Download Radiation

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