Moisture and temperature interact, and onecontrols the other. As temperature changes, so does the amount of evaporationand moisture, or humidity, in the air. Thus, temperature, evaporation andmoisture are interrelated environmental phenomena. Humidity increases astemperatures cool and air approaches its dew points. The dew point is thetemperature at which the atmosphere becomes saturated, and knowing it iscritical to being able to measure humidity.
Understanding the relationship betweentemperature and moisture requires a method to measure humidity. Relativehumidity (RH) expresses the amount of moisture in the air. It is calculated bythe percentage of actual humidity divided by how much moisture the air can holdat a given temperature. The result is the RH. For example, an RH of 50 percentmeans that the air contains half of the moisture it could contain at thecurrent temperature.
When air becomes saturated and condenses,it has reached its dew point. The dew point is calculated by cooling the airwhile at the same time accounting for evaporation. The temperature at which thecooled air reaches 100 percent of RH is its dew point. For example, if the RHat 21 degrees Celsius (70 degrees Fahrenheit) is 40 percent, its dew point willbe reached when the air is cooled to 7 degrees Celsius (44 degrees Fahrenheit).At this temperature the air will condense and “dew” will form.
Evaporation is the amount of water that isconverted to vapor, which then rises into the atmosphere. When RH is low,evaporation will increase because the air can hold more water vapor. Cooler airwill decrease the rate of evaporation since it reaches its dew, or saturationpoint, quicker. Conversely, warming the air will lower RH and diverge it fromits dew point. This is why a furnace in a home during the winter willdramatically lower RH in a house since inside air is warmed many degrees higherthan the cold air outside.
A psychrometer is a device that measuresRH. It has two thermometers, a dry bulb and a wet bulb. The dry bulbthermometer measures the current air temperature. The wet bulb thermometer issaturated and allowed to cool by evaporation. This evaporative cooling willprovide the dew point temperature. The dew point will be the coolesttemperature read by the wet bulb thermometer. A lower RH will evaporate the wetbulb more quickly. A humidity chart or calculator can then be used to calculateRH.