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The water molecule two hydrogen and one oxygen H20
The shape of the water molecule results in it forming the familiar six sided
snowflake structure. Also when water freezes the molecules arrange themselves
as shown above leaving empty space within the six-sided ring structure. This is
why 1.0 gram of ice is larger in volume than 1.0 gram of liquid water.
Water molecules continually evaporate from and condense back onto the liquid
water surface. If the evaporation rate is larger than condensation rate the humidity
increases and if the evaporation rate is smaller than the condensation rate then the
humidity decreases. The air is said to be saturated when the evaporation and
condensation rates are equal.
The air is said to be saturated when the evaporation and condensation
rates are equal. In a closed system the saturation humidity increases as
the temperature increases. The reason for this is that the evaporation
rates increase and hence the number of water vapor molecules the air
increases until the condensation rate matches this higher evaporation
rate.
Water Cycle: See hydrological Cycle Module
http://ga.water.usgs.gov/edu/watercycle.html
Figure 2. The mean distribution of precipitable water, or total atmospheric water vapor above the
Earth's surface, for 1992. This depiction includes data from both satellite and radiosonde
observations. (Image courtesy of Thomas Vonder Haar and David Randel, Colorado State
University, Fort Collins.) On average over the whole Earth there is about 1 inch (25 mm) of
precipitable water in the atmosphere at any given time. This is equivalent to about 1 week’s
worth of water use by humans.
There is typically more water near the surface than higher up.
The stratosphere is quite dry.
On average the near
surface equatorial
regions (the tropics)
have the largest
humidity of
anywhere on Earth.
Water vapor is extremely variable from place to place and time to
time. Water vapor amounts recorded from Satellites can help
forecasters determine heavy precipitation events and also help
estimate nighttime lows from radiative cooling.
Since water molecules emit infrared radiation IR detectors can be
used to sense water vapor amounts remotely.
Since water vapor is a greenhouse gas it can trap heat energy
absorbed by the sun close to the surface throughout the night. Les
water vapor implies cooler night-time temperatures. The
difference between day time and night time temperatures is
greatest in desert regions that have little water vapor.
In any given parcel of air there
are nitrogen oxygen, and water
molecules (water vapor) among
many other gases in small
quantities. The actual humidity
is a measure of how many water
molecules are actually in the air.
The actual humidity can be
measured in a variety of units.
Grams of water per kilogram of
air or grams of water per cubic
meter.
We will use vapor pressure as a measure of humidity. As the molecules
bounce around wildly they exert an outward pressure. At sea level this
outward pressure balance the inward pressure from the weight of all air
above a location. This is normal sea level pressure.
We will use vapor pressure as
a measure of humidity. As
the molecules bounce around
wildly they exert an outward
pressure. At sea level, this
outward pressure balances the
inward pressure from the
weight of all air above a
location. This is normal sea
level pressure and is about
1000 mb (milli-bars). Part of
this outward pressure comes
from the water vapor
molecules.
When the vapor pressure is 20 mb approximately 2% of all air molecules
are water vapor. When the vapor pressure is 10 mb approximately 1% of
all air molecules are water vapor. When the vapor pressure is 30 mb
approximately 3% of all air molecules are water vapor.
Water boils when the internal vapor pressure pushing out on a tiny bubble equals the
atmospheric pressure pushing inward on the bubble. The graph above shows the
saturation vapor pressure on the y-axis versus temperature on the x-axis. Water
normally boils at 100 oC at sea level because at that temperature the saturation vapor
pressure equals the sea level pressure.
At the top of Mt. Hood water boils at about 93 oC
Mt. Hood
Mt. Rainer
At the top of Mt. Rainer water boils at about 85 oC
Amazingly Vancouver Wa and Tucson Az have the same amount of
water vapor in the air during January.
Dew point Temperature
(directly linked to actual humidity)
Temperature that results in saturation or
dew forming on surfaces.
High dew point high actual humidity
Low dew point low actual humidity
Dew points in excess of 70 F will normally make
one uncomfortable.
The dew point temperature is directly linked to actual humidity
Vancouver Wa and Tucson Az also have roughly the same amount of
water vapor in the air during July.
Relative Humidity
Actual _ Humidity
RH 
Saturation _Humidity
The relative humidity describes how much water is in the atmosphere relative to the
maximum possible water vapor amount. In the above figure the dark blue columns
indicate the vapor pressure (actual) and the total column height up to the top of the light
blue is the maximum possible (saturation vapor pressure). Graphically we can see that the
far right column has the highest relative humidity and the middle has the lowest.
High relative humidity results in low evaporation rates so our body does not cool
itself very well and we more easily suffer from heat stroke. The Heat index is a
quantitative measure of this discomfort. The above example shows an air
temperature of 100 oF with relative humidity of 60% is equivalent to a dry 130 oF
day. During most years, heat stroke is the number 1 weather related cause of death in
These examples of cooling from evaporation work best in
dry climates.
Evaporative cooler, porous clay pot, and canvas canteen
If you soak the canvas wrapper the evaporative cooling will keep
you water cool.
However just wrapping your bottle in cloth with prevent
condensation from occurring on the bottle. Since water vapor
releases latent heat energy when it condenses, the cloth wrapping
isolates you water bottle from warming up too fast from the
condenstaion.
The RH will increase if more water vapor is added to the air.
Fig. 4-11a, p. 93
Since as the air temperature increases so does the saturation vapor pressure. This causes
the relative humidity decrease as the air temperature increases (denominator gets
bigger)
Fig. 4-11b, p. 93
RH is maximum when it is coolest. This is the most likely time for
fog, dew or frost to occur. RH is minimum when it is warmest.
Outside
Inside
T = -15 C
Td = -15 C
RH =100%
T = 20 C
Td = -15 C
RH = 8%
Inside the cabin the air is uncomfortably dry.
Air temp= 35oC
Dew point = 10 oC
RH =21 %
A dew point of 10 oC represents air
with a significant amount of water.
However since it is so warm here in
this desert the relative humidity is
quite low. Evapo-transpiration
occurs quite rapidly from surface
plants leaving the area relatively
dry. This same dew point with a
mean air temperature of 20 oC
(68F) would correspond to a
comfortable 52% relative humidity
and plant life would likely flourish
Air coming off the Pacific is relatively cool and contains
less water than the warm moist air flowing into the
Eastern US from the Atlantic and Gulf of Mexico.
Hygrometers measure humidity
A Sling Psychrometer is common
We’ll use the sling psychrometer in our week 4 lab.
Computer data collection has almost completely
replace the old style hydrographs.