Atmospheric Moisture and Precipitation by dfhdhdhdhjr

VIEWS: 4 PAGES: 13

									Chapter 5 Atmospheric Moisture and Precipitation
• The hydrologic cycle, p. 193
    Evaporation depends on:
• Temperature
• Wind
• Relative Humidity
          Measures of Humidity
Principle: the warmer an air mass, the higher its water
   vapor capacity
• specific humidity is the mass of water vapor (g) per mass
   of air (kg)
• dewpoint is the temperature that the air would have to be
   cooled off to in order to induce condensation.
• vapor pressure: the portion of total air pressure
   contributed by water vapor molecules, in millibars (mb) or
   inches of mercury. Higher amounts of humidity mean
   higher vapor pressure. Higher dwpts: higher vap. press.
• relative humidity

RH% = 100*actual water vapor pressure (determined by dewpt.)
          saturation vapor pressure (determined by air temp.)
Saturation vapor pressure & temperature

Air Temperature (°F)   Saturation Vapor
                        Pressure (“ Hg)
        20                   0.11
        30                   0.17
        40                   0.25
        50                   0.36
        60                   0.52
        70                   0.74
        80                   1.03
        90                   1.42
                    Air Temperature
                     and Saturation
                    Vapor Pressure
                  1.5
S.V.P. (in. Hg)



                   1
                  0.5
                   0
                        0             50              100
                            temperature (degrees F)
          CONDENSATION
• condensation: when air reaches
  saturation (~100% relative humidity).
  Process: either cool off air to saturate, or
  add enough water vapor
• condensation nuclei: include salt, dust,
  smoke, others
• case of dew & frost:
• clouds consist of water droplets and/or
  ice crystals
  Processes of growth of droplets
      into drops that can fall:
• ice crystal process - ice crystals behave as
  condensation nuclei: vapor droplets
  sublimate onto ice crystals
• coalescence process - large droplets fall
  faster than small droplets and collide/
  coalesce with them
          Atmospheric Stability & Lapse Rates
• stability condition of the atmosphere when rising air becomes
  cooler and denser than the surrounding air and is forced to
  subside.
• instability when rising air becomes warmer and less dense
  than the surrounding air and continues to rise.
• Environmental Lapse Rate: observed rate of temperature
  change in the atm. (average: 0.65 °C/ 100m)
• Dry Adiabatic Rate (DAR): rate at which unsaturated air cools
  as it is forced upward and expands. (1 °C/ 100m)
• Saturated Adiabatic Rate (SAR): rate at which saturated air
  cools as it is forced upward and expands. (~0.5 °C/100m) Also
  known as moist adiabatic rate.
• Example: chart with average Env. Lapse Rate and unsaturated
  air parcel.
• Stable conditions do not favor precipitation, unstable conditions
  do.
              FOG
• Advection
• Radiation
• Upslope
            Lifting mechanisms & precipitation

basic principle: air must be lifted and cooled to the dewpoint in order to
     induce condensation

1.   orographic precipitation: air forced upslope cools (Nov. 1)
2.   convectional: surface heating induces air to rise and cool
     example: thunderstorms, which have 3 stages: developing,
     mature, dissipating
3.   frontal
     -front: boundary between air masses of different temperature and
     humidity (examples)
4.   Low pressure systems: mid latitude cyclones, tropical weather
     systems and Inter Tropical Convergence Zone
5.   Global precipitation map: (see folder)
  GLOBAL PRECIPITATION PATTERNS
• general pattern: high pressure favors aridity (espec. W.
  Coasts), low pressure favors precipitation
• Areas of great precipitation
ITCZ (equatorial low) and related Monsoon lands, zone of
  sub polar lows, the littorals: (trade wind coasts &
  westerly coasts)
West side of Sub Tropical Highs: greater instability and
  precipitation than E side
                  Areas of low precipitation

• Stable east side of sub tropical highs (W. Coasts) coastal deserts:
  Atacama (SA), Baja Cal., Namibian coast (Africa)
• rain shadow deserts (eg. Mohave and Sonoran)
• Polar deserts: dominated by high pressure and low temps. ensure
  low moisture content of air and modest annual precip.



Seasonality of precipitation: most regions have their low precipitation
  season in the low sun season, with the important exception of mid-
  latitude west coasts (such as the west coast of the U.S., W.
  Australia)

								
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