MTR 2400 Introduction to Atmospheric Science
Lab Assignment #5
Moisture & Humidity
Introduction: Moisture and Humidity
Water vapor, by far, is the most variable gas in the atmosphere, ranging from zero to as
much as four percent. Water vapor plays a role in the energy balance of the atmosphereearth system, in keeping the average earth surface temperature at a moderate 15C
(59F). Also, water vapor is an essential weather element, which is regularly observed
and recorded. Forecasters and researchers are interested in the water vapor content
present in the atmosphere for a couple of reasons:
1) Water vapor condenses into a cloud droplet which eventually grows into a
raindrop. Forecasting for precipitation, including Quantitative Precipitation
Forecasts (QPF), remains a challenge for meteorologists today.
QPF is defined as a prediction of the amount of precipitation that will fall at a
given location in a given time interval.
2) Latent heat is released when water molecules changes phase from vapor-toliquid, liquid-to-ice, vapor-to-ice, etc. This source of “hidden” energy is
important in maintaining the earth’s energy budget.
Meteorologists, in general, think about moisture as the amount of water vapor in the
atmosphere. Moisture content can vary with height in different ways, but generally
speaking, water vapor content is at a maximum in the lowest kilometer of the atmosphere
and rapidly decreases with altitude. A casual observer might consider moisture as a
measure of the degree of wetness in the environment. One might say, “Gee, it sure is
muggy today,” and your first thought as an aspiring scientist would be, “what is the
relative humidity?” or “what is the dewpoint?” Both of these concepts would give a
measured value of the current moisture in the atmosphere. One gives you an exact
temperature at which saturation/condensation commences while the other is dependent on
the ambient temperature. As the temperature rises, the amount of water vapor that the
atmosphere can hold increases.
Moisture content in the atmosphere can be quantified in various ways. While no one way
is incorrect, learning the differences in their formulations and calculations can benefit
one’s knowledge of the inter-working between different processes in the atmosphere.
Dew Point Temperature
Dew point temperature (Td) is the temperature at which air becomes saturated and
therefore condensation occurs, without the introduction of more water vapor. This
moisture measurement has a unit similar to temperature either in Celsius (C) or
Fahrenheit (F). The dew point temperature can not exceed the ambient temperature.
Only when the environment is saturated are the dew point temperature and ambient
temperature equal. Equal lines of dew point temperature are called isodrosotherms.
Vapor Pressure and Saturation Vapor Pressure
Vapor pressure (e) is the amount of pressure exerted by water vapor molecules in the
atmosphere. This is sometimes called the partial pressure since water vapor is a minor
component of the overall atmospheric gas composition. Vapor pressure only changes
when the amount of water vapor in the air changes, so vapor pressure can be considered
as an absolute measure if the atmosphere’s vapor content.
Saturation vapor pressure (es) is the maximum amount of vapor pressure that the
atmosphere can support at a certain temperature and altitude. Saturation vapor pressure is
a relative measure of atmospheric moisture since temperature plays a direct role.
Mixing Ratio and Saturation Mixing Ratio
Mixing ratio (w) is the ratio between the mass of the water vapor over the mass of dry air.
This moisture measure has the unit of g kg-1. Mixing ratio can be thought of as the mass
of the water vapor in grams with respect to one kilogram of the dry air. It is dependent
on temperature and pressure.
Example: If the mixing ratio is 6 g kg-1, then for every 1 kg of dry air there is 6 g of
water vapor present in the atmosphere.
Saturation mixing ratio (ws) can be shown as the upper limit of the mixing ratio value.
Therefore, ws is defined as the maximum amount of water vapor that can be present in
one kilogram of dry air at any given temperature.
These moisture variables are a function of temperature and pressure. They can be found
by using a thermodynamic diagram, such as a skew-T log P diagram.
Relative Humidity
Relative humidity (RH) is defined by World Meteorological Organization (WMO) as the
ratio of the mixing ratio to the saturation mixing ratio. Relative humidity is usually
reported in percentage (%). Another way relative humidity can be defined is by the ratio
of the vapor pressure to the saturation vapor pressure with respect to water. At 100%
RH, the environment is said to be completely saturated.
Example: At Denver, where standard atmospheric pressure is around 830mb, if the dew
point temperature is 0C (32F), then the mixing ratio value is approximately 5 g kg-1.
So what that mixing ratio value is actually telling the observer is that for every 1 kg of
dry air there is 5 g of water vapor or for every 1 kg of dry air, there is 0.005 kg of water
vapor. If Denver’s ambient temperature is 15C (59F), then the saturation mixing ratio
value is slightly greater than 12 g kg-1. The RH value for these values would be 41.7%
RH 
w
 100 %
ws
RH 
5 g kg
RH 
12 g kg
 100 %
0.005 kg kg
0.012 kg kg
 100 %
RH  .417  100 %
RH  41.7 %
Wet-Bulb Temperature
The wet-bulb temperature (Tw) is defined as the minimum temperature to which the air
can be cooled by evaporating water into it. A sling psychrometer is used to measure the
wet-bulb temperature. The wet-bulb depression value is the difference between the drybulb temperature (the ambient temperature measured on a sling psychrometer) and the
wet-bulb temperature. The lower the wet-bulb depression value, the more saturated the
environment is. A wet bulb-depression value of 0 indicates a completely saturated
atmosphere.
Example:
Dry-bulb Temp: 4C
Wet-bulb Temp: -2C
Wet-bulb Depression = 4C – (-2C) = 6C
Name:_______________________
Lab Assignment #5
(66 points)
1. Assuming the moisture content remains constant; would you expect the relative
humidity to be greater in the morning or afternoon? Why? (5 pts)
2. The figure below shows the temperature (top) and dew point temperature (bottom) of
surface stations located in the Midwest. If the sky remains clear and light wind
conditions prevail, circle where would you expect fog to occur (dewpoint depression
within 3 degrees)? Assume the temperature will drop another 5 – 6°F (dropping as low
as, but not lower than, the dewpoint). Hatch the region(s) where you would expect fog to
appear. (10 pts)
3. Using the table below, plot the saturated vapor pressure (es) versus temperature in °F
on the graph provide on this page. (15 pts)
T (°F)
es (mb)
T (°F)
es (mb)
T (°F)
es (mb)
T (°F)
es (mb)
-20
.43
15
2.74
45
10.09
80
34.6
-15
.57
20
3.47
50
12.19
85
40.7
-10
.75
25
4.40
55
14.63
90
47.7
-5
.98
30
5.55
60
17.51
95
55.7
0
1.30
32
6.10
65
20.86
100
64.9
5
1.66
35
6.87
70
24.79
105
75.3
10
2.14
40
8.36
75
29.33
110
87.2
4. Using the saturation vapor pressure curve graph that you just made, answer the
following questions. Be sure to put down the correct units.
Example 1:
Given:
e = 8.36 mb
es= 17.56 mb
Find: RH, T, and Td
RH 
e
 100
es
RH 
8.36 mb
 100 %
17.51 mb
RH  .48  100 %
RH  48 %
To find T and Td, refer to the table of saturated vapor pressure. If e = 8.36 mb, then the
dew point must 40°F. If es = 17.51 mb, the air temperature must be 60°F
Example 2:
Given:
RH = 70%
es = 12.19 mb
Find: Td
RH 
e
 100
es
RH
e

100 es
.7 
e
es
(.7)(es )  e
(.7)(12.19 mb)  e
e  8.53 mb
Hence, if e = 8.53 mb then Td must be 41°F
4a) es = 20.86 mb and Td = 50°F
Find: RH (2 pts)
4b) w = 8 g/kg and RH = 62%
Find: ws (2 pts)
4c) RH = 50% and es = 8.36mb
Find: e, T, and Td (6 pts)
4d) RH = 80 % and e = 29.33 mb
Find: T (2 pts)
4e) e = 10.09 mb and es = 10.09 mb
Find: RH, T, and Td (6 pts)
4f) e = 2.74 mb and es = 4.40 mb
Find: RH, T, and Td (6 pts)
4g) w = 8 g/kg and RH = 9%
Find: ws (2 pts)
5) When you are a taking a hot shower and your bathroom fills up with steam, does w
increase or decrease? What about ws? What about the dew point depression? Explain
your answers to each. (10 pts)