Chapter 20

1. Chapter 20 Weather Patterns

2. This Week’s Weather – 5/4/14

3. 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

5. 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

7. 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

8. 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

9. Warm Front

10. Cold Front

11. Stationary Front

12. Occluded Front

13. 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

14. Mid Latitude Cyclones