Lecture 5 reading assignm: A&K finish Ch 3, Ch 4. Hartmann 3.8, 3.9, 3.10, 3.12 • Lapse rate, change in T with height Atmospheric water: • quantifying water vapor in the air • cloud formation • fog formation • high/middle/low clouds • radiative effects of clouds
Temperature inversion: when temperature increases w. height…………………………..
Forming a temperature inversion. Temperature inversions trap pollution near ground, can be important for formation of severe weather
Atmospheric Water Evaporation:process by which water is converted from liquid form to gas It takes energy! It occurs more easily over warmer surfaces. Evaporation is also greater when atmospheric pressure is low, wind speed is high and there is little water vapor in the air. Equilibrium when air is saturated
Measuring water vapor in the air • Mixing ratio (units :g/kg w.r.t. dry air) • Vapor pressure (at most 40 mb) • Saturation vapor pressure • Relative humidity: • Vapor pressure/saturation vapor press x 100 • Dew point: temp to which cool air at same pressure to cause condensation • Frost point: temp to which cool air at same pressure to cause frost to form
The source of water vapor is at the surface. That’s why it decreases sharply away from surface.
Condensation/deposition: Cloud formation, i.e., forming liquid droplets and solid ice crystals from water vapor Solute effect Curvature effect
Cloud formation • Nucleation (formation of cloud droplet around any type of particle.) • Homogeneous nucleation • Heterogeneous nucleation • Condensation nuclei • Hygroscopic nuclei • Hydrophobic nuclei • Ice nuclei
Fog – types and formation • Radiation fog. Forms similar to dew • Ground cools by radiation on long clear nights with light winds • Advection fog • When warm air advected over a cool surface • Off the coast of CA, cool coastal current • Gulf of Mexico in fall & winter • Evaporation (frontal or steam) fog • When water evaporates from rain that falls from warmer air into cold air near surface • Upslope fog
Lifting mechanisms forming clouds LCL updraft
Cloud classification • Layered clouds -- stratus (layered) • Convective clouds --- cumulo (heap) • Clouds are also classified by their altitude • Cirro (high composed of wispy ice crystals) • Alto (in mid-troposphere) • Nimbus (rain)
Nimbostratus (Ns) are deep layered clouds that bring precipitation and appear dark gray
Radiative effects of clouds • Clouds reflect solar radiation effectively • Thicker clouds reflect more –cooling effect • Clouds absorb & emit terrestrial radiation • The altitude of a cloud is important in determining radiative budget • Because the higher the cloud, the colder it is, and the less OLR it emits to space • Clouds can cool or warm the planet
Globally and annually averagedradiative effects of clouds • If clouds were removed without any other change then the Earth would gain approximately 20 W/m2 in net radiation and consequently it would warm.
Cloud composition: water clouds/ ice crystal clouds Whether clouds originate over land or ocean affects the number of CCNs Shape of crystal depends on the environmental temperature