Precipitation
	Form clouds
		Have condensation nuclei (silt, sea spray)
			have completely clean air? Need 800% RH to form raindrop 
			With hygroscopic particulates (sea salt)?  Can start at 76% RH
		Have freezing nuclei (silver iodide, clay)
			Less of these, work best at –10 to –20oC

•	Thus, if ‘tall’ enough, one cloud can have:
–	Ice crystals at top (why? way cold there, all freezes)
–	Middle layer with mix of ice and water
–	Lower layer, just water

•	Two key points
–	Water does not have to freeze at 0oC (hand warmer example?)
•	But ice does have to melt (should call it ‘melting point’)
–	Can go to –40oC
–	So, can have mix of water droplets, ice, AND, vapor (triple point)
•	Why ice? Freezing nuclei

•	Second, saturation vapor pressure 
–	Definition; when a volume (of air) has reached capacity for water vapor

•	Saturation vapor pressure over ice is lower than over water
–	Air over ice can hold less moisture than air over water
•	Why? Warm air can hold more moisture than cold air, but…if at same temperature…
•	Ice is a solid, water is not, so molecules more free to move from water (evaporate)
–	If there is enough vapor in air to keep droplet from evaporating away, then there is more than enough to keep ice crystals there

•	Bergeron
•	So, in our cloud…
–	Water molecules escape from droplets at similar rate that vapor condenses onto droplets (equilibrium, 100% RH)
–	BUT, with SVP lower over ice, some vapor deposits (gas to solid)
•	Meaning, the air over ice can’t hold the moisture, so the vapor just freezes on contact w/ ice 
–	As a result of ice ‘stealing’ some vapor, droplets begin to evaporate to restore equilibrium

•	Meaning, RH has dropped, so water droplets evaporate to make up for it
•	Then this new batch of vapor deposits onto ice making ice crystals enlarge more (outpaces sublimation from ice to vapor)
–	And the process continues…
•	Bottom line? Ice crystals grow at the expense of water droplets

Collision/coalescence
•	Greater rainfall associated with c/c in tandem with bergeron
–	Raindrop begins to fall after achieving enough weight (though can be updrafted back into cloud)
•	And achieve enough size to survive evaporation
•	As drop falls, collect more along the way
–	If proper charge (+ or -) 
–	And if droplets not swept away

•	Occur in conjunction with Bergeron (start with bergeron, melt, then C/C)
•	Primary method in warmer clouds (lower altitude) of tropics
•	Fewer condensation nuclei (thus bigger drops) in tropics…why? Less dust thrown into atmosphere; more veg, to anchor

•	Rain 
–	Drops of water falling from a cloud w/ diameter of at least 0.5mm
–	< 0.5mm? – drizzle, mist
–	Virga – rain that falls but does not reach ground

•	Snow 
–	Precipitation in form of ice crystals
•	Low temp = ‘powder snow’ individual crystals
•	Higher temp = larger agglomeration of flakes