1. Weather Patterns

2. Weather Changes Because of the movement of air and moisture in the atmosphere weather constantly changes.

3. Air Masses Air Mass; A large body of air that has the properties similar to the part of the Earth’s surface over which it develops.

4. Air Masses

5. When you observe a change in the weather from one day to the next, it is due to the movement of air masses.

6. Atmospheric Pressure Remember winds blow from areas of high pressure to areas of low pressure. Variation in atmospheric pressure affects the weather.

7. Low Pressure Low pressure systems at Earth’s surface are regions of rising air. Clouds form when air is lifted and cooled.

8. Low Pressure Areas of low pressure usually have cloudy weather.

9. High Pressure Air sinks in high pressure air masses. This sinking makes it difficult for air to rise and clouds to form.

10. High Pressure High pressure usually means good weather.

11. Fronts Front: A boundary between two air masses of different density, moisture, or temperature. Four types of fronts: 1.Cold 2.Warm 3.Occluded 4.Stationary

12. Cold Front A cold front is shown on a map as a blue line with triangles. A cold front occurs when colder air advances toward warm air. The cold air wedges under the warm air like a plow.

13. Cold Front As the warm air is lifted, it cools and water vapor condenses, forming clouds. When the temperature difference between the cold and warm air is large, thunderstorms and even tornadoes may form.

14. Warm Front Warm fronts occur when lighter, warmer air advances over heavier, colder air. A warm front is shown on weather maps as a red line with red semicircles.

15. Warm Front At the contact of the front the temperatures are warm, winds are light and drizzle may occur.

16. Occluded Front Occluded front involves three air masses. –Colder air –Cool air –Warm air

17. Occluded Front Occluded front forms when a cold air mass moves toward cool air with warm air in between. The colder air forces the warm air upward, closing off the warm air from the ground. Long steady rains can result.

18. Stationary Front A stationary front occurs when a boundary between air masses stops advancing. A stationary front may remain in place for several days producing light wind and precipitation.

19. Stationary Front A stationary front is represented by alternating red semicircles and blue triangles. The blue triangles point toward the warm air and the red semicircles point toward the cool air.

20. Thunderstorms Thunderstorms occur in warm, moist air masses along fronts. Warm moist air can be forced upward where is cools and condensation occurs, forming cumulonimbus clouds.

21. Thunderstorms Condensation creates water droplets or ice crystals. These small droplets collide to form larger droplets. As the droplets fall toward Earth they continue to collide and grow larger. The raindrops cool air around them. This cool dense air then sinks and spreads toward the Earth.

22. Thunderstorms The sinking, rain cooled air and strong updrafts of warmer air cause strong winds.

23. Thunderstorms

24. Thunderstorms may stall over a region, causing rain or hail to fall heavily for a period of time. Flooding may then occur. Strong winds or hail can also occur.

25. Lightning What causes lightning?

26. Lightning Inside a storm cloud, warm air is lifted rapidly as cooler air sinks. This movement of air can cause different parts of a cloud to become oppositely charged.

27. Lightning When current flows between regions of opposite electrical charge, lightning flashes. Lightning can occur within a cloud, between clouds, or between a cloud and the ground.

28. Thunder What causes thunder?

29. Thunder Thunder results from the rapid heating of air around a bolt of lightning. This extreme heat causes air around the lightning to expand rapidly. The heated air then cools quickly and contracts. The rapid movement of the molecules forms sound waves heard as thunder.

30. Tornadoes Tornado: A violently rotating column of air in contact with the ground.

31. Tornadoes In severe thunderstorms, wind at different heights blows in different directions and at different speeds.

32. Tornado Formation This difference in wind speed and direction, called wind shear, creates a rotating column parallel to the ground.

33. Tornado Formation A thunderstorm’s updraft can tilt the rotating column upward into the thunderstorm creating funnel cloud.

34. Tornadoes If the funnel touches the ground, it is called a tornado.

35. Tornadoes Tornadoes can be severely destructive.

36. Tornadoes What do you think causes the most injuries and deaths during a tornado?

37. Tornadoes

38. What is the most powerful storm?

39. Hurricanes

40. A hurricane turns heat energy from the ocean into wind. A storm must have winds of at least 119km or 74mph to be called a hurricane.

41. Hurricanes Similar storms in the Pacific are called typhoons and cyclones in the Indian Ocean.

42. Hurricanes Where do our hurricanes usually develop?

43. Hurricanes The strongest hurricanes usually develop as a low pressure system west of Africa. Steered by surface winds, these storms move west, gaining strength from the heat and moisture of the warm ocean water.

44. Hurricanes What aspect of a hurricane causes the most damage?

45. Storm Surge

47. Storm surge is water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more. In addition, wind driven waves are superimposed on the storm tide.

48. Hurricanes What can reduce the energy of a hurricane?

49. Hurricanes 1.Landfall 2.Cool ocean waters 3.Wind shear

51. Blizzards Blizzard: A winter storm with winds at 56k/h, the temperature low, the visibility is less than 400 m, in falling or blowing snow and these conditions must persist for at least three hours.

52. Blizzards

53. Snowfall