Ch 20: Weather Patterns and
Severe Storms
Overview
TAKE NOTES ON  bullet points!!
Air Masses and Weather
• Air Mass – immense body of air that is
characterized by similar temperatures and amounts
of moisture at any given altitude
• Because of its size, it may take several days for an air
mass to move over an area (giving that area fairly
constant weather)
• When an air mass moves out of the region over
which it formed, it carries its temperature and
moisture conditions with it
• As it moves, the characteristics of an air mass
change, and so does the weather in the area over
which the air mass moves
Influence of
a Canadian
Air Mass
Classifying Air Masses
• The area over which an air mass gets its characteristic properties of
temperature and moisture is called its source region
• Air masses are named according to their source region
• Polar (P) air masses (cold air) form at high latitudes, while tropical (T)
air masses (warm air) form at low latitudes
• In addition to their overall temperature, air masses are classified
according to the surface over which they form
• Continental (c) form over land (dry air) and maritime (m) form over
water (humid air)
• The four basic types of air masses in North America:
Continental Polar (cP) – cold and dry
Continental Tropical (cT) – warm and dry
Maritime Polar (mP) – cold and moist
Maritime Tropical (mT) – warm and moist
Air Mass Source Regions
Weather in North America
• Much of the weather in North America, especially weather east of
the Rocky Mountains, is influenced by continental polar (cP) and
maritime tropical (mT) air masses
• Continental polar air masses are uniformly cold and dry in winter and
cool and dry in summer; lake-effect snow is caused when one of
these air masses passes over the moisture of the lake and drops the
precipitation down on the leeward side of the lake
• Maritime tropical air masses are warm, loaded with moisture, and
usually unstable; they are the source of much of the precipitation in
the eastern two thirds of the U.S.
• During the winter, maritime polar (mP) masses that affect weather
in North America come from the North Pacific
• Continental tropical air masses have the least influence on the
weather of North America
• Only occasionally do continental tropical (cT) air masses affect the
weather outside their source region
Continental Polar and Maritime Polar Air
Masses
Formation of Fronts
• When two air masses meet, they form a front, which
is a boundary that separates two air masses
• Fronts can form between any two contrasting air
masses
• In contrast to the vast sizes of air masses, fronts are
narrow (~15-200 km wide)
• In the ideal case, the front would move in the same
direction with warmer air overlying cooler air
• However, the distribution of pressure across a front
causes one air mass to move faster than the other,
causing it to advance into the slow air mass
Warm Fronts
• A warm front forms when warm air moves into an
area formerly covered by cooler air
• The surface position of a warm front is shown by a
red line with semi-circles that point toward the
cooler air
• The first sign of the approaching warm front is the
appearance of cirrus clouds
• Because of their slow rate of movement and very
low slope, warm fronts usually produce light-tomoderate precipitation over a large area for an
extended period
• A gradual increase in temperature occurs with the
passage of a warm front, as does a wind shift from
the east to the southeast
Warm Front
Cold Fronts
• A cold front forms when cold, dense air moves into a
region occupied by warmer air
• On a weather map, the surface position of a cold front is
shown by a blue line edged with blue triangles that
point toward the warmer air mass
• As a cold front moves in, it becomes steeper and
advance more rapidly than warm fronts
• The forceful lifting of air along a cold front can lead to
heavy downpours and gusty winds
• As a cold front approaches, towering clouds can often
be seen in the distance
• The weather behind a cold front is dominated by a cold
air mass, so weather clears shortly after the front has
passed
Cold Front
Stationary Fronts
• Occasionally, the flow of air on either side of a front
is neither toward the cold air mass nor toward the
warm air mass, but almost parallel to the line of the
front
• The surface position of the front does not move,
and a stationary front forms
• On a weather map, stationary fronts are shown by
blue triangles on one side of the front and red
semicircles on the other
• Sometimes, gentle to moderate precipitation
occurs along a stationary front
Occluded Fronts
• When an active cold front overtakes a warm
front, an occluded front forms
• The weather associated with an occluded
front is complex: most precipitation is
associated with the warm air’s being forced
upward
• Remember, fronts, like all aspects of nature,
do not always behave as we expect them to
Formation of an
Occluded Front
Middle-Latitude Cyclones
• The main weather producers in the U.S. are middlelatitude cyclones, denoted by the letter L
• Middle-Latitude Cyclones – large centers of low
pressure that generally travel from west to east and
cause stormy weather
• The air in these weather systems moves in a
counterclockwise direction and in toward the center
of the low
• Most middle-latitude cyclones have a cold front, and
frequently a warm front, extending from the central
area
• Forceful lifting causes the formation of clouds that
drop abundant precipitation
MiddleLatitude
Cyclone
Model
Cloud
Patterns
Associated
With MiddleLatitude
Cyclones
The Role of Airflow Aloft
• Airflow aloft plays an important role in maintaining
cyclonic and anticyclonic circulation
• The rotating wind systems are actually generated by
upper-level flow
• Surface convergence must be offset by outflow
somewhere higher in the atmosphere
• As long as the spreading out of air high up is equal to or
greater than the surface inflow, the low-pressure system
can be sustained
• More often than not, air high up in the atmosphere
fuels a middle-latitude cyclone
• In an anticyclone, air spreading out at the surface is
balanced by air coming together from high up
Role of Airflow Aloft in Cyclonic Activity
Thunderstorms
• Thunderstorm – a storm that generates lightning and
thunder; frequently produce gusty winds, heavy rain, and
hail
• Thunderstorms may be produced by a single cumulonimbus
cloud, or it may be associated with clusters of
cumulonimbus clouds along a cold front
• At any given time, there are an estimated 2000
thunderstorms in progress on Earth (~45,000 a day and 16
million a year); the U.S. experiences ~100,000 a year
• Thunderstorms form when warm, humid air rises in an
unstable environment
• The life span of a single cumulonimbus cell within a
thunderstorm is only about an hour or two, but as the
storm moves, it is constantly getting fresh supplies of
warm, humid air
U.S. Distribution of Thunderstorms
Stages in the Development of a
Thunderstorm
Tornadoes
• Tornadoes – violent windstorms that take the form of a
rotating column of air called a vortex; the vortex extends
downward from a cumulonimbus cloud
• In the U.S., ~770 tornadoes are reported each year;
greatest occurrence between April and June
• Most tornadoes form in association with severe
thunderstorms
• An important process in the formation of many tornadoes
is the development of a mesocyclone (see diagram)
• Pressures within some tornadoes can be as much as 10%
lower than the surrounding area, causing air near the
ground to be “sucked” into the vortex
• One scale used to measure tornado intensity is the Fujita
tornado intensity scale
Tornado Suction Vortices
Formation of a Mesocyclone
U.S. Tornado Incidences
Hurricanes (2)
• Hurricanes – whirling tropical cyclones that produce
winds of at least 119 kilometers per hour (also known as
typhoons, cyclones, and tropical cyclones)
• At sea, they can generate 15-meter high waves capable of
destruction hundreds of kilometers away
• A hurricane is a heat engine that is fueled by the energy
given off when huge quantities of water vapor condense
• Hurricanes develop most often in the late summer when
water temperatures are warm enough to provide the
necessary heat and moisture to the air
• Eye Wall – doughnut-shaped wall that surrounds
the center of the storm, where the greatest wind
speeds and heaviest rainfall occur
• Eye – center of the storm where precipitation
ceases and winds subside
• The intensity of a hurricane is described using the
Saffir-Simpson scale
• Storm Surge – a dome of water about 65 to 80
km wide that sweeps across the coast where a
hurricane’s eye moves onto land
• A hurricane weakens when it moves over cooler
ocean water and land
Hurricane Cross-Section
Hurricane Katrina
Assignment
• Read Chapter 20
• Do Ch. 20 Assessment
– #1-37 (pg. 583-584)
– #1-4 (pg. 585)
• Study for Chapter 20 Quiz!