- Thus, if D = 60°,
- W = 55°,
- then dew point = 60 - {(60 - 55) 1·96}
- = 50°·2.
The dew-point may also be obtained by Apjohn’s formula; which for a pressure of about 30 inches is F = f - (D - W) ∕ 87
- D being dry and W wet bulb temperature,
- F elastic force of vapour corresponding to dew-point, and
- f, elastic force corresponding to wet bulb temperature (ascertained from a table of tensions).
The elastic force of aqueous vapour, i.e. the amount of barometric pressure due to the vapour present in the air is dependent upon the temperature of the dew-point. It is given for every tenth of a degree of temperature in Table VI. (p. 42) of Marriott’s Hints.
The relative humidity is a term expressing the percentage of saturation of the air with water vapour. It is obtained from Table VI. (above) as follows:—
| Relative |  | = | Elastic force of water vapour at the temperature of the dew-point |
| Humidity | Elastic force of water vapour at the temperature of the air (i.e. the dry-bulb reading.) |
- Thus elastic force with dry bulb = 55° is ·433 in. in the table.
- Thus elastic force with dew-point = 46°·5 is ·317 in.„
- ·317 ∕ ·433 = ·73.
- If saturation = 100, relative humidity is 73.
In Table VII. of Marriott’s Hints, a table is given which enables the relative humidity to be found by mere inspection. Thus if the dry bulb temperature is 58°·5, wet-bulb 51°·7, and the difference 6°·8, the relative humidity given in the table is 62.
Fig. 54.
Snowdon Pattern Rain-Gauge.
A. Copper Upper Part of Gauge. B. Funnel. C. Bottle. To the right is shown the glass measure inverted.