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Chapter 17

Weather Patterns and

Severe Storms

I. Air masses

A. Characteristics

1. Large body of air

a. 1600 km (1000 mi.) or more across

b. Several kilometers thick

2. Similar temperature at any given altitude

3. Similar moisture at any given altitude

4. Move and affect large part of a continent

B. Source region - the place where an air mass acquires its properties


C. Classification of air mass

1. Two criteria are used to classify

a. By the nature of the surface in the source region

1. Continental ( c )

a. Form over land

b. Likely to be dry

2. Maritime (m)

a. Originate over water

b. Humid air

b. By the latitude of the source region

1. Polar ( P )

a. High latitude

b. Cold

2. Tropical ( T )

a. Low latitudes

b. Warm


2. Four basic types of air masses

a. Continental polar ( cP )

b. Continental tropical ( cT )

c. Maritime polar ( mP )

d. Maritime tropical ( mT )


D. Air masses and weather

1. cP and mT air masses are the most important in North America, especially east of the Rockies

2. North America ( east of the Rocky Mountains )

a. Continental polar ( cP )

1. From northern Canada and interior of Alaska

a. Winter - brings cold, dry air

b. Summer - brings cool relief


Air masses and weather continued

2. Responsible for lake-effect snows

a. cP air mass crosses the Great Lakes

b. Air picks up moisture from lakes

c. Snow occurs on the leeward shores of the lakes

b. Maritime tropical ( mT )

1. From the Gulf of Mexico and the Atlantic Ocean

2. Warm, moist, unstable air

3. Brings precipitation to the eastern United States


Air masses and weather continued

3. Continental tropical ( cT )

a. Southwest and Mexico

b. Hot, dry

c. Seldom important outside the source region

4. Maritime polar ( mP )

a. Brings precipitation to the western mountains

b. Occasional influence in the northeastern United States causes the " Northeaster " in New England with its cold temperature and snow.


III. Fronts

A. Boundary that separates air masses

1. Air masses retain their identities

2. Warmer, less dense air forced aloft

3. Cooler, denser air acts as wedge


B. Types of fronts

1. Warm front

a. Warm air replaces cooler air

b. Shown on map with semicircles

c. Small slope (1:200)

d. Clouds become lower as the front nears

e. Slow rate of advance

f. Light-to-moderate precipitation

g. Gradual temperature increase with the passage of the front


2. Cold Front

a. Cold air replaces warm air

b. Shown ona map with triangles

c. Twice as steep as warm fronts (1:100)

d. Advances faster than a warm front

e. Weather more violent than a warm front

1. Intensity of precipitation is greater

2. Duration of precipitation is shorter

f. Weather behind the front is dominated by

1. Cold air mass

2. Subsiding air

3. Clearing conditions


III. Middle - latitude cyclones

A. Primary weather producer in the middle-latitudes

B. Life cycle

1. Originate along a front where air masses are moving parallel to the front in opposite directions

a. Continental polar (cP) air is often north of the front

b. Maritime tropical (mT) air is often south of the front

2. Frontal surface takes on a wave shape with low pressure centered at the apex of the wave

3. Warm front and cold front form

4. Cold front catches up to warm front and produces an occlusion

5. Warm sector is forced aloft

6. Fronts discontinue

7. Storm comes to an end


C. General characteristics

1. Large center of low pressure

a. Counterclockwise air circulation

b. Air flows inward toward center

2. Travel west to east guided by the westerlies

3. Last a few days to more than a week

4. Extending from the center of the low are

a. Cold front, and

b. Frequently a warm front

5. Convergence and forceful lifting cause

a. Cloud development

b. Abundant precipitation


D. Idealized weather

1. Middle-latitude cyclones move eastward across the U.S.

a. First signs of their approach are in the west

b. Require two to four days to pass

2. Largest weather contrasts occur in the spring


3. Changes in weather associated with the passage of a middle-latitude cyclone

a. Changes depend on the path of the storm
b. Weather associated with fronts
1. Warm front
a. Clouds become lower and thicker
b. Light precipitation
1. Perhaps
2. Perhaps prolonged duration
c. After the passage of a warm front
1. Winds become more southerly

2. Warmer temperature (mT air mass)

2. Cold Front

a. Wall of dark clouds

b. Heavy precipitation

1. Narrow band along the front

2. Short duration


Idealized weather continued

C. After the passage of a cold front

1. Wind becomes north to northwest

2. Drop in temperature as a cP air mass moves in

3. Clearing sides


E. Role of airflow aloft

1. Cyclones and anticyclones

a. Generated by upper-level air flow

b. Maintained by upper-level air flow

2. Cyclone

a. Low pressure system

b. Surface convergence

c. Outflow (divergence) aloft sustains the low pressure

3. Anticyclone

a. High pressure system

b. Associated with cyclones

c. Surface divergence

d. Convergence aloft


IV. Severe weather types

A. Thunderstorm

B. Tornado

C. Hurricane


A. Thunderstorm

1. Features

a. Cumulonimbus clouds

b. Heavy rainfall

c. Lightning

d. Occasional hail


Thunderstorm continued

2. Occurrence

a. 2000 in progress at any one time

b. 100,000 per year in the united States

c. Most frequent in

1. Florida

2. Eastern Gulf Coast region


Thunderstorm continued

3. Stages of development

a. All thunderstorms require

1. Warm air

2. Moist air

3. Instability (lifting)

a. High surface temperatures

b. Most common in the afternoon


Thunderstorm continued

b. Require continual supply of warm air

1. Each surge causes air to rise higher

2. Updrafts and down drafts form

c. Eventually precipitation forms

1. Most active stage

2. Gusty winds, lightning, hail

3. Heavy precipitation

d. Cooling effect of precipitation marks the end of thunderstorm activity


B. Tornado

1. Local storm of short duration

2. Features

a. Violent windstorm

b. Rotating column of air

c. Extends down from a cumulonimbus cloud

d. Low pressures inside causes the air to rush into

e. Winds approach 480 km (300 miles) per hour


Tornado continued

3. Occurrence and development

a. Average of 780 each year in the U.S.

b. Most frequent from April through June

c. Associated with severe thunderstorms

d. Exact cause for tornadoes is not known

e. Conditions for the formation of tornadoes

1. Occur most often along a cold front

2. During the spring months

3. Associated with intense thunderstorms


Tornado continued

4. Characteristics

a. Diameter between 150 and 600 meters

b. Speed across landscape is about 45 kilometers per hour

c. Cut about a 10 km long path

d. Most move toward the northeast

e. Maximum winds are about 480 km/hr

f. Intensity measured by the Fujita intensity scale


Tornado continued

5. Tornado forecasting

a. Difficult to forecast because of their small size

b. Tornado watch

1. To alert people

2. Issued when the conditions are favorable

3. Covers 65,00 square kilometers

c. Tornado warning is issued when a funnel cloud is sighted or is indicated by radar

d. Use of Doppler radar helps increase the accuracy by detecting the air motion


C. Hurricane

1. Most violent storm on Earth

2. To be called a hurricane

a. Wind speed in excess of 119 kilometers per hour

b. Rotary circulation

3. Features

a. Tropical cyclone

1. Low pressure (lowest pressure system in the Western Hemisphere)

2. Steep pressure gradient

3. Rapid, inward-spiraling wind


Parts of a Hurricane

1. Eyewall

a. Near the center

b. Rising air

c. Intense convective activity

d. Wall of cumulonimbus clouds

e. Greatest wind speeds

f. Heaviest rainfall


Parts continued

2. Eye

a. At the very center

b. About 20 km diameter

c. Precipitation ceases

d. Wind subsides

e. Air gradually descends and heats

f. Warmest part of the storm

c. Wind speeds reach 300 km/hr

d. Generate 50 foot waves at sea


Hurricane continued

4. Cause great damage on land

5. Known by different names

a. Typhoon in western Pacific

b. Cyclone in Indian Ocean

6. Frequency

a. Most (20 / year) occur in the North Pacific

b. Fewer than 5 occur in the warm North Atlantic

7. Hurricane formation and decay

a. Form in all tropical waters except the

1. South Atlantic

2. Eastern South Pacific


Hurricane continued

B. Energy comes from condensing water vapor

c. Develop most of in late summer when warm water temperatures provide energy and moisture

d. Initial stage is not well understood

1. Tropical depression-winds do not exceed 61 kilometers per hour

2. Tropical storm-winds 61 to 119 km/hr

e. Diminish in intensity whenever

1. They move over cooler ocean water

2. They move onto land

3. The flow aloft is unfavorable


Hurricanes continued

8. Destruction from a hurricane

a. Factors that affect amount of hurricane damage

1. Strength of storm (the most important factor)

2. Size and population density

3. Shape of the ocean bottom near the shore

b. Categories of hurricane damage

1. Wind damage

2. Storm surge

a. Large dome of water

b. 65 to 80 kilometers long

c. Where eye makes landfall

3. Inland freshwater flooding from torrential rains


The End

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