GY205 Weather and Climate Lecture 4

Atmospheric Stability Atmospheric Stability Reviewed

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. 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 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 Snow

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

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 ( mph) winds Near the top of the troposphere Formed by steep pressure/temp gradients

The Jet Stream and Rossby Waves

Global Atmospheric Circulation Model

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 drySummer 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

GY205 Weather and Climate End of Lecture 4