This calculator shows the
inter-relationship of air temperature and moisture content with dew point and
wet-bulb temperature as measured by a psychrometric thermometer or sling
psychrometer.
The
amount of moisture air can hold is dependent on its temperature and pressure.
The warmer the air the greater the quantity of water vapor it can contain. The
air temperature is measured with a normal thermometer this is the Dry-Bulb
reading. The actual amount of moisture known as the mixing ratio
is measured in grams of water per kilogram of dry air. When air at a certain
temperature is saturated it cannot hold any more moisture. The relative
humidity of the air is the ratio of the actual amount of moisture in
the air to the saturated amount.
We
can use evaporation to measure the amount of moisture in the air. A wet cloth
is placed over the bulb of a thermometer and then air blown over the cloth
until the water evaporates. Since evaporation takes up heat, the thermometer
will cool to a lower temperature than a thermometer with a dry bulb at the same
time and place. The depression in Wet-Bulb temperature allows
the humidity to be calculated. If the air is fully saturated (100% relative
humidity) the water cannot evaporate, so both the wet and dry bulb temperatures
are the same. The calculator will show the saturation mixing ratio
in grams of water per kilogram of dry air of a given temperature, if the dry
and wet bulb temperatures are set to the same value.
If
partly saturated air is cooled without changing its pressure or amount of water
vapor, a point is reached when it becomes saturated. The moisture will be given
up as dew or ice crystals. This temperature is the Dew Point.
Condensation will form on a bottle of cold beer, if the air in its vicinity is
cooled below its dew point. Meteorological reports usually quote the
temperature and dew point as well as the station pressure, moisture content and
relative humidity can be calculated from these figures.