4. What Causes Wind?  Air is a fluid – it moves easily air pressure  Movement of air caused by changing air pressure igh pressure to low pressure  Wind is movement of air from high pressure to low pressure

5. Air Movement

6. Global Winds

10. Types of Air Masses An air mass forms when a large amount of air stays in the same place over an area Continental air masses form over land and are dry Maritime air masses form over water and are moist Polar air masses are cold Tropical air masses are warm

11. Types of Air Masses There 4 air masses affecting North America cT- continental tropical cP – continental polar mT – maritime tropical mP – maritime polar

14. Fronts Air masses move and bump into each other Fronts appear at the edges of two air masses where they are colliding with each other

15. Fronts There are 4 types of fronts: Cold fronts Warm fronts Stationary fronts Occluded fronts

16. Cold front Occurs when a cold air mass overtakes a warm air mass Cold air forces warm air to rise quickly Resulting in strong winds, thunderstorms, and large amounts of precipitation

17. Cold fronts usually bring cooler weather, clearing skies, and a sharp change in wind direction.

18. Cold Front

19. Cold Front Diagrams

20. Warm Fronts Occurs when a warm air mass overtakes a cold air mass Warm air slowly rises over the cold air Starts with steady rain (8-12 hours) but eventually clears when temperatures rise Warm fronts tend to move slowly are far less violent then cold fronts

21. After warm front passes, weather becomes warm and humid, but generally clear

22. Warm Front

23. Warm Front Diagrams

24. Stationary Front When neither air mass can move the other Similar to warm fronts and can remain in same place days Light wind and precipitation occur across entire region affected

27. Occluded front When a warm air mass gets trapped between two cold air masses Strong winds and heavy precipitation occur