Relative Humidity - DAVIS Vantage Pro User Manual & Setup Manual

Relative Humidity

Humidity itself simply refers to the amount of water vapor in the air. However,
the amount of water vapor which the air can contain varies with air tempera-
ture and pressure. Relative humidity takes into account these factors and offers
a humidity reading which reflects the amount of water vapor in the air as a per-
centage of the amount the air is capable of holding. Relative humidity, there-
fore, is not actually a measure of the amount of water vapor in the air, but a
ratio of the air's water vapor content to its capacity.
It is important to realize that relative humidity changes with temperature, pres-
sure, and water vapor content. A parcel of air with a capacity for 10 g of water
vapor which contains 4 g of water vapor, the relative humidity would be 40%.
Adding 2 g more water vapor (for a total of 6 g) would change the humidity to
60%. If that same parcel of air is then warmed so that it has a capacity for 20 g
of water vapor, the relative humidity drops to 30% even though water vapor
content does not change.
Relative humidity is an important factor in determining the amount of evapo-
ration from plants and wet surfaces since warm air with low humidity has a
large capacity for extra water vapor.
Vantage Pro Manual
T A1: H C E
ABLE EAT AND OLD FFECTS
Weather Conditions Measured/Calculated
Page 49