2.  Weather depends on what type of air mass is over an area and whether or not the spot is under a front.  The air masses and how they interact will determine if the weather is constant or will be changing in temperature, wind, or precipitation.

3. Air Mass  An air mass is a large body of air that has nearly the same temperature and humidity.  It has properties similar to the area over which it developed.

4.  They most commonly form over polar and tropical regions.  The temperate zone (where the US is) is too unstable for air masses to form.

5.  An air mass will carry its characteristics as it travels and will share it with whatever it is over.  This cold, dry air mass from the polar region is cooling down all the cities in its path.

6.  The polar vortex was the result of the arctic air mass moving further south than usual

7. Classifying Air Masses  Four major types of air masses influence the weather in North America: –maritime tropical –continental tropical – maritime polar –continental polar.

8. North American Air Masses

9.  Polar air masses flow towards the equator  Tropic air masses flow towards the poles.  They move due to pressure differences.

10.  When 2 or more different air masses meet, this is called a front.

11.  When 2 air masses have different temperatures and different humidity levels, they will have different densities.  Objects with different densities do not mix easily  One air mass will be lifted above another

12.  When air masses are lifted, clouds may form, precipitation may fall, wind and thunderstorms may occur.  Fronts are the main cause of stormy weather.

13.   The type of front depends on both the direction in which the air mass is moving and the characteristics of the air mass.

14.  There are four main types of fronts

15. Cold Front Occurs when colder air (more dense) advances toward warm air (less dense).Occurs when colder air (more dense) advances toward warm air (less dense). Cold air wedges under warmer air.Cold air wedges under warmer air.

16. Cold Front  Warm air is lifted, it cools, and water vapor condenses, forming clouds.  If there is a large temperature difference, thunderstorms and tornadoes may form

17. Cold Front Click to see animation!see

18.   Clouds can indicate what type of front is occurring.

19. Warm Front  Forms when lighter, warmer air advances over heavier, colder air.

20. Warm Front  May result in hours or days of wet weather http://stream2.cma.gov.cn/pub/comet/FireWeather/S290Unit6Atmosp hericStability/comet/fire/s290/unit6/media/flash/2-2.4.0.pop2.htm

21. Warm Front

22.  Clouds can help us figure out what type of front is approaching. http://www.classz one.com/books/ea rth_science/terc/c ontent/visualizatio ns/es2002/es2002 page01.cfm?chap ter_no=visualizati on

23. Stationary Front  Occurs when a boundary between air masses stops advancing.

24. Stationary Fronts

25.  May stay in place for days  Light wind and precipitation Stationary Front

26. Occluded Front  Forms when air masses of three different temperatures meet up.  A cold air mass moves toward a cool air with warm air between the two. The colder air forces the warm air upward.

27. Weather can be especially fierce at the occlusionWeather can be especially fierce at the occlusion OcclusionFront COOL AIR

28. Comparing and Contrasting  compare and contrast the four types of fronts by completing a table like the one below. Cold frontA cold air mass overtakes a warm air mass. FrontHow It Forms Types of Weather Clouds, possibly storms with heavy precipitation Warm frontA warm air mass overtakes a cold air mass. Clouds, light precipitation Stationary frontCold and warm air masses meet, but neither can move the other. Clouds, precipitation Occluded frontA warm air mass is caught between two cold air masses. Clouds, precipitation

29. Compare the fronts- animations  http://www.phschool.com/atschool/phsciexp/ active_art/weather_fronts/ http://www.phschool.com/atschool/phsciexp/ active_art/weather_fronts/ http://www.phschool.com/atschool/phsciexp/ active_art/weather_fronts/

30. Summary of Fronts

33. Air Mass  An air mass is a large body of air that has nearly the same temperature and humidity.  It has properties similar to the area over which it developed.

34. Classifying Air Masses  Four major types of air masses influence the weather in North America: –maritime tropical –continental tropical – maritime polar –continental polar.

35.  When 2 or more different air masses meet, this is called a front.

36. High and Low Pressure Areas High pressure causes air to sink Usually results in several days of clear sunny skies Air rises in low pressure areas and forms water droplets Usually results in rain and storms

38.  http://video.nationalgeographic.com/video/w eather basic info http://video.nationalgeographic.com/video/w eather http://video.nationalgeographic.com/video/w eather  http://www.phschool.com/atschool/phsciexp/ active_art/weather_fronts/ diagram of fronts http://www.phschool.com/atschool/phsciexp/ active_art/weather_fronts/ http://www.phschool.com/atschool/phsciexp/ active_art/weather_fronts/