WOU ES 476 - Overview of Precipitation Meteorology

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1 G476/576 Overview of Precipitation Meteorology I. HUMIDITY, CONDENSATION, AND PRECIPITATION A. Terminology 1. Humidity- amount of water vapor in air 2. Vapor Pressure- total atmospheric pressure that can be attributed to water vapor content 3. Vapor Saturation- maximum amount of water vapor the air can hold, a. rate of evaporation = rate of condenstation b. "Vapor Capacity"- measure of amount water vapor air can hold c. Temperature dependent (1) >T, air can hold more moisture (>expansion of volume, > motion of molecules) (2) <T, air can hold less moisture (< expansion of volume) Temp. C Vapor Capacity (gm/kg) -40 0.1 -20 0.75 0 3.5 10 7 20 14 30 26.5 40 47 (3) Thus by taking an unsaturated system to lower temperature, system can become saturated 4. Specific Humidity- amount of water vapor contained in unit of air (expressed as wt. of vapor/mass of air = gm/kg) a. not affeted by changes in pressure or temperature 5. Relative Humidity- ratio of airs water vapor content to its water vapor capacity at a given temperature for given temp. R.H. = water vapor x 100% vapor capacity E.g. referring to table above, given a temp of 20 C, vapor capacity of air is 14 gm/kg. Assume a specific humidity of 7 gm/kg, relative humidity = 7/14 x 100% = 50% a. Temperature influence on relative humidity Relative humidity changes with temperature according to vapor capacity of air. In example above at 20 C, vapor capacity is 14 gm/kg, assuming a specific humidity of 7 gm/kg, relative humidity = 7/14 x 100% = 50%. By decreasing the temperature to 10 C, vapor capacity of air is now 7 gm/kg, assuming the same specific humidity of 7 gm/kg, the relative humidity = 7/7 x 100% = 100% humidity.2 Vapor capacity must be exceeded to oversaturation for condensation/precipitation to occur (1) In sum: a decrease in air temperature will result in an increase in relative humidity, and an increase in air temperature will result in decrease in relative humidity (2) <T, > R.H.; >T, < R.H. (3) In terms of absolute air moisture... (a) cold air contains less specific humidity than warm air, although cold air relative humidity and warm air relative humidities may be identical (e.g. 85%), the vapor capacities are different, and the warm air will have much more moisture than cold air at same relative humidity 6. Dew Point Temperature a. Temperature at which air would have to be cooled in order to reach saturation (1) at temperatures colder than dew point, air vapor capacity is exceeded and condensation would occur 7.Atmospheric Precipitation – condensation of atmospheric moisture below dew point a.condensating nuclei in atmosphere (dust) b.aggradation of microscopic cloud droplets into rain drops c.measurement – rain gage collection device at point locations i.measured in inches or mm of precip as collected in gage II. Evaporation A.Defined – addition of heat energy to water surface, transformation from liquid to vapor phase 1.Heat of Vaporization = 590 cal / gram to transform from liquid to vapor 2. Evaporation of water at surface-air interface (lakes, rivers, etc) 3. evaporation driven by solar radiation B.Evaporation = portion of outflow from drainage basin C. Measurement of Evaporation 1.Pan Evaporation – water placed in shallow pans, measure volume of evap. a. varies according to temperature and wind speed III. FORCEFUL LIFTING (A Mechanism for Lifting and Cooling Air) A. Air may be forced upward regardless of stability of air mass or adiabatic processes B. Methods of Forceful lifting of air3 1. Convergence- flowing of air masses together, occupies less space, air column forced to rise vertically a. air forced to rise upward b. enhances instability c. E.g. Florida: on warm summer days (1) Atlantic air flow westward over land (2) Gulf air flow eastward over land (a) convergence + vertical uplift + intense solar heating = high rate of thunderstorm occurrence (greatest occurrence in U.S.) 2. Orographic Lifting- sloping terrain/mountain slopes act as barriers to air flow, forces air to ascend a. Rain fall on windward side, rising moist air masses, <Temp during ascent, >saturation point.... rain (1) e.g. Mt. Waialeale Hawaii: 38 Ft rain/yr b. Lee sides: dry air descends... orographic deserts (1) Rain shadow deserts (2) e.g. Columbia Basin of Wash, Mojave of Calif. 3. Frontal Wedging- cool air acts as a barrier over which warm, less dense air rises a. Responsible for rainfall patterns over much of continental U.S. (1) Cold Fronts moving in from w-nw, forcing warm continental air above (a) Arctic/Canandian cold fronts moving into US (2) Warm Fronts moving in from w-nw, riding over cold IV. FACTORS AFFECTING WIND A. General 1. Wind = horizontal movement of air (advective motion) 2. Wind and Pressure a. basic gas law: air of higher pressure moves towards air of lower pressure b. wind = drive toward equilbrium of air pressure 3. Pressure Differences on Earth's Surface a. Caused by unequal heating of atmosphere by sun b. Variable solar insolation due to tilt of earth, orbital path, latitudinal changes B. Factors Influencing Air Motion 1. General : if earth did not rotate and there were no friction of air motion, wind would be simply controlled by air motion from high pressure to low pressure 2. Influencing factors a. Pressure Gradient Force- degree of pressure changes per unit distance b. Coriolis Effect- motion on rotating objects/centrifugal force c. air friction- resistance to flow C. Pressure Gradient Force4 1. Pressure variation = wind a. > press. diff, > wind speed 2. Mapping air pressure a. pressure contour maps b. isobars = lines on constant air pressure 3. Pressure Gradient = change in pressure unit distance a. gradient to hill (1) steeper the hill (pressure gradient) (a) > press. gradient, > wind acceleration (b) > close spacing of isobars (2) gentler the hill slope (pressure gradient) (a) < press. gradient, < wind acceleration (b) widely spaced isobars 4. Coriolis Effect a. Coriolis = apparent shift due to rotation and velocity variation by latitude (1) Earth rotating in counterclockwise direction as viewed from north pole (2) Net result: (a) Northern Hemisphere: air deflected to right in the direction of travel (b) Southern Hemisphere: air


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