1. Air Masses and Fronts

2. What is an air mass?
Large body of air where temperature and moisture content are similar throughout
The moisture content and temperature of an air mass are determined by the area over which the air mass forms

3. Air masses that affect weather in North America:
Maritime (m) forms over water; wet
Continental (c) forms over land; dry
Polar (P) forms over the polar regions; cold
Tropical (T) develops over the Tropics; warm

4. Most of the cold winter weather in the United States is influenced by 3 polar air masses
A continental polar air mass forms over northern Canada, which brings extremely cold winter weather to the U.S.
In the summer, a cP air mass generally bring cool, dry weather

5. A maritime polar air mass that forms over the North Pacific Ocean is cool and very wet. It brings cool, foggy weather in the summer
A maritime polar mass that forms over the North Atlantic Ocean brings cool, cloudy weather and precipitation to New England. In the summer, the air mass brings cool weather and fog

6. Four warm air masses influence the weather in the United States:
Maritime Tropical air masses develop over the Pacific Ocean, Gulf of Mexico, and the Atlantic Ocean
These air masses bring hot and humid weather
Thunderstorms and hurricanes develop

7. In the winter, they bring mild, cloudy weather
A continental tropical air mass forms over the deserts of northern Mexico and the southwestern U.S.
This brings clear, dry, and hot weather

8. Fronts
The area in which 2 air masses meet

9. Cold Front
Forms where cold air move under warm air, which is less dense, and pushes the warm air up
Cold fronts move quickly and bring thunderstorms, heavy rain, and snow

10. Warm Front
A warm front forms where warm air moves over cold, denser air
In a warm front, the warm air gradually replaces the cold air
Warm fronts bring drizzly rain and are followed by clear and warm weather

11. Occluded Front
Forms when a warm air mass is caught between 2 colder air masses
The coldest air mass moves under and pushes up the warm air mass
The coldest air mass moves forward until it meets a cold air mass that is warmer and less dense

12. Occluded Fronts continued
Sometimes, the 2 colder air masses mix
An occluded front has cool temperatures and large amounts of rain and snow

13. Stationary Front Forms when a cold air mass meets a warm air mass
Both air masses do not have enough force to lift the warm air mass over the cold air mass
The 2 air masses remain separated
A stationary front often brings many days of cloudy, wet weather

14. Cyclones
Areas that have lower pressure than the surrounding areas and have winds that spiral toward the center

15. Anticyclone
Areas that have high pressure

16. Low pressure areas -- stormy weather
High pressure areas – fair weather