Chapter 19
Learning Objectives
After reading, studying, and
discussing the chapter, students should be able to:
·
Explain what an
air mass is.
·
Describe how air
masses are classified.
·
Describe the
general weather associated with each air mass type.
·
Discuss the
differences between warm fronts and cold fronts.
·
Describe the
primary mid-latitude weather-producing systems.
·
List the
atmospheric conditions that produce thunderstorms, tornadoes, and hurricanes.
Chapter Outline___________________________________________________________________
I. Air masses
A.
Characteristics
1.
Large body of air
a. 1600 kilometers (1000 miles) or more
across
b.
Perhaps several kilometers thick
2.
Similar temperature at any given altitude
3.
Similar moisture at any given altitude
4.
Moves and affects a large portion of a continent
B.
Source region—the area where an air mass acquires its properties
C.
Classification of an air 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
2.
North America (east of the
a.
Continental polar (cP)
1.
From northern
a.
Winter brings cold, dry air
b.
Summer brings cool relief
2.
Responsible for lake-effect snows
a.
cP air mass crosses the
b.
Air picks up moisture from the lakes
c.
Snow falls on the leeward shores of the lakes
b.
Maritime tropical (mT)
1.
Sources are the Gulf of Mexico and the
2.
Warm, moist, unstable air
3. Brings precipitation to the eastern
3.
Continental tropical (cT)
a.
Southwest and
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
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 at 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 that is forced aloft
III.
Middle-latitude cyclone
A.
Primary weather producer in the middle-latitudes
B. Life
cycle
1.
Forms 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
4.
Warm front and cold front form
5.
Cold front catches up to warm front and produces an occlusion
6.
Warm sector is displaced aloft
7.
Pressure gradient weakens and fronts fade
8.
Storm comes to an end
C.
Idealized weather
1. Middle-latitude cyclones move eastward
across the
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 front passes, 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.
Temperature drops 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
c.
Most frequent in
1.
2.
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 in
d.
Winds approach 480 kilometers (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
b.
Most frequent from April through June
c.
Associated with severe thunderstorms
d.
Exact cause of tornado 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-kilometer (6-mile) -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.
Alerts the public to the possibility of tornadoes
2.
Issued when the conditions are favorable
3.
Covers 65,000 square kilometers (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.
Necessary conditions
a.
Wind speed in excess of 119 kilometers (74 miles) per hour
b.
Rotary cyclonic circulation
3.
Profile
a.
Form between 5° and 20° latitude