Earth
Science, 10th edition
Chapter
18: Weather Patterns and Severe Storms
I. Air masses
A. Characteristics
1. Large body of air
a.
1600 km (1000 mi.) or more across
b. Perhaps several kilometers
thick
2. Similar temperature at any given
altitude
3. Similar moisture at any given
altitude
4. Move and affect a large portion
of a continent
B. Source region – the area where an air
mass acquires its properties
C. Classification of air an mass
1. Two criteria are used to classify
air masses
a. By
the latitude of the source region
1. Polar (P)
a. High latitudes
b. Cold
2. Tropical (T)
a. Low latitudes
b. Warm
b. 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
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 air masses 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
2. Responsible for
lake-effect snows
a. cP air mass crosses the Great Lakes
b. Air picks up
moisture from the 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
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 temperatures and
snow
II. Fronts
A. Boundary that separates air masses of
different densities
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 a map by a line
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 on a map by a line
with triangles
c. Twice as steep (1:100) as
warm fronts
d. Advances faster than a warm
front
e. Associated weather is 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
3. Stationary front
a. Flow of air on both sides
of the front is almost parallel to the line of the front
b. Surface position of the
front does not move
4. Occluded front
a. Active cold front overtakes
a warm front
b. Cold air wedges the warm
air upward
c. Weather is often complex
d. Precipitation is associated
with warm air being forced aloft
III. Middle-latitude cyclone
A. Primary weather producer in the
middle-latitudes
B. Life cycle
1. Form 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. Flow of air is counterclockwise
cyclonic circulation
3. Warm front and cold front form
4. Cold front catches up to warm
front and produces an occlusion
5. Warm sector is displaced aloft
6. Pressure gradient weakens and
fronts discontinue
7. Storm comes to an end
C. Idealized weather
1. Middle-latitude cyclones move
eastward across the United States
a. First signs of their
approach are in the western sky
b. Require two to four days to
pass over a region
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
c. After the passage
of a warm front
1. Winds become
more southerly
2. After
passing, warmer temperature is experienced (mT air
mass)
2. Cold front
a. Wall of dark
clouds
b. Heavy
precipitation
1. Hail
2. Occasional
tornadoes
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
skies
D. Role of airflow aloft
1. Cyclones and anticyclones
a. Generated by upper-level
air flow
b. Maintained by upper-level
air flow
c. Typically are found
adjacent to one another
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. Thunderstorms
1. Features
a. Cumulonimbus clouds
b. Heavy rainfall
c. Lightning
d. Occasional hail
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
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 and early evening
b. Require continuous supply of
warm air and moisture
1. Each surge causes air
to rise higher
2. Updrafts and downdrafts
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. Tornadoes
1. Local storm of short duration
2. Features
a. Violent windstorm
b. Rotating column of air that
extends down from a cumulonimbus cloud
c. Low pressures inside causes
the air to rush into
d. Winds approach 480 km (300
miles) per hour
e. Smaller suction vortices
can form inside stronger tornadoes
3. Occurrence and development
a. Average of 770 each year in
the United States
b. Most frequent from April
through June
c. Associated with severe
thunderstorms
d. Exact cause of tornadoes
formation 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 huge
thunderstorms called supercells
4. Characteristics
a. Diameter between 150 and
600 meters (500 and 2000 feet)
b. Speed across landscape is
about 45 kilometers (30 miles) per hour
c. Cut about a 10 km (6 miles)
long path
d. Most move toward the
northeast
e. Maximum winds range beyond
500 kilometers (310 miles) per hour
f. Intensity measured by the
Fujita intensity scale
5. Tornado forecasting
a. Difficult to forecast
because of their small size
b. Tornado watch
1. To alert the public to
the possibility of tornadoes
2. Issued when the
conditions are favorable
3. Covers 65,000 square km
(25,000 square miles)
c. Tornado warning is issued
when a tornado is sighted or is indicated by weather radar
d. Use of Doppler radar helps
increase the accuracy by detecting the air motion
C. Hurricanes
1. Most violent storms on Earth
2. To be called a hurricane
a. Wind speed in excess of 119
kilometers (74 miles) per hour
b.
Rotary cyclonic circulation
3. Profile
a. Form between the latitudes
of 5 degrees and 20 degrees
b. Known as
1. Typhoons in the western
Pacific
2. Cyclones in the Indian
Ocean
c. North Pacific has the
greatest number per year
d. 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
2. Eye
a. At the very center
b. About 20 km (12.5
miles) diameter
c. Precipitation
ceases
d. Winds subsides
e. Air gradually
descends and heats by compression
f. Warmest part of
the storm
c. Wind speeds reach 300 km/hr
d. Generate 50 foot waves at
sea
4. Hurricane formation and decay
a. Form in all tropical waters
except the
1. South Atlantic and
2. Eastern South Pacific
b. Energy comes from
condensing water vapor
c. Develop most often 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 (38 miles) per hour
2. Tropical storm – winds
between 61 to 119 km (38 and 74 miles) per hour
e. Diminish in intensity
whenever
1. They move over cooler
ocean water
2. They move onto land
3. The large-scale flow
aloft is unfavorable
5. 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 of the area affected
3. Shape of the ocean
bottom near the shore
b. Saffir-Simpson
scale ranks the relative intensities of hurricanes
c.
Categories of hurricane damage
1.Storm surge - large dome of water 65 to 80
kilometers (40 to 50 miles) wide
sweeps
across the coast where eye makes landfall
2. Wind damage
3. Inland flooding from
torrential rains