Air Masses and Fronts

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Table of Contents

• Air Masses and Fronts
• Storms
• Predicting the Weather

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Classifying Air Masses
- Air Masses and Fronts

• Four major types of air masses influence the
  weather in North America maritime tropical,
  continental tropical, maritime polar, and
  continental polar.

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Types of Air Masses
- Air Masses and Fronts

• Air masses can be warm or cold, and humid or dry.
  As an air mass moves into an area, the weather
  changes.

4
How a Front Forms
- Air Masses and Fronts

• The boundary where unlike air masses meet is
  called a front.

5
Types of Fronts
- Air Masses and Fronts

• Colliding air masses can form four types of
  fronts cold fronts, warm fronts, stationary
  fronts, and occluded fronts.

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Weather Fronts Activity
- Air Masses and Fronts

• Click the Active Art button to open a browser
  window and access Active Art about weather fronts.

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Cyclones and Anticyclones
- Air Masses and Fronts

• Winds spiral inward towards the low-pressure
  center of a cyclone. Winds spiral outward from
  the high-pressure center of an anticyclone.

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Comparing and Contrasting
- Air Masses and Fronts

• As you read, compare and contrast the four types
  of fronts by completing a table like the one
  below.

Types of Weather
Front
How It Forms
Cold front
A cold air mass overtakes a warm air mass.
Clouds, possibly storms with heavy precipitation
Warm front
A warm air mass overtakes a cold air mass.
Clouds, light precipitation
Stationary front
Cold and warm air masses meet, but neither can
move the other.
Clouds, precipitation
Occluded front
A warm air mass is caught between two cold air
masses.
Clouds, precipitation
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End of SectionAir Masses and Fronts
10
Thunderstorm Formation
- Storms

• A thunderstorm forms when warm, humid air rises
  rapidly within a cumulonimbus cloud.

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Tornado Formation
- Storms

• Tornadoes can form when warm, humid air rises
  rapidly in thick cumulonimbus cloudsthe same
  type of clouds that bring thunderstorms.

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Tornado Alley
- Storms

• Tornadoes in the U.S. are most likely to occur in
  a region known as Tornado Alley.

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Structure of a Hurricane
- Storms

• In a hurricane, air moves rapidly around a
  low-pressure area called the eye.

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Hurricane Andrew
- Storms

• The path of Hurricane Andrew over three
  consecutive days can be seen below.

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Lake-Effect Snow
- Storms

• As cold dry air moves across the warmer water, it
  becomes more humid as water vapor evaporates from
  the lake surface. When the air reaches land and
  cools, snow falls.

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Sequencing
- Storms

• As you read, make a flowchart like the one below
  that shows how a hurricane forms. Write each step
  of the process in a separate box in the order in
  which it occurs.

Hurricane Formation
Begins as a low-pressure area over warm water,or
a tropical disturbance.
Warm, humid air rises and begins to spiral.
As air rises, more warm, moist air is drawn into
the system and the hurricane gains energy.
As winds spiral inward, bands of high windsand
heavy rains form.
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More on Thunder and Lightning
- Storms

• Click the Planet Diary button for an activity
  aboutthunder and lightning.

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Tornadoes
- Storms

• Click the Video button to watch a movie about
  tornadoes.

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Hurricanes
- Storms

• Click the Video button to watch a movie about
  hurricanes.

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End of SectionStorms
21
Red Sky
- Predicting the Weather

• A red sky is one kind of observation that helps
  people to predict the weather.

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Computer Weather Forecasting
- Predicting the Weather

• Scientists use computers to develop different
  models of how a front may move. These predictions
  are then used to make weather forecasts. As more
  data become available, some models are found to
  be incorrect, while others are found to closely
  fit the predicted conditions. The upper graph
  shows the predicted air pressure from two models.
  The lower graph shows actual data for air
  pressure.

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Computer Weather Forecasting
- Predicting the Weather

• Reading Graphs
• What two variables are being graphed?

• Time of day and air pressure

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Computer Weather Forecasting
- Predicting the Weather

• Interpreting Data
• How is air pressure predicted to change according
  to each model in the top graph?

• According to model A, air pressure will drop
  slightly then increase. According to model B, air
  pressure will steadily decrease.

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Computer Weather Forecasting
- Predicting the Weather

• Inferring
• Which computer model most closely matches the
  actual air pressure data?

• Model B

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Computer Weather Forecasting
- Predicting the Weather

• Predicting
• What weather would you forecast for Monday and
  Tuesday? Explain.

• Stormy weather, clouds, and precipitation
  accompany low air pressure.

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Reading Weather Map Symbols
- Predicting the Weather

• The figure below shows what various weather
  symbols mean.

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Reading Weather Maps
- Predicting the Weather

• This is the type of weather map produced by the
  National Weather Service. It shows data collected
  from many weather stations.

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Reading Weather Maps
- Predicting the Weather

• Weather maps in newspapers use symbols to show
  fronts, high- and low-pressure areas, and
  precipitation. Color bands indicate different
  temperature ranges.

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Previewing Visuals
- Predicting the Weather

• Before you read, preview Figure 21, a weather
  map. Then write four questions that you have
  about the map in a graphic organizer like the one
  below. As you read, answer your questions.

Previewing Figure 21
Q. What type of front is located west of
Okalahoma City?
A. A cold front
Q. What do the stick symbols indicate?
A. Amount of cloud cover, atmospheric pressure,
wind direction and speed, and temperature
Q. What are the slender, curvy lines?
A. Isobars, which join places with the same air
pressure
Q. What does the symbol to the east of Florida
mean?
A. A hurricane
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More on Weather Maps
- Predicting the Weather

• Click the Planet Diary button for an activity
  aboutweather maps.

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More on Doppler Radar

• Click the PHSchool.com button for an
  activityabout Doppler radar.

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End of SectionPredicting the Weather
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Graphic Organizer

Typical Time of Year
Type of Storm
Where Forms
Safety Rules
Within large cumulonimbus clouds
Seek shelter, avoid trees and water.
Spring or Summer
Thunderstorms
Move to a storm shelter or basement if possible
stay away from windows and doors.
Tornado
Spring, early summer
Cumulonimbus cloud
Evacuate or move inside a well-built building.
Over warm ocean water
Late summer and into autumn
Hurricane
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End of SectionGraphic Organizer