Transcript 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