ESCI 106 – Weather and ESCI 106 – Weather and ClimateClimate

Lecture 7Lecture 7

9-29-20119-29-2011

Jennifer D. Small Jennifer D. Small

Weather Fact of the Day: September 29

1938: Five tornadoes struck Charleston, South Carolina, killing 32 people and causing $2 million in damages

National Watches and Warnings

“ “ Chapter 7- Circulation of the Chapter 7- Circulation of the Atmosphere”…Con’tAtmosphere”…Con’t

MonsoonsThe greatest seasonal change in Earth’s global circulation is

the MONSOON.

Monsoon does NOT mean rainy season.

Refers to a wind system that exhibits a pronounced seasonal reversal in direction.

Winter : Winds blow off the continents (dry) Summer: Winds blow from the sea towards the land (moist)

The Summer Monsoon brings all the rain.

Asian MonsoonA seasonal reversal in weather patternsAn alternation between two types of weather

patters Ex: India – Wet hot summer, dry cool(ish) winter

A seasonal reversal of wind also

F

SUMMER MONSOON WINTER MONSOON

L H

LHL

H

H

L

Warm Ocean Warm OceanHot Indian Continent

COLD

Down sloping air = No clouds

Asian Monsoon

SUMMER MONSOON WINTER MONSOON

Siberian High

North American Monsoon

Weaker than the Asian Monsoon

Hot temperatures over the desert cause an intense low to form

Moisture for summer precipitation comes from Gulf of Mexico and California

Westerlies

General west to east flow of air aloft

Due to the effects of the PGF and Coriolis Force

Wind speeds increase with height until the tropopause then decrease.

Jet Streams

Weaker than the Asian Monsoon

Hot temperatures over the desert cause an intense low to form

Moisture for summer precipitation comes from Gulf of Mexico and California

Jet Streams

Wind speeds of Jet Stream Winter 120-240 mph Summer ~100 mph

Height (altitude) ~7 to 12 km above the

surface

High winds, high in the atmosphereLocation of jet stream influences local weatherRegion of the upper atmosphere where a narrow

band of air moving REALLY fastLARGE temperature constasts

Jet Streams

Polar Jet Stream (Mid-Latitude) Where the Polar Front is located Also called the mid-latitude jet stream Where the cold polar easterlies interact with the warm westerlies It’s a Geostrophic Wind since it’s high up in the atmosphere

Meanders west to eastCan exceed 500 km an hour

(300 mph) In winter on average it

travels at 125 kph (75 mph) In summer ~65 kph

Jet Streams

Slower than the polar jet.Still travels west to eastUsually at about 25 NAltitude of 13 km.

Subtropical Jet Stream Semi-permanent jet that exists over the subtropics Is mainly a wintertime phenomenon. Due to the weak summertime temperature gradient, the

subtropical jet is relatively weak during the summer.

Waves in the Westerlies

The Jet Streams do not travel in a straight lineThey follow wavy paths at long wave lengths, these

waves are called Rossby Waves.The waves move slowlyHas seasonal changes in strength and location

Global Winds and Ocean Currents

Winds are the driving force for ocean currentsA relationship exists between ocean and atmosphere

circulation.

El Nino / Southern Oscillation

Ocean phenomenon that occurs in the Equatorial Pacific

As the southeast trade winds decrease in strength (weaken) the warm water can make it farther across the Pacific towards South American

El Nino / Southern Oscillation

Named El Nino because it usually starts during Christmas.

Usually happen every 3-7 years.

El Nino / Southern Oscillation

Marked by abnormal weather patterns Especially for Equador and Peru Cold-nutrient rich water is blocked by

the warm water.

In California we have increase severe winter storms.

Strongest ones in recent years: 1982-83 and 1997-98

San Francisco, County

Santa Cruz, County

La Nina

The opposite of an El Nino

When you have colder than average temperatures in the Pacific.

“ “ Chapter 8- Air MassesChapter 8- Air Masses

What is an air mass?

Air Mass really big blob of air with similar properties Usually 1600 km (1000 miles) across Several km thick

Change in weather when one air moves out and a new air moves in Also known as a FRONTAL PASSAGE Brings changes in T, P, RH and Winds

Area between different air masses is known as FRONT

How is an airmass formed?

Air masses form when they move over large regions that exhibit very similar properties Just land or water Has to stay put for a while Be stationary or slow moving…. Like anti-cyclones

If more heat is added than escapes an air mass, then it warms up

If more moisture is added than precipitates, then its humidity increases

Obviously, the opposites are also true.

Air Mass Source Regions

Polar and tropical regions tend to exhibit such properties and therefore are good source regions.

The mid-latitudes tend to be strongly varies and therefore are not good source regions

Types of Air Masses

Polar (P) air masses form over northern Canada and the Arctic. The tend to be COLD.

Tropical (T) air masses form to the south of the United States. These tend to be WARM.

Maritime (m) Air masses form over ocean (either the Pacific, North Atlantic, subtopical Pacific, or Gulf of Mexico). These tend to be HUMID.

Continental (c) air masses form over land (northern Canada or northern Mexico). These tend to be DRY.

Air masses are designated by two-letter combinations describing whether they are m or c, P or T. Describes basic info about T and RH.

Air Mass Designations

All the possible combinations are: mP: N Pacific or N Atlantic mT: Subtropical Pacific or Gulf of Mexico cP: Northern Canada cT: Northern Mexico and South West USA

cP (continental polar)

Also continental Arctic (cA)

Generally cold and dry

During winter, cooled by the land, creating stable air mass.Mostly cloud free

In summer, warmer and more humid than in winterTend to remain farther north and therefore do not influence

as much of North American as during winter

cP (continental polar)

Land warms the air mass slightly, so can be slightly unsettled, resulting in fair weather cumulus.

Related to lake effect snows around the Great Lakes

Can affect large regions of central and northeastern US during winter, much less during the summer.

mP (maritime polar)

Because systems move W to E, generally mP from N Pacific is more influential.

In general, this is where CA gets our bad weather from

mP (maritime polar)

Humid and coolGenerates low level stratus clouds, which maintain

instability through cloud-top radiative coolingThis leads to somewhat unpleasant showers in Pacific

NW, and to much of our bad winter weather.

mP (maritime polar)

Upon orographic lifting, get heavy snow and rain (in the Sierras)

Tend to remain farther N during the summer (recall the jet stream average latitude in summer is 50N, the US/Canada border).

mT (maritime tropical)

Hot and dry, forming over the desert regions of North Mexico and South West US.

Intense surface heating creates very unstable air, but generally cloud-free because of extremely dry conditions

If however, some moisture is acquired, intense thunderstorm can develop.

Occasionally bring drought to the Plains states.

cT (continental tropical)

Warm and moist

Can be unstable because of high heating from warm waters perfect for precipitation.

cT (continental tropical)

In summer, land is even warmer creating even more instability, often triggering thunderstorms.

In winter, often warmer than underlying land (more stable), bringing widespread fog and low level precipitation.

Strongly influences southeastern US much of the year. During summer, influence extends further north.