Ch. 20: Weather Patterns & Severe Storms

Earth-Space Science

Bremen High School

Teacher: Aaron McNeely

Arizona monsoon storm from Kitt Peak

Observatory

Air Masses

An air mass is an immense body of air that has similar temperatures and humidity throughout

Homogenous Produce “air mass weather”, similar

conditions for a few days of time

Classifying Air Masses

The area over which an air mass gets its characteristic temperature and humidity is called its source region

Air masses are named according to their source region and temperature

Source Regions

Polar (P) air masses form at high latitudes toward earth’s poles

Tropical (T) air masses form at low latitudes closer to earth’s equator

Maritime and Continental

Surfaces: Land or water Continental (c) air masses form over

land Maritime (m) air masses form over

water

Air Masses

Polar (P)

(cold)

Tropical (T)

(warm)

Continental (c)

(land)

cP

(land, cold)

cT

(land, warm)

Maritime (m)

(water)

mP

(water, cold)

mT

(water, warm)

North American Air Masses

Air masses are classified by the region over which they form Nor’easters

SW monsoonsIndian Summer

Lake Effect Snow

When cP air masses move over warmer lake water (Great Lakes), the air mass draws heat and moisture from the lake and precipitates it as lake effect snow on the leeward shores

Leeward cities such as Rochester and Buffalo, New York are among the snowiest in the country

Great Lakes Lake Effect

•NASA Satellite view (12-5-00) of lake effect snow over Michigan and Northern Indiana

•Where is Bremen?

http://antwrp.gsfc.nasa.gov/apod/ap041130.html

SW Monsoons

The monsoons occur in Southern California, Arizona, and New Mexico during midsummer

mT air masses moving inland from over the ocean bring warm, moist air with storms and occasional heavy precipitation

Despite being a desert, monsoons make Arizona one of the lightning capitals of the United States (Florida is the top lightning capital)

Arizona Monsoon

http://www.solarhaven.org/MONSOON.htm

Indian Summer

cT air masses form in the SW United States and Mexico during summer

In the fall, cT air masses can create the Indian Summer, a short period of warm weather within the cooler autumn

FrontsSec 20.2

When two air masses meet, they form a front

A front is a boundary that separates two air masses of differing temperatures and amounts of moisture

Types of Fronts

Types of fronts Warm Cold Stationary Occluded

Warm Fronts

A warm front forms when warm air moves into an area formerly covered by cooler air

(all front symbols) http://members.aol.com/pakulda/images/

Warm Front

Cold Fronts

A cold front forms when cold, dense air moves into a region occupied by warmer air

Cold Front

Stationary Fronts

Occasionally, the flow of air on either side of a front is parallel to the front

In such cases, the surface position of the front does not move, and a stationary front forms

Occluded Front

When an active cold front overtakes a warm front, an occluded front forms

The occluded front forms when an advancing cold front wedges beneath a warm front and pushes it upward

Occluded FrontsA cold front moves toward a warm front, forcing warm air aloft

A cold front merges with the warm front to form an occluded front that drops heavy rain

Because occluded fronts can move slowly, light precipitation falls for many days

Middle-Latitude Cyclones

Middle-latitude cyclones are the main weather producing systems for the United States

Low pressure West to east

L

Cyclonic System (Low Pressure)

Note the warm, cold, and occluded fronts

Satellite View of a Cyclone

Note the counter-clockwise spiral over the continental US

ThunderstormsSec 20.3

Thunderstorms produce lightning and thunder, and frequently gusty winds, heavy rain, and hail

Cumulonimbus clouds, fronts

Occurrence of Thunderstorms

The US experiences about 100,000 thunderstorms per year mostly in the Gulf of Mexico area and Florida

Any part of the US experiences 30-100 thunderstorms per year

Development of Thunderstorms

Three stages of development:

1. Cumulus stage Strong updrafts of moist, warm air

2. Mature stage Heavy precipitation, most active stage

3. Dissipating stage Down moving air dominates, storm dies

Stages of Thunderstorm Development

Time

Tornadoes

Violent rotating windstorms Column of rotating air called a vortex The vortex extends downward from a

cumulonimbus cloud Single or multiple vortices

Occurrence of Tornadoes

United States, tornado capital About 770 tornadoes occur each year

in the US Most occur between April and June Most tornadoes form in association

with severe thunderstorms

1925 Tri-State Tornado

The deadliest tornado in US history occurred on March 18, 1925

The Tri-State or Great Tornado killed 695 people as it raced across parts of Missouri, Illinois, and Indiana

http://www.geocities.com/Heartland/7847/tornado2.htm

The Great Tornado

Hurricanes

Hurricanes are the most violent storms on earth

Generally form in the tropics Whirling tropical cyclones that produce

winds 75 mph

Hurricane Damage

Destructive waves up to 45 feet high Flooding Heavy winds Storm surges, a dome of water 40-50

miles wide that sweeps over land 50% of the US population lives within

50 miles of a coast

Cross-Section of an Hurricane

Hurricane Names

World Meteorological Association, annual list of names

Names originally all short, female; Male names were introduced in 1978

Names are given when a disturbance becomes a tropical storm (winds 38-74 mph)

2008 Hurricane Names

ArthurBertha

CristobalDolly*

EdouardFay

Gustav*

HannaIke*

JosephineKyle

LauraMarcoNana

* US landfall, major hurr’

OmarPalomaReneSally

TeddyVicky

Wilfred

2009 Hurricane Names

AnaBill

ClaudetteDannyErikaFred

Grace

HenriIda

JoaquinKateLarryMindy

Nicholas

OdettePeterRoseSam

TeresaVictor

Wanda

http://www.nhc.noaa.gov/aboutnames.shtml

Hurricane Katrina

Deadliest hurricane in US history 1,836 deaths $81.2 billion in damage, the costliest

natural disaster in US history Katrina flooded 80% of New Orleans,

devastated other cities such as Biloxi MS and Mobile AL

Katrina—August 28, 2005

New Orleans freeway submerged

Katrina NASA satellite view

http://www.katrinahelp.com/hurricane-katrina-pictures.html