Processes: Evaporation, condensation, and precipitation

Evaporation

• Dalton's law: evaporation rate, E, controlled by two factors, the windspeed and the saturation deficit:
  E = u ( e_s-e_a )
  u: function of windspeed, e_a : current vapor pressure, e_s : saturation pressure at that temperature

• this formula is useful particularly over the oceans, over land the situation is more complicated
• excercise: estimate the average annual evaporation rate over the oceans in m/y using the residence time of the ocean of approximately 3000 years.

Condensation/Precipitation

• gravity concentrates the atmospheric gases near the surface, at 5.5km, atmospheric pressure half that at the surface
• 90% of water vapor content is confined to the lower 6km
• air rising -> expansion -> adiabatical (=no heat exchange with environment) cooling -> condensation
• condensation and precipitation
• at T>0^oC: warm cloud process: condensation, gradual growth of water droplets by condensation and coalescence
• at T<0^oC: cold cloud process: involves also the formation and growth of ice crystals
• two extra factors are needed to form precipitation:
• sufficient moisture supply
• sufficient vertical motion
• warm cloud process
• a moisture laiden air parcel rises, cools at dry adiabatic lapse rate (~1^oC/100m) until it reaches the dewpoint, at which point condensation occurs. After that, any further rise causes cooling at the moist adiabatic lapse rate (0.5 - 0.9^oC/100m), because of the released latent heat. (Fig)
• super saturation: relative humidity > 100%
• condensation nuclei are needed to increase condensation
• most efficient particles: Aitken nuclei (0.01-0.1 micro m)
• typical source: dust from land, sea spray (hygroscopic!)
• 5 million/l over land, 1 million/l over the ocean
• experiment: salt crystals as condensation nuclei (Fig)
• experiment: when a beer bottle is opened, a cloud forms in the neck. If temp. of the bottle is 5^oC, temperature drops to ~-36^oC when bottle is opened (Fig)
• experiment: when beer is poored into a glass, bubbles form on scratches and dust particles, adding salt can increase the bubble formation: clouds in a glass of beer
• excercise: condensation on a mirror in the bathroom (Fig); condensation on windshields
• condensation only creates droplets < 100 micro m radius, while raindrops are of the order of 1mm
• clouds are continuously forming and dissipating, some live only 5 to 15 minutes
• excercise: how many cloud droplets form one rain drop?
• droplets merge due to direct impact and collision in the wake of falling drops
• cold cloud process
• saturation vapor pressure is lower over ice than water -> ice crystals grow in favor of liquid droplets (Bergeron-Findeisen process)
• ice crystals are very efficient condensation nuclei
• most efficient in mid latitudes (temperatures low enough, but enough instability in the atmosphere)
• precipitation amounts
• kinds of precipitation: drizzle, rain, ice pellets, snow, hail
• terminal velocity (v) is achieved when gravitational acceleration is counterbalanced by the friction of the air, for 1mm diameter drop: v = 4 m/s
• raindrops break up at 5mm diameter, snow can reach 40mm, and hailstones over 50mm
• moisture in atmosphere: 25% condenses, 75% forms ice and snow; only 5% of that falls as snow and ice crystals, the rest melts; a lot of the precipitation re-evaporates before it reaches the ground
• wettest place on Earth: Mt. Waialeale, Kauai; 460 inches/y (11.68 m/y)(Fig)
• dryest place on Earth: Calama in Atacama desert, Chile, rain has never been recorded (in US: Greenland ranch, Death Valley: 1.78 inches/y (45 mm/y))
• distribution of precipitation in the United States (Fig); isolines reveal precip sources, orgraphic effects

Atmosphere/Oceans

• Global circulation patterns in the atmosphere (Fig)
• Hadley cells, tradewinds, westerlies, Coriolis force....
• Global circulation patterns in the oceans (Fig)
• gyres, thermohaline circulation, cold/warm water currents

Exercise: Look at patterns of ocean currents (Fig), atmospheric circulation (Fig), global precipitation (Fig), and evaporation (Fig). Group 1: look at precipitation, group 2: look at evaporation. Do you see any patterns in evaporation or precipitation? Can you explain any of the patterns by comparing with ocean currents or atmospheric dynamics?

• Evaporation, global patterns (Fig)
• E higher over oceans than land (except for areas covered by sea ice)
• E increases with temperature
• oasis effect
• Precipitation, global patterns (Fig)
• P peaks in low and mid latitudes, zones of atmospheric convergence (ITCZ and moving depressions)
• zones of divergence are dry
• orographic precipitation
• monsoons: seasonal reversals of wind patterns
• The Atlantic Ocean is loosing water by evaporation (Fig), this loss is balanced by transport of ocean water from the Pacific ocean

Runoff

• 1/3 of global runoff is not gauged
• climatological method of estimating global runoff: RO = P -E
• estimates are very uncertain

Regional water resources

• only 1/3 of total runoff is stable or baseflow runoff
• per capita runoff range from 1300 (asia) to 19,000 (Australasia) m³/y (Jones, Table 2.5)

Resources

Bohren, C.F. (1987) Clouds in a glass of beer.