Relative Humidity vs Dewpoint see pages 134 137

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 Physical comfort, precipitation forecasting Typical Daily T, Td, RH Cycle

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)

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

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

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

Key Figures § 4. 16, 4. 18, 4. 51, 8. 13, 8. 32, 8. 33, 8. 34