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Relative Humidity vs Dewpoint: see pages 134-137

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. Dewpoint Absolute measure of moisture in air If temperature drops to the dewpoint air is saturated

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Relative Humidity vs Dewpoint: see pages 134-137

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  1. 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 Dewpoint Absolute measure of moisture in air If temperature drops to the dewpoint air is saturated Physical comfort, precipitation forecasting Typical Daily T, Td, RH Cycle Relative Humidity vs Dewpoint: see pages 134-137

  2. Cloud Formation • Air 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 it • Rising Air • Convection (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 shadows • Condensation Nuclei: surfaces upon which water can condense • Salt, smoke, dust • Grass blade tips, car surfaces (dew, not clouds)

  3. Precipitation Processes • How do we get from tiny cloud droplets to large raindrops or snowflakes? (See pp. 321-324) • 1) Warm rain process: collision, coalescence • Droplets collide and stick together to form bigger drops, which fall and collide with more droplets on the way down, forming large rain drops • Tropics • 2) Cold (sub-freezing) clouds • Bergeron-Findeisen Process • Much of the United States

  4. Bergeron-Findeisen Process I • All 3 phases of water exist in same cloud • Just like water droplets need condensation nuclei, ice crystals need ice nuclei • Resemble ice crystals • Much less abundant than condensation nuclei • This is why liquid water is found in clouds as cold as –40 degrees • Thus: cloud initially has a lot of liquid water and very little ice

  5. Bergeron-Findeisen Process II • Air within the cloud is saturated, but… • SVP over water is higher than SVP over ice • Ice molecules more tightly bonded • In-cloud air is supersaturated with respect to ice and “subsaturated” w.r.t. liquid water • Ice crystals will grow at liquid water’s expense • Crystals grow heavy enough to fall • Shatter and create more ice nuclei, starting process anew • Catch droplets on the way down and reach the ground as rain or snow

  6. Key Figures • 4.16, 4.18, 4.51, 8.13, 8.32, 8.33, 8.34

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