Chapter 13 Section 3Chapter 13 Section 3

Air Masses and Air Masses and FrontsFronts

Essential Questions Ch.13 Essential Questions Ch.13 S3S3

1.1. What are the major types of air What are the major types of air masses in North America? How do masses in North America? How do they move? they move?

2.2. . What are the main types of . What are the main types of fronts? fronts?

3.3. What type of weather is What type of weather is associated with cyclones and associated with cyclones and anticyclones? anticyclones?

Air MassesAir Masses

An air mass is an immense (huge) body of air that is characterized by similar temperatures and amounts of moisture at any given altitude (height)

Usually covers hundreds of Usually covers hundreds of thousands of square miles (over thousands of square miles (over millions of square kilometers and millions of square kilometers and can be up to 10 kilometers deep)can be up to 10 kilometers deep)

Classified according to 2 Classified according to 2 characteristics:characteristics:

TemperatureTemperature HumidityHumidity4 Major types of Air Masses that 4 Major types of Air Masses that influence the weather in North influence the weather in North

AmericaAmerica Maritime tropicalMaritime tropical Continental tropicalContinental tropical

Maritime polarMaritime polar Continental polarContinental polar

The characteristics of an air mass The characteristics of an air mass depend on the temperatures and depend on the temperatures and moisture content of the region over moisture content of the region over which the air mass forms. Temperature which the air mass forms. Temperature affects air pressure.affects air pressure.

Cold, dense air has a higher pressure , Cold, dense air has a higher pressure , while warm, less dense air has lower while warm, less dense air has lower pressure.pressure.

Tropical or warm air forms in the tropics Tropical or warm air forms in the tropics and has low pressureand has low pressure

Polar or cold air mass forms north of the Polar or cold air mass forms north of the 50 degrees latitude or South of the 50 50 degrees latitude or South of the 50 degrees latitude and has high pressure.degrees latitude and has high pressure.

Whether an air mass is humid Whether an air mass is humid (moist) or dry depends on if it (moist) or dry depends on if it forms over land or water.forms over land or water.

Maritime air masses form over Maritime air masses form over oceans and the evaporated oceans and the evaporated water from the oceans makes water from the oceans makes the air humid.the air humid.

Continental air masses form over Continental air masses form over land and have less exposure to land and have less exposure to moisture and therefore are dry.moisture and therefore are dry.

Weather in North America

Air Masses Air Masses

Continental Polar Air Masses• Continental polar air masses are

uniformly cold and dry in winter and cool and dry in summer. Maritime Tropical Air Masses

• Maritime tropical air masses are warm, loaded with moisture, and usually unstable. • Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

Weather in North America

Air Masses Air Masses

Maritime Polar Air Masses• Maritime polar air masses begin as cP

air masses in Siberia. The cold, dry continental polar air changes into relatively mild, humid, unstable maritime polar air during its long journey across the North Pacific.• Maritime polar air masses also originate in the North Atlantic off the coast of eastern Canada.

Weather in North America

Air Masses Air Masses

Continental Tropical Air Masses• Only occasionally do cT air masses

affect the weather outside their source regions. However, when a cT air mass moves from its source region in the summer, it can cause extremely hot, drought like conditions in the Great Plains.

• Movements of cT air masses in the fall result in mild weather in the Great Lakes region, often called Indian summer.

How Air Masses MoveHow Air Masses Move When an air mass moves into an area When an air mass moves into an area

and interacts with other masses, it and interacts with other masses, it causes the weather to change.causes the weather to change.

In the continental United States, air In the continental United States, air masses are commonly moved by the masses are commonly moved by the prevailing westeriles and jet streams.prevailing westeriles and jet streams.

The prevailing westerlies, the major The prevailing westerlies, the major wind belt over the U.S., generally push wind belt over the U.S., generally push air masses from west to east.air masses from west to east.

The jet streams are embedded The jet streams are embedded in the prevailing westerlies and in the prevailing westerlies and are bands of high speed winds, are bands of high speed winds, As the jet streams blow from As the jet streams blow from west to east, air masses are west to east, air masses are carried along their tracks.carried along their tracks.

FrontsFronts When two air masses meet, they form

a front, which is a boundary that separates two air masses. Air masses don’t easily mix.

Storms and changeable weather often develop along fronts.

Types of FrontsTypes of Fronts Colliding air masses can form four Colliding air masses can form four

types of fronts: types of fronts: cold fronts, warm cold fronts, warm fronts, stationary fronts, and fronts, stationary fronts, and occluded fronts.occluded fronts.

The kind of front that develops The kind of front that develops depends on the characteristics of the depends on the characteristics of the air masses and how they are moving.air masses and how they are moving.

Cold frontCold front

A cold front forms when cold, dense air moves into a region occupied by warmer air.

Cold fronts tend to move quickly and can cause abrupt weather changes (including thunderstorms).

After a cold front passes through an area, colder, drier air moves in. Can bring clear skies, a shift in wind, and lower temperatures.

Warm FrontWarm Front

A warm front forms when warm air moves into an area formerly covered by cooler air.

Clouds and precipitation accompany warm fronts

Weather may be rainy or cloudy for several days

After a warm front passes through an area, the weather is likely to be warm and humid.

Stationary FrontsStationary 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. In such cases, the surface position of the front does not move, and a stationary front forms.

Neither one can move the other --a “standoff”

If it remains stalled over an area, it may bring days of clouds and precipitation.

Occluded FrontsOccluded Fronts

The most complex weather situation occurs at an occluded front, where a warm air mass is caught between two cooler air masses.

The warm air mass is cut off (occluded) from the ground.

As the warm air cools and its water vapor condense, the weather may turn cloudy and rain or snow may fall.

CyclonesCyclones The term The term cyclonecyclone, in common use, is , in common use, is

sometimes applied to a tornado. In sometimes applied to a tornado. In the science of meteorology, however, the science of meteorology, however, the term has a different meaning. For the term has a different meaning. For meteorologists, a cyclone — and its meteorologists, a cyclone — and its counterpart, an anticyclone — is a counterpart, an anticyclone — is a large-scale system of air circulation in large-scale system of air circulation in the atmosphere in the zones between the atmosphere in the zones between the equator and either of the poles. the equator and either of the poles.

Middle-latitude cyclones are large centers of low pressure that generally travel from west to east and cause stormy weather.

It can be considered as either It can be considered as either producing or resulting from differences producing or resulting from differences in air pressure in those zones. In a in air pressure in those zones. In a cyclone the central air pressure is cyclone the central air pressure is lower than that of the surrounding lower than that of the surrounding environment, and the flow of environment, and the flow of circulation is clockwise in the Southern circulation is clockwise in the Southern Hemisphere and counterclockwise in Hemisphere and counterclockwise in the Northern Hemisphere. Cyclones are the Northern Hemisphere. Cyclones are also characterized by low-level also characterized by low-level convergence and ascending air within convergence and ascending air within the system. the system.

Cyclones and decreasing air pressure are Cyclones and decreasing air pressure are associated with clouds, winds, and associated with clouds, winds, and precipitation.precipitation.

AnticyclonesAnticyclones

An anticyclone system has An anticyclone system has characteristics opposite to that of a characteristics opposite to that of a cyclone. That is, an anticyclone's cyclone. That is, an anticyclone's central air pressure is higher than central air pressure is higher than that of its surroundings, and the that of its surroundings, and the airflow is counterclockwise in the airflow is counterclockwise in the Southern Hemisphere and clockwise Southern Hemisphere and clockwise in the Northern Hemisphere. in the Northern Hemisphere. Anticyclones are usually Anticyclones are usually characterized by low-level divergence characterized by low-level divergence and subsiding air. and subsiding air.

The descending air in an anticyclone The descending air in an anticyclone generally causes dry, clear weather.generally causes dry, clear weather.

ReviewReview

1.1. What are the major types of air What are the major types of air masses in North America? How do masses in North America? How do they move? they move?

Maritime tropical– moves NorthMaritime tropical– moves NorthContinental polar – moves south Continental polar – moves south

and collides with maritime and collides with maritime tropicaltropical

Maritime polar – moves southeastMaritime polar – moves southeastContinental tropical moves north Continental tropical moves north

and occasionally northeastand occasionally northeast

2.2. What are the main types of What are the main types of fronts?fronts?

Cold frontsCold frontsWarm frontsWarm frontsStationary frontsStationary frontsOccluded FrontsOccluded Fronts

3.3. What type of weather is What type of weather is associated associated

with cyclones and with cyclones and anticyclones?anticyclones?

Cyclone—clouds, wind, and Cyclone—clouds, wind, and precipitationprecipitation

Anticyclone – dry clear weatherAnticyclone – dry clear weather