Atmospheric Moisture and Precipitation by dfhdhdhdhjr


									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
S.V.P. (in. Hg)

                        0             50              100
                            temperature (degrees F)
• 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
• 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
• 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)
• 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.

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