Atmospheric Moisture
CH 5. pp. 122-152
Hydrologic Cycle
Water is an Odd Molecule

Boiling point is much higher than one would expect
from molecular weight.
Substance
Oxygen
Nitrogen
Water
Carbon
Dioxide
Molecular
weight
32
28
18
44
Boiling Temp
-183 oC
-196 oC
100 oC
-79 oC
Water is a Polar Molecule
Net positive charge on one side and a negative side on the other.
Polar Molecule
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Electrical negative will attract positive charge.
The electrical attraction of the polar molecules is
quite strong.
The attraction makes water molecules join
together, raising the temperature of the boiling
and freezing point.
Polar attraction also makes water a good liquid
solvent. Other molecules bind to water in
solution.
Water one of the very few substances that is
densest in its liquid state
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Implications in the atmosphere?
Ice Crystal
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105 degree separation of hydrogen
causes formation of a six sided crystal.
Water Vapor
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As the number of water vapor molecules in the
air increase in number, they will have more
chance of being close to another molecule and
being attracted to each other.
After there are a certain number of molecules in
the air, any more molecules will cause the
molecules to clump together forming a liquid.
This point is the saturation point.
Saturation Point
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Saturation point dependent upon:
Temperature (the higher the temperature the
more the molecules can escape from each
other’s influence)
 Pressure (the higher the pressure, the higher
the number of molecules present)
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Atmospheric Moisture
Methods of achieving saturation
1. Adding water vapor to the air
2. Mixing cold air with warm, moist air
3. Lowering the temperature to the dew point
Let’s talk about #3
Air Temperature can change by : Diabatic & Adiabatic Processes
Atmospheric Moisture
Diabatic Processes
Processes that involve the removal/input of heat
Increase Heat; Increase Temp.; Increase volume
Adiabatic Processes
Processes that do not involve the removal/input of heat
Expansion of air; Increase volume; Decreases Temp.
Evaporation/Condensation
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Imagine a container of
water covered with a lid
Some molecules go from
liquid to gas
(evaporation), and some
go from gas to liquid
(condensation).
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The air above the liqud
surface is at saturation
when the number of
molecules escaping equals
the number of molecules
reentering the water.
Saturation
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Saturation occurs when
evaporation=condensation (the air can’t
hold any more water vapor)
Increasing the temperature of the air increases the amount of water
vapor it can hold (it takes more water vapor to reach saturation point)
Moisture Measurements
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There are a number of measurements we can use to
specify the amount of moisture (also referred to as
humidity) in the air:
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absolute humidity
specific humidity
vapor pressure
saturation vapor pressure
relative humidity
mixing ratio
saturation mixing ratio
wet-bulb temperature
dew-point temperature
Absolute Humidity

Absolute Humidity = mass of water
vapor/volume of air
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So, absolute humidity is like a water vapor
density, commonly express in grams/m3
Abs humidity is not a useful measurement for
humidity--because it changes with volume and
temperature changes that occur in the
atmosphere. Why?
Expansion/Compression
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What happens to the
volume of a rising or
sinking air parcel?
Consider a parcel of air at
1000mb
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The parcel exerts 1000 mb
of outward pressure to
counteract the atmospheric
pressure acting on the parcel
If no energy is added or
taken away from the parcel,
then the force of the
molecules bumping into the
side of the parcel will be
constant
Expansion
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Now, we raise
the parcel to
500 mb
The outside
pressure on the
parcel decreases,
so the volume of
the parcel
increases as the
parcel expands
Specific Humidity
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Specific humidity = mass of water vapor/total
mass of air
For example, in a parcel, the mass of water
vapor is 1g
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The total mass of the parcel (N2, O2, Ar, H2O, other
trace gasses) is 1 kg
Mass H20/Total mass of the parcel of air (including
the water vapor)
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Specific Humidity is 1g/kg
Specific humidity is not effected by expansion
and compression changes in the air parcel
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Much more useful in meteorology
Latitudinal Distribution of Specific
Humidity
Figure 5.9
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High at the equator
Low at the poles
Absolute & Specific Humidity
Figure 5.7
Specific humidity is concerned
with the mass of vapor to mass of
air, and is not affected by changes
in parcel volume.
For a given mass of water vapor in
an air parcel, the absolute humidity
changes as the parcel volume
changes (lifts or descends).
Figure 5.8
Mixing Ratio
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Mixing ratio = mass of water vapor/mass
of dry air
For example: In a parcel, the mass of
water vapor is 1g; and the mass of the dry
air in the parcel is 1.0 kg
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The mixing ratio is 1g/kg
Vapor Pressure
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Remember from Chap 1 that pressure is the
force of collisions of molecules against a
surface
The total pressure is the sum of the pressures
of the different molecules
If the total pressure in
this parcel was 1000 mb-Nitrogen would contribute
780 mb, Oxygen 210 mb,
and Water vapor 10 mb.
Actual Vapor Pressure
(Dalton’s Law)
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The total pressure of
the air parcel is due to
the sum of partial
pressures of each of
the gasses comprising
the parcel
The pressure due to
water vapor is called
the actual vapor
pressure
Distribution of Actual Vapor
Pressure
January
July
Higher in moist regions
Higher in warmer areas and warmer season
Very low in cold air
Saturation Vapor Pressure
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Actual vapor pressure tells us the total water
vapor content of the air..
Saturation vapor pressure indicates how much
water vapor pressure is present when the air is
saturated
Dependent on air temperature
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It takes less moisture to saturate a cold parcel
Warm air can hold much more water vapor than cold
air
Relative Humidity
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How much water vapor is there divided by how much it can hold (X
100)=RH
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Relative Humidity can be calculated by:
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Basically: Content/capacity
Actual vapor pressure/saturation vapor pressure
Actual mixing ratio/saturation mixing ratio
We can change RH in two ways:
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Change the amount of vapor in the air OR
Change the air temperature
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Relative Humidity is affected by both temperature and pressure
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Relative humidity is high at equator and poles
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In most places, vapor content changes only
slightly over a day, yet RH varies widely each
day
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This is due to daily change in air temperature.
Lowest temperatures of early morning result in
highest RH values, warm afternoon result in lowest
RH values
Relative and Specific Humidity
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Relative humidity
(RH) as an indicator
of saturation reveals
that desert air is far
from saturated, and
that cold polar air
nears saturation.
Graphs of RH
contrast with specific
humidity in the
deserts and poles.
Specific Humidity
Relative Humidity
Figure 5.14
Dew Point Temp (TDP)
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The temperature that the air must be cooled to
(assuming no changes in water vapor or
pressure) in order for saturation to occur
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If the dew point is < 32oF, it’s called Frost Point
Important measurement to predict formation of
dew, frost, fog and minimum air
temperatures…as well as a good indicator of
severe weather, cumulus cloud ceiling heights
High TDP is a good indicator of the air’s actual
vapor content.
Seasonal Dew Point Maps
Figure 5.12A
Figure 5.12B
January, July Average Dew Point Temperatures
-High Dew Points=plentiful moisture
-Low Dew Points=very dry air
Dew Point Reality
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Can you feel dew point?
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On a typical summer day the following is true:
Dew Point(F)
Perception
75+..........................Extremely uncomfortable
70-74........................Very humid, quite uncomfortable
65-69........................A bit uncomfortable for most people
60-64........................Ok for most, but everyone begins to
feel the humidity
55-59........................Comfortable
50-54........................Very comfortable
<=49........................”Feels like the west”, very pleasant,
feels a bit dry to some
Wet Bulb Temperature
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The lowest temperature that can be reached by
evaporating water into the air
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Note: the wet bulb temperature will always be less
than or equal to the temperature
If you are a runner:
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A good measure of how cool the skin can become by
sweating
T = 90°F, RH = 90%...High wet-bulb temperature.
T = 90°F, RH = 10%...Low wet-bulb temperature.
You feel more comfortable when wet-bulb
temperature is low
Wet-bulb temperature is related to the heat index
Measuring Moisture
Moisture Measurements
Hair Hygrometer
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Human hair lengthens with increasing
RH. (why you have a “bad hair day”)
Levers amplify the change in length.
Measures Relative Humidity directly.
Relatively cheap instrument.
Moisture Measurements
Sling Psychrometer
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Two thermometers; one wet, one dry. Move
through the air (either fan or swing around).
Wet thermometer measures wet bulb
temperature
Moisture measurements
Dew Point
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Cool mirror until dew
(or frost) forms on
mirror.
Used by NWS at all
automated surface
observing stations.
Moisture Measurements
Electrical Hygrometer
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Uses a chemical
film that absorbs
moisture which
changes the
electrical
resistance.
Used in
radiosonde
measurements.