Introduction:
Atmospheric Temperature is one of the most frequently considered of the many meteorological
elements. This influence of air temperature on the growth and well-being of earth’s life forms is
paramount. Temperature is basically a measure of the molecular kinetic energy of a substance.
(Or how fast the molecules are moving). Therefore reference points upon which compare
molecular activities or temperatures are needed. The freezing point and boiling points of water
are used as 32OF or 0OCis the freezing point and 212OF or 100OC is the boiling point.
Water is a basic necessity for the life forms here on Earth. At a given moment, only about
1/100,000 part of the earth’s supply of water is found in the atmosphere, yet it is this atmospheric
water that brings moisture to the surface of the earth and makes it habitable. In the hydrologic
cycle, water is constantly being moved from the earth’s surface into the troposphere where it
gives rise to clouds and precipitation, thereby bringing the water back to the surface again.
Humidity is defined as the amount of water vapor in the atmosphere. It is measured in several
ways. In one method, the mass of the water vapor is measured per unit volume, usually a cubic
meter (m3). This measure of humidity is referred to as absolute humidity and reported as grams
H2O/m3.
The humidity may also be expressed as a mixing ratio. The mixing ratio represents the mass of
water vapor in the atmosphere measured in grams per kilogram mass (g/kg) of dry air.
Specific humidity, yet another way to measure and report humidity, is essentially the same as the
mixing ratio, differing only in that the sample mass of air includes the mass of H2O vapor
contained within it.
While each of the methods just described to report humidity involve measuring the mass of H2O
vapor within a sample unit of air, there is yet another way to measure the amount of water vapor
in the atmosphere that utilizes a different approach. The concept of partial vapor pressure breaks
out the amount of the total atmospheric pressure attributable solely to H2O vapor. For instance if
the atmospheric pressure is 1000 millibars, and H2O vapor represents 3% of the mass of the air
(which is manifested as air pressure) we could describe the amount of H2O vapor present by
reporting the actual vapor pressure as 30 millibars.
Relative humidity, the most widely reported measure of humidity, is not a direct measure of the
amount of water vapor in the atmosphere, but rather the ratio of the air's water vapor content to
its water vapor capacity at a given temperature. The capacity is the amount of H2O vapor needed
to produce saturation at that particular temperature.
The various formulae for relative humidity are:
amount of water vapor in atmosphere (per unit)
water vapor capacity of atmosphere (per unit)
or
X 100
mixing ratio
saturation mixing ratio
X 100
or
actual vapor pressure X 100
saturation vapor pressure
Humidity can be expressed in several ways. One measure of humidity is vapor pressure. Every
gas existing in air is contributing to part of the pressure being exerted by the air. The vapor
pressure is the part of the total atmospheric pressure being exerted by the water vapor in the air.
Therefore, it is a measure of the actual vapor content or absolute humidity. If the air is holding
the maximum amount of vapor capable of holding, it is said to be saturated and the vapor content
can be expressed as the saturated vapor pressure. There is a significant relationship between the
temperature of the air and its capacity to hold moisture. The higher the temperature the greater
of the air to hold moisture. This relationship between temperature and saturated vapor pressure.
Relationship between Air Temperature & Saturation Vapor Pressure
60
Saturation Vapor Pressure
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
Series1 45
Temp C
Simple Calculations:
Using the formulas stated above in this lab perform the following calculations and answer the
questions. You must show your work.
1 If the water vapor capacity of the atmosphere in a given location is 25 grams per kilogram of
air sampled, and the actual amount of water vapor present is 5 grams per kilogram of air
sampled, what is the relative humidity?
2 If the water vapor capacity of the atmosphere in a given location is 12 grams per kilogram of
air sampled, and the actual amount of water vapor present is 3 grams per kilogram of air
sampled, what is the relative humidity?
3 If the mixing ratio of a sample of air is 5 grams/kilogram, and the temperature of the sample is
15 degrees Celsius, yielding a saturation mixing ratio of 10 grams/kilogram, what is the relative
humidity of the sample?
4 If the mixing ratio of a sample of air is 2 grams/kilogram, and the temperature of the sample is
25 degrees Celsius, yielding a saturation mixing ratio of 20 grams/kilogram, what is the relative
humidity of the sample?
5 If the vapor pressure is 10 millibars and the saturation vapor pressure is 40 millibars, what is
the relative humidity of the atmosphere?
6 Why was the relative humidity in question 2 higher than that calculated for question 1, even
though the amount of water vapor present in the atmosphere was less?