see pages 105106 Relative Humidity Ratio of moisture in air to moisture in air if saturated VP SVP x 100 Relative to air temperature Evaporation rate drying times forest fire danger fog formation ID: 1024858
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1. Relative Humidity vs Dewpoint: see pages 105-106Relative HumidityRatio of moisture in air to moisture in air if saturated (VP / SVP) x 100%Relative to air temperatureEvaporation rate, drying times, forest fire danger, fog formationDewpointAbsolute measure of moisture in airIf temperature drops to the dewpoint air is saturatedPhysical comfort, precipitation forecastingTypical Daily T, Td, RH Cycle
2. Cloud FormationAir must be cooled to saturation point (dewpoint), and then slightly below it (net condensation)Radiational cooling (dew, fog): the ground radiates away more energy than it absorbs and cools the air above itRising AirConvection (showers, thunderstorms)Convergence into low pressure (rain, snowstorms)Orographic lifting: ram air into the side of a mountain where it is then forced to rise (windward vs leeward side, rain shadowsCondensation Nuclei: surfaces upon which water can condenseSalt, smoke, dustGrass blade tips, car surfaces (dew, not clouds)
3. Precipitation ProcessesHow do we get from tiny cloud droplets to large raindrops or snowflakes? (See pp. 321-324)1) Warm rain process: collision, coalescenceDroplets collide and stick together to form bigger drops, which fall and collide with more droplets on the way down, forming large rain dropsTropics2) Cold (sub-freezing) cloudsBergeron-Findeisen ProcessMuch of the United States
4. Bergeron-Findeisen Process IAll 3 phases of water exist in same cloudJust like water droplets need condensation nuclei, ice crystals need ice nucleiResemble ice crystalsMuch less abundant than condensation nucleiThis is why liquid water is found in clouds as cold as –40 degreesThus: cloud initially has a lot of liquid water and very little ice
5. Bergeron-Findeisen Process IIAir within the cloud is saturated, but…SVP over water is higher than SVP over iceIce molecules more tightly bondedIn-cloud air is supersaturated with respect to ice and “subsaturated” w.r.t. liquid waterIce crystals will grow at liquid water’s expenseCrystals grow heavy enough to fallShatter and create more ice nuclei, starting process anewCatch droplets on the way down and reach the ground as rain or snow
6. Precipitation FormationWarm Cold
7. Key Figures4.13, 4.15, 8.30, 8.31, 8.32