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Air Masses Revisited
If a blob of air sits over the same place
long enough it acquires temperature
and moisture characteristics of the
surface below
Associated with high pressure areas
(source regions)
Continental = Dry
Maritime = Moist
Polar = Cold or Cool
Tropical = Hot or Warm
cP = dry and cold; mP = moist and cool
cT = dry and hot; mT = moist and warm
cA = dry and (very) cold [A =Arctic]
Front = Boundary between Two
Air Masses
Axis of lower (air, barometric)
Clouds and precipitation in vicinity
Large temperature, moisture
Convergent surface winds (winds a
few feet above the earth’s surface,
generally measured about 30 feet, or
10 meters, above the surface)
Moisture: The Hydrologic Cycle
The journey water takes from the earth’s
surface and beneath to the atmosphere
and back again
Closed system
Water is Unique
• All 3 phases exist naturally in normal
atmospheric temperature ranges
• All 3 phases can and do exist at the same
Phase Changes of Water
Evaporation: liquid to gas
• Requires a relatively large amount of
Condensation: gas to liquid
• Releases a relatively large amount of
Law of Conservation of Energy
Melting/Freezing: require/release
somewhat smaller amounts of energy
Sublimation/Deposition: minor players
Amount of moisture in the air
• The temp. at which dew forms; the
temp. to which the air must be cooled
for net condensation to begin
• An absolute measure of humidity
• Comfort levels, precipitation
• Air is saturated when temperature =
Humidity (continued)
Relative humidity: (VP/SVP)*100%
• A relative measure of humidity,
dependent upon moisture and
• Typical range from 0% (bone dry air, no
moisture) to 100% for saturated air (T
= Td)
Saturated NOT equal to “liquid” air!
How to Relate RH, Td
The larger the difference between T
and Td the lower the RH
For every 20 degree F difference
between temperature and dewpoint,
the relative humidity is halved
• T >= Td in general
• So, if T = Td, RH = 100% (saturation)
• If T = 80F, Td = 60F; RH = 50%
• If T = 80F, Td = 40F; RH = ?
• If T = 95F, Td = 35F; RH = ?
• Typical Daily Cycle of T, Td, RH
Key Figures
4.11, 4.12, 4.30, 4.31