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Air Masses Revisited

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

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Air Masses Revisited

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  1. 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]

  2. Front = Boundary between Two Air Masses • Axis of lower (air, barometric) pressure • Clouds and precipitation in vicinity • Large temperature, moisture gradients • Convergent surface winds (winds a few feet above the earth’s surface, generally measured about 30 feet, or 10 meters, above the surface)

  3. 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 temperature

  4. Phase Changes of Water • Evaporation: liquid to gas • Requires a relatively large amount of energy • Condensation: gas to liquid • Releases a relatively large amount of energy • Law of Conservation of Energy • Melting/Freezing: require/release somewhat smaller amounts of energy • Sublimation/Deposition: minor players

  5. Humidity • Amount of moisture in the air • Dewpoint • 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 forecasting • Air is saturated when temperature = dewpoint

  6. Humidity (continued) • Relative humidity: (VP/SVP)*100% • A relative measure of humidity, dependent upon moisture and temperature • Typical range from 0% (bone dry air, no moisture) to 100% for saturated air (T = Td) • Saturated NOT equal to “liquid” air!

  7. How to Relate RH, Td (Approximate) • 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

  8. Key Figures • 4.11, 4.12, 4.30, 4.31

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