Science ~ chapter 8 weather

1. Science ~ chapter 8weather Miss Nelson

2. Section 3 Air Masses and Fronts

3. Anticipatory set Which is denser… warm air, or cold air? What do you think happens when warm air and cold air collide?

4. standards S 6.4.e – Students know differences in pressure, heat, air movement, and humidity result in change in weather

5. The big idea What are the major types of air masses in North America, and how do they move? What are the main types of fronts? What type of weather is associated with cyclones and anticyclones?

6. Key terms • Air mass – a huge body of air that has similar temperature, humidity, and air pressure throughout • Tropical – a warm air mass that forms in the tropics and has low air pressure • Polar – a cold air mass that forms north of 50 degrees latitude or south of 50 degrees latitude and has high air pressure • Maritime – a humid air mass that forms over oceans • Continental – a dry air mass that forms over land

7. Key terms • Front – the boundary where unlike air masses meet but do not mix • Occluded – cut off, as in a front where warm air mass is caught between two cooler air masses • Cyclone – a swirling center of low air pressure • Anticyclone – a high-pressure center of dry air

8. Air masses and fronts • Weather can be influenced by air from thousands of kilometers away • A huge body of air that has similar temperature, humidity, and air pressure at any given height is called an air mass • A single air mass may spread out over millions of square kilometers and be up to 10 kilometers deep

9. Types of air masses Four major types of air masses influence the weather in North America: • Maritimetropical • Continental tropical • Maritime polar • Continental polar

10. Types of air masses The characteristics of an air mass depend on the temperatures and moisture content of the region over which the air mass forms

11. Types of air masses • Temperatures affects pressure • Cold, dense air has a higher pressure • Warm, less dense air has a lower pressure

12. Types of air masses Polar air masses: • Cold air masses • Form north of 50° and south of 50° • Have high air pressure Tropical air masses: • Warm air masses • Form in the tropics • Have low air pressure

13. Types of air masses Whether an air mass is humid or dry depends on whether it forms over water or land

14. Types of air masses • Maritime air masses form over water • The air can become very humid • Continental air masses form over land • Have less exposure to moisture from water – are drier

15. Maritime tropical • Warm, humid air masses form over tropical oceans • Gulf of Mexico, Atlantic Ocean • In summer time, they bring hot, humid weather • Summer showers and thunderstorms • In winter, bring heavy rain or snow

16. Maritime polar • Cool, humid air masses form over the icy cold North Pacific and North Atlantic Oceans • Affect the west coast more than the east coast • Even in summer, they bring fog, rain, and cool temperatures to the west coast

17. Continental tropical • Hot, dry air masses form mostly in summer over dry areas of the Southwest and northern Mexico • Cover a smaller area than other air masses • Occasionally move northeast, bringing hot, dry weather to the southern Great Plains

18. Continental polar • Form over central and northern Canada and Alaska • Can bring bitterly cold weather with very low humidity • In winter, bring clear, cold, dry air to much of North America • In summer, the air mass is milder • Storms may occur when continental polar air masses move south and collide with maritime tropical air masses moving north

19. Types of air masses Read Types of Air Masses on pages 311-312 of your textbook

20. How air masses move In the continental United States, air masses are commonly moved by the prevailing westerlies (winds) and jet streams

21. Prevailing westerlies • The major wind belts over the continental United States • Generally push air masses from west to east

22. Jet streams • Embedded within the prevailing westerlies are jet streams • Bands of high speed winds about 10 kilometers above Earth’s surface • Blow from west to east

23. fronts • As huge air masses move across the land and the oceans, they collide with each other • They do not easily mix • Think about oil and water! • Less dense oil floats on top of the denser water • Something similar happens when two air masses with different temperatures and humilities collide

24. fronts • The boundary where these air masses meet is called a front • Storms and changeable weather often develop along fronts View the picture on page 313 of your textbook

25. How air masses move Read How Air Masses Move on page 313 of your textbook

26. Types of fronts Colliding air masses can form four types of fronts: • Cold fronts • Warm fronts • Stationary fronts • Occluded fronts

27. Types of fronts The type of front that develops depends on the characteristics of the air masses and how they are moving

28. Cold front • Cold air is dense and tends to sink • Warm air is less dense and tends to rise • 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 pushed upward along the leading edge of the colder air View figure 14 on page 314 of your textbook

29. Cold front • As the warm air rises, it expands and cools • Warm air can hold move water vapor than cool air • The rising air reaches the dew point (the temperature at which water vapor in the air condenses into droplets of water) • Clouds form • If there is a lot of water vapor in the warm air, heavy rain or snow may fall • If the warm air mass contains only a little water vapor, then the cold front may be accompanied by only cloudy skies

30. Cold front Read Cold Fronts on page 314 of your textbook

31. Warm fronts • Clouds and precipitation also accompany warm fronts • At a warm front, a fast-moving warm air mass overtakes a slowly moving cold air mass • Cold air is denser than warm air, the warm air moves over the cold air • If the warm air is humid, light rain or snow falls along the front • If the warm air is dry, scattered clouds form • Because warm fronts move slowly, the weather may be rainy or cloudy for several days • After a warm front passes through an area, the weather is likely to be warm and humid

32. Warm fronts Read Warm Fronts on page 315 of your textbook

33. Stationary fronts View the diagram on page 315 of your textbook Read Stationary Fronts on page 315 of your textbook

34. occluded fronts View the diagram on page 315 of your textbook Read Occluded Fronts on page 315 of your textbook

35. Cyclones and anticyclones • As air masses collide to form fronts, the boundary between the fronts sometimes becomes distorted • Can be caused by surface features such as mountains, strong winds, or jet streams • When this happens the air begins to swirl • Swirling air can cause a low pressure center to form

36. cyclones • When you look at a weather map you will see areas marked with an L • Stands for “low” pressure • Swirling center of low air pressure is called a cyclone • From the Greek word meaning “wheel” • Cyclones play a large part in the weather of the United States • As air rises in the cyclone, the air cools, forming clouds and precipitation

37. cyclones Cyclones and decreasing air pressure are associated with clouds, winds, and precipitation

38. anticyclones • The opposite of a cyclone • High pressure centers of dry air • Called “highs” and marked with an H on a weather map • Winds spiral outward from the center of an anticyclone, moving toward areas of low pressure

39. anticyclones The descending air in an anticyclone generally causes dry, clear weather

40. Check for understanding What two characteristics are used to classify air masses? Of the four types of air masses, which are dry, and which are humid?

41. Check for understanding What two characteristics are used to classify air masses? The two characteristics used to classify air masses are temperature and humidity. Of the four types of air masses, which are dry, and which are humid? Maritime tropical and maritime polar air masses are humid, while continental tropical and continental polar air masses are dry.

42. Guided practice What type of air mass would form over the northern Atlantic Ocean? What are the four types of fronts?

43. Guided practice What type of air mass would form over the northern Atlantic Ocean? The type of air mass that would form over the northern Atlantic Ocean is a maritime polar air mass. What are the four types of fronts? The four types of fronts are cold front, warm front, stationary front, and occluded front.

44. Independent practice Complete Weather 8-3 Independent Practice