1. Chapter 3 Section 1
Air Masses and Fronts

2. Four Major Types of Air Masses in North America
Maritime Tropical
Maritime Polar
Continental Tropical
Continental Polar

3. Maritime Tropical
Warm, humid air masses form over oceans near the tropics
Influence weather in the central & eastern United States when formed over the Gulf of Mexico & the Atlantic Ocean
Influence weather on the West Coast when formed over the Pacific Ocean
Summer – hot, humid weather
Winter – humid air can bring heavy rain or snow

4. Maritime Polar
Cool, humid air masses form over the icy cold North Pacific & North Atlantic oceans
Affect the West Coast more than the East Coast
In summer, often bring fog, rain, & cool temperatures to the West Coast

5. Continental Tropical
Hot, dry air masses form only in summer over dry areas of the Southwest & northern Mexico
Covers a smaller area than other air masses
Move northeast, brings hot, dry weather to the southern Great Plains

6. Continental Polar
Large masses that form over central & northern Canada & Alaska
Brings cool or cold air
In winter, bring clear, cold, dry air to much of North America
Can bring bitterly cold weather with very low humidity.
In summer, storms may occur when air masses move south & meet maritime tropical air masses moving north

7. Four Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts

8. Cold Fronts
When a rapidly moving cold air mass runs into a slowly moving warm air mass, the denser cold air slides under the lighter warm air
The warm air is then pushed upward resulting in a cold front
Cold fronts move quickly & can cause quick changes in the weather, including violent thunderstorms
Once a cold front moves through an area, cool, dry air moves in and often brings clear skies & cooler temperatures

9. Warm Fronts
A moving warm air mass collides with a slowly moving cold air mass
As in a cold front, the warm air rises above the cold air
If the warm air is humid, showers & light rain fall along the front when the two air masses meet
If the warm air is dry, scattered clouds form
Warm fronts move much slower than cold fronts, therefore, the weather may be rainy or foggy for several days
After a warm front passes through an area, warm & humid weather will follow
In winter, warm fronts bring snow

10. Stationary Fronts
Sometimes cold & warm air masses meet, but neither has enough force to move the other; the result a stationary front
When the two air masses meet, water vapor in the warm air condenses into rain, snow, fog, or clouds
If the front is stalled, the weather may be days of clouds & precipitation

11. Occluded Fronts The most complex of the four fronts
At an occluded front, a warm air mass is caught between two cooler air masses
The two cooler air masses meet & may mix
Temperature near the ground becomes cooler
The warm air mass is cut off (occluded) from the ground
As the warm air cools & its water vapor condenses, the weather may turn cloudy & rainy or snowy

12. Cyclones An area of relatively low air pressure is called a cyclone
Low air pressure is represented on a weather map as a L
Cyclone spin counterclockwise in the Northern Hemisphere
Cyclones play a large part in the weather of the United States
As air rises in a cyclone, the air cools, forming clouds & precipitation
Cyclones & decreasing air pressure are associated with storms & precipitation

13. Anticyclones Opposite of cyclones in most ways
Anticyclones are high-pressure centers of dry air
High pressure areas are represented on a weather map as a H
Anticyclones spin clockwise in the Northern Hemisphere
Cool air moves downward from higher in the troposphere
As the cool air falls, it warms up, so its relative humidity drops
Descending air in an anticyclone causes dry, clear weather