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 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.)

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

CONDENSATION condensation: when air reaches saturation (100% relative humidity). Process: either cool off air to saturate, or add 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 (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.

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)

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

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)

FOG Advection Radiation Upslope