Chapter 20Weather Patterns

This Week’s Weather – 5/4/14

Section 1: Air Masses• Changes in weather patterns are caused by the movement of

air masses• An air mass is a large body of air with similar

temperatures and amounts of moisture at a given altitude•Weather inside the area with the air mass will be different

from weather outside of the air mass• As it moves, the characteristics of the air mass change and

so does the weather in the area that the mass moves over

Types of air masses

•Air masses are classified by their temperature and the area where they form•Polar air masses are cold• Tropical air masses are warm•Continental air masses form over land•Maritime air masses form over water

Types of Air Masses•There are 4 basic types of air masses•Continental polar air mass is dry and cool•Continental tropical air mass is dry and warm•Maritime polar air mass is wet and very cold•Maritime tropical air mass is wet and hot

•Most of the weather in the eastern United States is affected by continental polar and maritime tropical air masses

Section 2: Fronts• A front is a boundary that forms when two air masses meet• A warm front forms when warm air moves into an area formerly covered by

cool air• Brings precipitation and warmer weather

• A cold from forms when cold air moves into an area formerly covered by warm air• Cold fronts bring more violent, windy weather and leave clear skies when

they pass• Stationary fronts form when cold and warm air masses move next to each

other • Stationary fronts bring light precipitation

• Occluded fronts form when a cold air mass over takes a warm air mass • Precipitation forms when warm air is forced upward

Warm Front

Cold Front

Stationary Front

Occluded Front

Cyclones•Middle latitude cyclones are large centers of low pressure

that travel from west to east and cause stormy weather•A cold front moves toward a warm front, forcing warm

air up• The cold front merges with the warm front forming an

occluded front that drops heavy rains•Occluded fronts move slowly causing several days of

light rain

Mid Latitude Cyclones