represents a new value of the quantity a applicable to this condition. The quantity eD represents, as before the saturation vapour pressure of liquid water (supercooled) at the dew-point temperature, since by international agreement, both dew-point temperature and relative humidity are defined with respect to liquid water at ALL temperatures. With ice bulb temperatures below 0°C, a_i becomes 5.6759 * 10^-4 (°C)^-1 for a ventilated psychrometer, and 6.8112 * 10^-4 (°C)^-1 for a non-ventilated psychrometer.

Tables for use at stations at elevations less than 305 metres are calculated using a pressure of 1010 hectopascals, for stations at elevations of 305 metres to 760 metres, 950 hectopascals, and for stations more than 760 metres, 890 hectopascals.

Goff-Gratch vapour pressures with respect to water and ice are employed.

Values of relative humidity are determined from the equation:

Relative Humidity = 100 * (e_D/e_T)