GY205 Weather and GY205 Weather and ClimateClimate

Lecture 4Lecture 4

Atmospheric Stability Atmospheric Stability

Atmospheric Stability Reviewed

PrecipitationPrecipitation• 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 GrowHow 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.Forms of Precip.• 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 RainMeasuring Rain• Raingauges

• Doppler radar

• 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

Measuring SnowMeasuring Snow

Cloud SeedingCloud 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 CirculationAtmospheric 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 BeltsPressure 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

Trade WindsTrade 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 WesterliesThe 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 FrontThe 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 StreamsJet Streams• High-speed (50-100+mph) winds• Near the top of the troposphere• Formed by steep pressure/temp gradients

The Jet Stream and The Jet Stream and Rossby Waves Rossby Waves

The Jet Stream and Rossby Waves

Global Atmospheric Global Atmospheric Circulation Model Circulation Model

Global Atmospheric Circulation Model

Seasonal Pressure and Seasonal Pressure and Precipitation Patterns Precipitation Patterns

Seasonal Pressure and Precipitation Patterns

Major Wind SystemsMajor 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 NiEl Niñoño• 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

El Niño and La Niña

GY205 Weather and GY205 Weather and ClimateClimate

End of Lecture 4