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Chapter 5 Atmospheric Moisture and Precipitation

Chapter 5 Atmospheric Moisture and Precipitation. The hydrologic cycle. Evaporation depends on:. Temperature Wind Relative Humidity. Measures of Humidity. Principle: the warmer an air mass, the higher its water vapor capacity

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Chapter 5 Atmospheric Moisture and Precipitation

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  1. Chapter 5 Atmospheric Moisture and Precipitation

  2. The hydrologic cycle

  3. Evaporation depends on: • Temperature • Wind • Relative Humidity

  4. Measures of Humidity Principle: the warmer an air mass, the higher its water vapor capacity • dewpoint is the temperature that the air would have to be cooled off to in order to induce condensation. • vapor pressure: the share of total air pressure contributed by water vapor molecules. Higher amounts of humidity mean higher vapor pressure. Higher dwpts: higher vapor press. • relative humidity RH% = 100*actual water vapor pressure(determined by dewpt.) saturation vapor pressure (determined by air temp.)

  5. Saturation vapor pressure & temperature

  6. CONDENSATION • condensation: when air reaches saturation (100% relative humidity). Process: either cool off air to saturate, or add water vapor • condensation nuclei: includesalt, dust, smoke, others • case of dew & frost: • clouds consist of water droplets and/or ice crystals

  7. 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

  8. Lifting mechanisms & precipitation basic principle: air can be lifted and cooled to the dewpoint in order to induce condensation • orographic precipitation: air forced upslope by winds cools • convectional: surface heating induces air to rise and cool example: thunderstorms, which have 3 stages: developing, mature, dissipating 3. Low pressure systems: mid latitude cyclones, tropical weather systems and Inter Tropical Convergence Zone 4. frontal -front: boundary between air masses of different temperature and humidity, mid latitude cyclone example Global precipitation map: (see folder)

  9. GLOBAL PRECIPITATION PATTERNS • view precip. map in 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 exception: greater instability and precipitation than E side

  10. 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 exception of mid-latitude west coasts (such as the west coast of the U.S., W. Australia)

  11. Atmospheric Stability & Lapse Rates • stability condition of the atmospherewhen 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 (ELR):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) • Example: chart with average Env. Lapse Rate and unsaturated air parcel. • Stable conditions do not favor precipitation, unstable conditions do.

  12. FOG • Advection • Radiation • Upslope

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