GY205 Weather and
Climate
Lecture 4
Atmospheric Stability Reviewed
Atmospheric Stability
Precipitation
• Solid and liquid water that falls from clouds
• Clouds are made of cloud droplets
• Cloud droplets are too small to fall to earth, they fall very
slowly and evaporate just below the cloud
• Cloud droplets must increase in volume ~1,000,000X to
become raindrops
How Cloud Droplets Grow
• Collision-coalescence process – cloud droplets
collide and combine with other cloud droplets
• Occurs in warm clouds (tropics and subtropics)
• Bergeron process – water evaporates from
supercooled cloud droplets and is deposited
onto ice crystals
• Occurs in cool and cold clouds (mid-latitudes
and above)
Forms of Precip.
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Snow – ice crystals
Rain – liquid water from melting snow (even in summer)
Graupel – small ice pellets
Hail – large ice pellets
• Sleet – rain that has frozen on the way down
• Freezing rain (glaze) – rain that freezes as soon
as it hits the ground
Measuring Rain
• Raingauges
• Doppler radar
Measuring Snow
• Measuring snow more complicated
• Depth – average several measurements
• Water equivalent – depth of water if snow
melted
• Water equivalent averages 10-to-1
• 10 inches snow equivalent to 1 inch of rain
Cloud Seeding
• Attempt to produce precip. by introducing
materials into a cloud
• Substances try to jump-start the Bergeron
process
• Dry ice and silver iodide used
• Overall poor results, very limited success
Atmospheric Circulation
• Single-cell model – non-rotating earth
• Single cell model – with earth rotating
• Three-cell model
• Three cells due to earth’s rotational speed
Pressure and Wind Belts
• Contrasting temps between land and water
break the belts up a bit
• The belts “follow the sun,” moving north during
summer, south during winter
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Trade Winds
Used by sailors to cross to the New World
Hot, rising air near the equator creates the low-pressure
Intertropical Convergence Zone (ITCZ) (the Doldrums)
Air at the surface moves toward ITCZ from the north
and south, creating the Trade Winds between ~0-30°
latitude
The Westerlies
• Wind belt of the middle latitudes
• Between ~30-60° latitude
• Air at the surface flows from the subtropical high
pressure belt (Horse latitudes) toward the subpolar low
pressure belt
The Polar Front
• Extremely cold, dense air sinks at the Polar
Highs and flows across the surface toward the
Subpolar Low pressure belt
• Steep temp/pressure gradient along the polar
front produces the polar jet stream
Jet Streams
• High-speed (50-100+mph) winds
• Near the top of the troposphere
• Formed by steep pressure/temp gradients
The Jet Stream and Rossby Waves
The Jet Stream and
Rossby Waves
Global Atmospheric Circulation Model
Global Atmospheric
Circulation Model
Seasonal Pressure and Precipitation Patterns
Seasonal Pressure and
Precipitation Patterns
Major Wind Systems
• Monsoons – seasonal reversals of wind direction
• Asian monsoon most well known
• Also occurs in SW US to lesser degree
Winter monsoon is dry
Summer monsoon is wet
• Foehn, Chinook, Santa Ana Winds
• Warm, dry winds set in motion by pressure
differences
• Caused by the compression and adiabatic
warming of air flowing down mountain slopes
• World record fastest temp change was due to a
chinook: from -4°F to 45°F in just two minutes!
January 22, 1943 in Spearfish, SD
• Katabatic winds (mistrals, boras)
• Very cold, dense air builds up on high plateaus
• The air periodically flows downslope under the
influence of gravity
• A cold wind
• Sea and Land Breezes
• Convective circulation caused by temp
difference between large bodies of water and the
adjacent land mass
• Mountain and Valley Breezes
• Slopes warm during the day and air rises,
drawing warm air upslope from the valley
• At night, mountains cool rapidly, and cold air
sinks into valley
El Niño
• Warming of eastern Pacific off the coast of South America
• Every 2-5 years; starts around Christmas
• Caused by weakening or reversal of Trade Winds, which ends
upwelling of deep, cold water off of Peru
• El Niño events affect weather in US: wetter in California and
SE; milder, drier in NE
Upwelling off coast of Peru
El Niño and La Niña
El Niño and La Niña
GY205 Weather and
Climate
End of Lecture 4