24.5 Weather Patterns
A cold front moving in brings stormy weather. The
arrow shows the direction that the front is moving.
24.5 Weather Patterns
Air Masses
How do air masses form?
An air mass forms when a large body of air
becomes fairly stationary over a region of
Earth’s surface or as air moves over a large,
uniform region like an ocean.
24.5 Weather Patterns
Air Masses
An air mass is a large body of air that has
fairly uniform physical properties, such as
temperature and moisture content, at any
given altitude.
24.5 Weather Patterns
Air Masses
Air masses can cover large portions of
continents. The air is strongly influenced by
the properties of the region.
• The air above the Gulf of Mexico contains more
water vapor than the air above northwestern
Canada.
• Each type of air mass reflects the characteristics
of the region where it formed.
24.5 Weather Patterns
Air Masses
• An air mass that forms over water, for
example, contains more water vapor than
one that forms over land.
• An air mass that forms over the Arctic is
much colder than one that forms over the
tropics.
24.5 Weather Patterns
Air Masses
Air masses are classified according to
whether they form over water or land, and the
latitude where they form.
• A maritime air mass forms over water.
• A continental air mass forms over land.
• A polar air mass forms north of 50° north
latitude or south of 50° south latitude.
• A tropical air mass is an air mass that originates
in the tropics.
24.5 Weather Patterns
Air Masses
The air masses that most affect weather in
North America include maritime polar air
masses, continental polar air masses,
maritime tropical air masses, and continental
tropical air masses.
24.5 Weather Patterns
Air Masses
Air masses that form in different regions affect
North American weather.
24.5 Weather Patterns
Fronts
What are the four types of fronts?
There are four types of fronts: cold fronts,
warm fronts, stationary fronts, and occluded
fronts.
24.5 Weather Patterns
Fronts
Air masses move with the circulation of air
throughout the world.
• As an air mass moves, it tends to retain the
properties of the region where it formed.
• When different air masses meet, they ordinarily
don’t mix. Instead, a boundary forms between
them.
• The sharply defined boundary that forms when
two unlike air masses meet is called a front.
24.5 Weather Patterns
Fronts
For example, when a continental polar air
mass moves southeast and collides with a
maritime tropical air mass, a front forms
between the two.
Clouds and precipitation often accompany
fronts.
24.5 Weather Patterns
Fronts
Cold Fronts
A cold front occurs when a cold air mass
overtakes a warm air mass.
• Cold air is denser than warm air, so the cold air mass
remains close to the ground and moves underneath
the warm air mass.
• The cold air mass forces the warm air mass to rise.
• Cold fronts usually move quickly and are relatively
steep, so the warm air mass is often lifted rapidly.
24.5 Weather Patterns
Fronts
A cold front forms when a cold air mass
overtakes a warm air mass.
Rising
warm
air
Direction
of front
Cold air mass
sinks under the
warm air mass.
Precipitation
24.5 Weather Patterns
Fronts
The rapid rise of warm air often produces
cumulus and cumulonimbus clouds.
• Cold fronts often bring strong winds, severe
thunderstorms, and large amounts of
precipitation.
• The slope of a cold front is steep, and it travels
quickly, so the severe weather usually lasts for
only a short time.
• After the front passes, skies clear and
temperatures drop.
24.5 Weather Patterns
Fronts
Warm Fronts
A warm front occurs when a warm air mass
overtakes a cold air mass.
• The cold air is denser, so it remains close to the
ground.
• The advancing warm air mass rises up over the cold
air mass.
• A warm front has a gentle slope because the
advancing warm air mass gradually moves up and
over the cold air mass.
24.5 Weather Patterns
Fronts
A warm front occurs when a warm air mass
overtakes a cold air mass.
Clouds form as
rising moist, warm
air condenses.
Rising
warm air
Direction
of front
Cold air mass
sinks under the
warm air mass.
Precipitation
24.5 Weather Patterns
Fronts
As the warm air rises, stratus clouds form.
• If the warm air mass contains a large amount of
moisture, there will be a steady rain.
• Occasionally heavy showers or thunderstorms
occur.
• Once the front has passed, the warm air mass
settles in, the skies are mostly clear, and
temperatures rise.
24.5 Weather Patterns
Fronts
Stationary Fronts
Sometimes when two air masses converge,
neither air mass overruns the other.
• A front forms and either stays where it formed or
moves slowly.
• When two unlike air masses have formed a boundary
but neither is moving, the front is called a stationary
front.
• Stationary fronts often result in clouds and steady
rain or snow for several days.
24.5 Weather Patterns
Fronts
Occluded Fronts
An occluded front forms when a warm air mass
is caught between two cooler air masses.
• The colder air masses force the warm air mass to
rise, trapping it between the two cold air masses.
• Water vapor typically condenses from the warm air.
• Occluded fronts usually bring cloudy skies and
precipitation.
24.5 Weather Patterns
Low- and High-Pressure Systems
What types of weather are associated with
cyclones and anticyclones?
Cyclones are associated with clouds,
precipitation, and stormy weather.
The weather associated with an anticyclone
includes clear skies, very little precipitation,
and generally calm conditions.
24.5 Weather Patterns
Low- and High-Pressure Systems
Air masses are part of larger weather
systems.
Weather systems are organized around either
a center of high air pressure or a center of low
air pressure, and may include more than one
type of air mass.
24.5 Weather Patterns
Low- and High-Pressure Systems
Cyclones
A weather system with a center of low air pressure
is called a cyclone, or sometimes simply a low.
• As the warm air at the center of a cyclone rises,
pressure decreases.
• Because air moves from high-pressure areas to lowpressure areas, air spirals in toward the center of a
cyclone.
24.5 Weather Patterns
Low- and High-Pressure Systems
• Clouds form and precipitation develops.
• Because air is converging in a cyclone, air
masses collide and fronts develop.
• A cyclone generally moves in the direction
of global winds, and the weather of the
cyclone moves along with it.
• Westerly winds typically cause fronts and
storms to move across the United States
from west to east.
24.5 Weather Patterns
Low- and High-Pressure Systems
Winds and heavy rain are common in lowpressure areas, also called cyclones.
24.5 Weather Patterns
Low- and High-Pressure Systems
Anticyclones
A weather system with a swirling center of high air
pressure is called an anticyclone, or sometimes
simply a high.
• Winds in an anticyclone circulate clockwise in the
Northern Hemisphere.
• Because the center of an anticyclone has high
pressure, air flows away from that center.
24.5 Weather Patterns
Low- and High-Pressure Systems
• As air flows away from the center, air from
higher in the troposphere descends and
replaces it.
• When air descends, it warms, and its
relative humidity decreases.
24.5 Weather Patterns
Low- and High-Pressure Systems
A. In the Northern
Hemisphere, winds
in a cyclone blow
counterclockwise
and converge.
B. In an anticyclone,
winds blow
clockwise and
diverge.
24.5 Weather Patterns
Storms
How do thunderstorms and tornadoes form?
Thunderstorms form when columns of air
rise within a cumulonimbus cloud.
A tornado forms when a vertical cylinder of
rotating air develops in a thunderstorm.
24.5 Weather Patterns
Storms
Thunderstorms
A thunderstorm is a small weather system that
includes thunder and lightning.
It is usually accompanied by strong winds and
heavy rain or hail.
24.5 Weather Patterns
Storms
When a cold front forms, the cold air mass
quickly lifts the warm air mass up, and
thunderstorms often result.
Thunderstorms can also occur on hot summer
days when buoyant air rises as part of a
convection current within an air mass.
If the rising air is cooled to the dew point and
the convection is strong enough, a
thunderstorm can result.
24.5 Weather Patterns
Storms
Thunderstorms are associated with tall cumulonimbus
clouds where there are strong updrafts of warm air.
24.5 Weather Patterns
Storms
One striking feature of a thunderstorm is
lightning, which is a sudden electrical
discharge in the atmosphere.
• Positive charges tend to develop near the top of
a cumulonimbus cloud.
• Negative charges tend to develop near the base
of the cloud.
• When the potential difference between the
positive and negative charges becomes great
enough, a discharge occurs.
24.5 Weather Patterns
Storms
Thunder is the sound produced by rapidly
expanding air along the path of a lightning
discharge.
When lightning occurs, the air around it is
heated, expands explosively, and produces
sound waves.
You hear thunder after you see lightning
because light travels much faster than sound.
24.5 Weather Patterns
Storms
Tornadoes
A tornado is a small but intense windstorm that
takes the form of a rotating column of air that
touches the ground.
Tornadoes most often form at the leading edge of
an advancing cold front or along a line of
thunderstorms where winds are converging to a
center of very low pressure.
24.5 Weather Patterns
Storms
A tornado appears as a twisting funnel cloud
descending from the bottom of a
cumulonimbus cloud.
A tornado’s winds can sometimes be more
than 500 kilometers per hour, although they
are more typically about 150 kilometers per
hour.
A large tornado can cause death and
tremendous destruction.
24.5 Weather Patterns
Storms
A tornado crosses a farm field.
24.5 Weather Patterns
Storms
For a tornado to develop, there must be great
differences between the properties of colliding
air masses.
In the United States, where most of the
world’s tornadoes develop, these storms
occur mainly in the Midwest and the South.
24.5 Weather Patterns
Storms
Tropical Storms and Hurricanes
Thunderstorms and tornadoes are often
associated with cyclones of the middle latitudes.
Cyclones also develop in the tropics, where they
can develop into tropical storms or hurricanes.
A hurricane is a large tropical cyclone with winds
of at least 119 kilometers per hour (74 miles per
hour).
24.5 Weather Patterns
Assessment Questions
1. A cold front occurs when
a.
b.
c.
d.
a cold air mass moves in a uniform direction.
a cold air mass collides with another cold air mass.
a cold air mass converges with a warm air mass.
a cold air mass overtakes a warm air mass.
24.5 Weather Patterns
Assessment Questions
1. A cold front occurs when
a.
b.
c.
d.
a cold air mass moves in a uniform direction.
a cold air mass collides with another cold air mass.
a cold air mass converges with a warm air mass.
a cold air mass overtakes a warm air mass.
ANS: D
24.5 Weather Patterns
Assessment Questions
2. An occluded front is generally accompanied by
a.
b.
c.
d.
cloudy skies and precipitation.
violent thunderstorms.
cool, dry weather.
warm, dry winds.
24.5 Weather Patterns
Assessment Questions
2. An occluded front is generally accompanied by
a.
b.
c.
d.
cloudy skies and precipitation.
violent thunderstorms.
cool, dry weather.
warm, dry winds.
ANS: A
24.5 Weather Patterns
Assessment Questions
3. A weather system associated with high air
pressure is called a(n)
a.
b.
c.
d.
occluded front.
warm front.
cyclone.
anticyclone.
24.5 Weather Patterns
Assessment Questions
3. A weather system associated with high air
pressure is called a(n)
a.
b.
c.
d.
occluded front.
warm front.
cyclone.
anticyclone.
ANS: D
24.5 Weather Patterns
Assessment Questions
1. A hurricane forms when a vertical cylinder of
rotating air develops in a thunderstorm.
True
False
24.5 Weather Patterns
Assessment Questions
1. A hurricane forms when a vertical cylinder of
rotating air develops in a thunderstorm.
True
False
ANS:
F, tornado