Chapter 8: Weather

1. Chapter 8: Weather

2. Two Important Terms for This Chapter • Meteorology is the scientific study of the atmosphere. • Weather is the short-term, day-to-day condition of the atmosphere, and should not be confused with climate, which is the long-range weather conditions in a region.

3. Cloudsare fundamental indicators of overall atmospheric conditions: stability, moisture content, and weather.

4. A cloud is comprised of tiny water droplets and/or ice crystals, a snowflake is an aggregate of many ice crystals, and rain is just liquid water.

5. Cloud formation

8. Warm, moist air can form advection fog.

9. The topography of an area determines what type of fog forms.

10. Convergent lifting is the widespread, broad lifting of an entire layer of air. Convergence is an atmospheric condition that exists when there is a horizontal net inflow of air into a region. When air converges along the earth's surface, it is forced to rise since it cannot go downward. Large scale convergence can lift a layer of air hundreds of kilometers across.

11. Convectional lifting refers primarily to atmospheric motions in the vertical direction. As the earth is heated by the sun, bubbles of hot air (called thermals) rise upward from the warm surface. A thermal cools as it rises and becomes diluted as it mixes with the surrounding air, losing some of its buoyancy (its ability to rise).

12. When air is confronted by a mountain, it is lifted up and over the mountain, cooling as it rises. If the air cools to its saturation point, the water vapor condenses and a cloud forms. These types of clouds are called "orographic clouds", which develop in response to lifting forced by the topography the earth.

13. Frontal lifting occurs along frontal boundaries, which separate air masses of different density. In the case of a cold front, a colder, denser air mass lifts the warm, moist air ahead of it. As the air rises, it cools and its moisture condenses to produce clouds and precipitation. Due to the steep slope of a cold front, vigorous rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms.

14. In the case of a warm front, the warm, less dense air rises up and over the colder air ahead of the front. Again, the air cools as it rises and its moisture condenses to produce clouds and precipitation. Warm fronts have a gentler slope and generally move more slowly than cold fronts, so the rising motion along warm fronts is much more gradual. Precipitation that develops in advance of a surface warm front is typically steady and more widespread than precipitation associated with a cold front.

15. Severe thunderstorms often develop in groups of many individual storms clustered together. When these clusters occur as elongate bands, they are known as squall lines.

16. Warm sector of middle latitude cyclones 60 to 180 miles in advance of cold front Develop from a combination of warm, moist air near the surface and an active jet stream aloft Can form along a boundary called a dryline, a narrow zone along which there is an abrupt change in moisture Two Areas of Squall Line Development

18. Mid-latitude cyclones cause of most of the stormy weather in the United States, especially during the winter. They are areas of low pressure located between 30 degrees and 60 degrees latitude. Since the continental U.S. is located in this latitude belt, these cyclones have a major impact on our weather. A mid-latitude cyclone looks very distinct on a satellite image. It is often identifiable by a comma-shaped cloud mass.

19. These are all examples of some type of cyclone – they have some similarities, yet they are not the same thing.

20. Cyclogenesis

21. Thunderstorms are simply storms that generate lightning and thunder. They frequently produce gusty winds, heavy rain, and hail.

23. Lightning and Thunder

24. Lightning is caused when a separation of charge occurs. Part of a large cumulonimbus cloud develops an excess negative charge, and part develops an excess positive charge. Lightning equalizes these differences by producing a negative flow of current from one region to the region with the opposite excess charge.

25. Air is a poor conductor of electricity. As a result, the electrical potential, or charge difference, must be very high before lightning will occur.

26. Lightning rarely occurs before the cloud has penetrated the 5 km level, where there is sufficient cooling to generate some ice crystals.

27. Some scientists believe that the charge separation occurs during the formation of ice pellets.

29. The channel of every lightning flash, at a temperature of 30,000 to 50,000 degrees F, is extremely bright white. The apparent colors of lightning, then, are caused by intervening dust, moisture, and other particles in the air.

33. Ice pellets called hail generally form within cumulonimbus clouds and are often associated with severe thunderstorms.

34. Tornadoes

35. Tornadoes are violent windstorms that take the form of a rotating column of air, or vortex, that extends downward from a cumulonimbus cloud

36. Maximum winds can approach 480 km, or 300 miles, per hour.

37. The average tornado has a diameter between 150 and 600 meters.

38. Tornadoes can travel at speeds close to 45 kilometers per hour

39. The vertical wind profile in a supercell may cause the updraft to rotate. This column of cyclonically rotating air is called a mesacyclone, and is where tornadoes often form.

40. Tornadoes can form over water, creating beautiful but deadly waterspouts.

41. Although April through June is the period of greatest tornado activity, tornadoes can and do occur during every month of the year.

43. Most tornado damage is caused by the tremendously strong winds associated with the storm.

44. The Fujita Scale Minor roof damage; breaks branches off trees; push over shallow-rooted trees; damage to sign boards.Frequency 29% F0 40-72 MPH The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations, overturned or torn apart; moving autos thrown from roads.Frequency 40% F1 73-112 MPH

45. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated.Frequency 24% F2 113-157 MPH Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown. Frequency 6% F3 158-205 MPH

46. Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown 300 yards or more and large missiles generated.Frequency 2% F4 207-260 MPH Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobile sized missiles fly through the air in excess of 100 meters(109 yds); trees debarked; steel reinforced concrete structures badly damaged; incredible phenomena will occur.Frequency less than 1% F5 261-318 MPH

47. The Fujita scale actually goes to F10 (the speed of sound), however, no known tornado has exceeded F5 – not even this one.

49. Tropical cyclones originate entirely within tropical air masses.

50. Hurricanes