1. AIR MASSES and FRONTS

2. Air Masses
A large body of air throughout which temperature and moisture are similar.

3. Air moves from areas of high pressure to areas of low pressure.

4. Directions of Air Mass Movements in North America

5. Types of Air Masses
Air masses are classified according to their source regions. These regions also determine the temperature and humidity of the air mass
Source Region
Type of air
Symbol
Continental
Dry c
Maritime
Moist m
Tropical
Warm T
Polar
Cold P

6. Continental Air Masses
Form over large land masses
Level of humidity is very low
Air mass will remain over its source region for days or weeks

7. Two Types of Continental Air Masses
Continental polar air masses masses move into the northern US, occasionally reach to the Gulf Coast brings cool, dry weather in summer brings very cold weather to the northern US in winter

8. Continental Tropical bring dry hot weather in the summer
Continental Tropical bring dry hot weather in the summer do not form in the winter

9. Maritime Air Masses Form over oceans or other large bodies of water
Humidity tends to be high
Commonly bring precipitation and fog

10. Two Types of Maritime Air Masses
Maritime polar air masses bring rain and snow to Pacific Coast in winter move inland and eastward over the Cascades, the Sierra Nevada, and the Rocky Mountains (lose moisture, warm slightly, bring cool, dry weather to the central United States

11. Maritime tropical air masses
bring mild, often cloudy weather in the winter unless it collides with cold polar air
bring hot, humid weather and thunderstorms in the summer

12. FRONTS

13. FRONTS
A front is a boundary between air masses of differing densities.
A typical front is several hundred kilometers long, some can be
several thousand kilometers long.
Fronts do not exist in the tropics because no air masses with significant temperature
differences exist there.

14. TYPES OF FRONTS
For a front to form, one air mass must collide with another air mass. The kind of front that forms is determined by how the air masses move in relationship to each other.

15. COLD FRONTS Cold air mass overtakes a warm air mass.
The moving cold air lifts the warm air
Brings thunderheads followed by cool, fair weather.

16. WARM FRONTS Cold air mass retreats from an area
Less dense warm air rises over the cooler air
Brings gentle rain or light snow followed by warmer, milder weather.

17. STATIONARY FRONTS
Two air masses meet, the cold air moves parallel to the front and neither air mass is displaced Air masses move very slowly or not at all Brings many days of clouds and precipitation

18. OCCLUDED FRONTS
Fast-moving cold front overtakes a warm front The warm air is lifted completely off the ground Brings strong winds and heavy precipitation.