Atmospheric moisture, condensation, and precipitation

Moisture in the atmosphere

• water undergoes huge expansion during evaporation: 1 g of water equals 1 ml volume in liquid form and 42 l as vapor (at 25^oC)
• gravity concentrates the atmospheric gases near the surface, the pressure drops to 1/e (=  37%) at about  8 km elevation
• P/Po = exp(-h/8000m)
  
• 90% of water vapor content is confined to the lower 6 km
• water vapor pressure as a function of temperature (svp = saturation water vapor pressure) (Fig), can explain many phenomena in the atmosphere.
• absolute humidity (or water vapor mixing ratio): mass of vapor per unit volume of air, in g m^-3
• at 30^oC, air has a svp of 42.43 hPa (hPa = mbar) and can contain up to 30 g m^-3, at 0^oC svp is only 4.5 g m^-3
• relative humidity: actual water vapor pressure / svp in %; or: actual water vapor content / absolute humidity
• formation of fog, clouds, mixing clouds, can be understood in the framework of the vapor pressure diagram

Condensation and Precipitation

• transition from vapor phase to liquid phase
  

• deposition of liquid water droplets and ice particles that are formed in the atmosphere and grow to a sufficient size so that they are returned to the Earth's surface by gravitational settling. Solid and liquid. Dew and fog do not count as precipitation (can add 5-10% to precipitation in the Pacific Northwest)

clouds and precipitation

explanantion of processes through the vapor pressure diagram (Fig): air rising => expansion => adiabatical (= no heat exchange with environment) cooling => condensation

example: humidity and temperature for Black Rock Forest (Fig)

at T>0^oC: warm cloud process: condensation, gradual growth of water droplets by condensation, collision  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 air over land, 1 million/l air 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
• ice crystals are very efficient condensation nuclei
• most efficient in mid latitudes (temperatures low enough, but enough instability in the atmosphere)

Precipitation patterns

• 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 = 9 miles/hour
  
• raindrops break up at 5 mm diameter, snow flakes 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
• most precipitation comes from bordering oceans, but up to 40% can come from local ET.
• extremes in US: Kauai: 12,000 mm/y, Death Valley: 40mm/y
• dryest place on Earth: Calama in Atacama desert, Chile, rain has never been recorded
• average annual precipitation (global (Fig) and US (Fig)) onto the continents is a function of:
• (a) latitude (precipitation highest in latitudes of rising air-0° and 60° north and south-and lowest in latitudes of descending air- 30° and 90° north and south);
• global circulation patterns in the atmosphere (Fig)
• (b) elevation (due to orographic cooling, precipitation usually increases with elevation (Fig) ;
• (c) distance from moisture sources (precipitation is usually lower at greater distances from the ocean);
• (d) position within the continental land mass;
• (e) prevailing wind direction;
• global circulation patterns in the atmosphere (Fig)
• (f) relation to mountain ranges (windward sides typically cloudy and rainy, with leeward sides typically dry and sunny)
• (g) relative temperatures of land and bordering oceans
• global circulation patterns in the oceans (Fig)
• excercise: spatial and temporal variability of precipitation

Point measurements of precipitation

• Obviously precipitation data are extremely important in hydrology
• need to measure at many points and need to extrapolate
• point measurements performed by recording and non-recording gages
• tipping bucket rain gage
• rain gauge at Black Rock Forest (Fig)
• snow depth measurements by telemetry, 500 remote sites in US
• precipitation typically measured as depth
• many stations all over the world (Fig)
• the record of hourly precipitation over time is called a hyetograph and shows that precipitation is organized into discrete storms (Figure 2.3, a station in North Carolina)

Spatial characteristics of precipitation and radar estimation

• averaging over an area using point measurements at stations (Fig2.4)
• measurement by radar, radar is reflected from raindrops
• storm track and total rainfall accumulation during a storm on June 27, 1995 based on radar measurements (Fig2.5)
• current radar image
• radar image of previous fieldtrip (Fig)

Temporal characteristics of precipitation

• our ability to forecast this temporal variation even a few hours in advance is
  
limited and our ability to forecast several days in advance is almost zero
• if you examined all of the rainfall data for a given region, you would find an upper limit to
  
the depth of rainfall per time (precipitation intensity) for a given duration. (Envelope Curve for Precipitation)
• hydrologists apply a technique called frequency analysis to describe the temporal characteristics of precipitation
• we assume that precipitation data are samples of a random variable characterized by a probablility density function
• only mean annual precipitation appears to be normally (or Gaussian) distributed (Fig2.7)
• if normally distributed, precipitation can be described by a mean and and a standard deviation (Fig2.6)
• this information is useful to determine the exceedence probability (the probability that a certain annual precipitation value is exceeded in a given year) or the return period (the inverse of the exceedence probability).
• If data are normal distributed, EXCEL's NORMINV function can be used to determine the value of a particular parameter (e.g. annual precipitation) for a particular exceedence probability and recurrence interval
• (Excercise: Explore2, global variability of precipitation), determination of exceedence probablity using standard deviation, mean and the normal distribution. Denver, Seattle and LGA/NY as an example (Fig2.6). (Fig)
  

Resources

• Hornberger et al., chapter 2, and Appendix 3
• Bohren, C.F. (1987) Clouds in a glass of beer.
• NCDC Climate Visualization (CLIMVIS)
• Intellicast NYC weather page