stated humidity is simply the amount of water vapor held in
the air. Water vapor is the gaseous
state of water. As the temperature of the air increases
more water vapor can be held since the movement of molecules
at higher temperatures prevents condensation from occurring.
are three main measurements of humidity: relative, absolute
and and specific.
humidity (units are grams of water vapor per cubic meter
volume of air) is a measure of the actual amount of water
vapor in the air, regardless of the air's temperature. The
higher the amount of water vapor, the higher the absolute
humidity. For example, a maximum of about 30 grams of water
vapor can exist in a cubic meter volume of air with a temperature
in the middle 80s.
humidity, expressed as a percent, is a measure of the
amount of water vapor that air is holding compared the the
amount it can hold at a specific temperature. Warm air can
possess more water vapor (moisture) than cold air, so with
the same amount of absolute/specific humidity, air will have
a higher relative humidity. A relative humidity of 50% means
the air holds on that day (specific temperature) holds 50%
of water needed for the air to be saturated. Saturated air
has a relative humidity of 100%.
humidity of an air-water mixture is also defined as the ratio
of the partial pressure of water vapor in the mixture to the
saturated vapor pressure of water at a given temperature (See
is vapor pressure). Thus the relative humidity of
air is a function of both water content and temperature.
Specific humidity refers to the
weight of water vapor contained in a unit weight (amount)
of air (expressed as grams of water vapor per kilogram of
air). Absolute and specific humidity are quite similar in
What is Dew Point?
Dew Point is the temperature at which air
is saturated with water and condensation begins. The higher
the dew point rises, the greater the amount of moisture in
What is the relationship between Dew
Point and Relative Humidity?
Compared to relative humidity, dew point is
frequently cited as a more accurate way of measuring the humidity
and comfort of the air, since it is an absolute measurement
(unlike relative humidity).
The relative humidity is 100 percent when
the dew point and the temperature are the same. If the temperature
drops any further, condensation will result, and liquid water
will begin to form.
If the relative humidity is 100 percent (i.e.,
dew point temperature and actual air temperature are the same),
this does not necessarily mean that precipitation will occur.
It simply means that the maximum amount of moisture is in
the air at the particular temperature the air is at. Saturation
may result in fog on the ground and clouds aloft (which consist
of tiny water droplets suspended in the air).
While dew point gives one a quick idea of
moisture content in the air, relative humidity does not since
the humidity is relative to the air temperature. In other
words, relative humidity cannot be determined from knowing
the dew point alone, the actual air temperature must also
Relative humidity is also approximately
the ratio of the actual to the saturation vapor pressure.
RH = (Actual Vapor Pressure) / (Saturation
Vapor Pressure) X 100%
Where actual vapor pressure is a measurement
of the amount of water vapor in a volume of air and
increases as the amount of water vapor increases.
Saturated vapor pressure is the maximum
VP that can exist at any given temperature.
Air which is at 100% relative humidity
(RH) contains water vapor whose VP is its SVP at the
given temperature. This corresponds to air which is
in equilibrium with liquid water. RH is the ratio VP/SVP
expressed as a percentage. 'Dry' air will contain water
vapor with a VP which is less than the SVP at the given
How is humidity measured?
A device to measure relative humidity is called
a hygrometer. The simplest hygrometer - a sling psychrometer
- consists of two thermometers mounted together with a handle
attached on a chain. One thermometer is ordinary. The other
has a cloth wick over its bulb and is called a wet-bulb thermometer.
How does a psychrometer measure relative
also called a sling psychrometer has two thermometers attached.
One is dry (often called the dry bulb thermometer) and measures
the actual air temperature. The other called the wet bulb
thermometer has a has a wet cloth at the tip. As water molecules
evaporate from the surface of the wet bulb they will take
heat with them lowering the reading on the thermometer. The
rate of evaporation depends on the vapor pressure or amount
of water vapor in the air. At 100% relative humidity no water
will evaporate from the wet bulb and the readings on both
thermometers will be the same. Comparing the two temperatures
in a chart will give the relative humidity.
Sper Scientific Sling Psychrometer
to Measure Humidity
Top is wet bulb
Bottom is dry bulb
Table of Relative Humidity, %---Difference Between Readings
of Wet & Dry Bulbs
DETERMINING RELATIVE HUMIDITY WITH A SLING PSYCHROMETER