1. The temperature of the dew point.
2. The elastic force of aqueous vapour.
3. The relative humidity.
The dew point temperature is that temperature at which the outside air at the time the observation is taken will deposit the moisture contained in it. It is the temperature at which the air is saturated with moisture. It is calculated from the readings of the wet and dry bulb thermometers
(a) by Glaisher’s tables; (b) by Apjohn’s formula.
Glaisher’s tables are based on a series of numbers called Greenwich or Glaisher’s factors, which he determined by comparison between observations made with the dry and wet bulb thermometers and with Daniell’s hygrometer. The formula for using the factors is as follows:—
d = D - {(D - W) × f}
where d = dew point, D = dry bulb temperatures, W = wet bulb temperature, and f = factor.
The following examples are from Glaisher’s table of factors.
| READING OF DRY BULB THERMOMETER FAHR. | FACTOR. |
|---|---|
| 55° | 1·96 |
| 56° | 1·94 |
| 57° | 1·92 |
| 58° | 1·90 |
| 59° | 1·89 |
| 60° | 1·88 |