1. Chapter 13

2.  How thunderstorms form:  1. Moisture: There must be an abundant source of moisture in the lower levels of the atmosphere.  2. Lifting: Some mechanism must lift the air so that the moisture can condense and release latent heat.  3. Instability: The portion of the atmosphere through which the cloud grows must be unstable.

3.  Thunderstorms are often classified according to the mechanism that caused the air to rise:  If the air rose because of unequal heating of Earth’s surface within one air mass, the thunderstorm is called an air-mass thunderstorm. ▪ 1. mountain thunderstorms occur when an air mass rises as a result of orographic lifting. ▪ 2. sea-breeze thunderstorms are caused by extreme temperature differences between the air over land and the air over water.

5.  The second main classification of thunderstorms is frontal thunderstorms, which are produced by advancing cold fronts and warm fronts.

6.  A thunderstorm usually has 3 stages:  1. cumulus stage – warm air starts to rise nearly vertically upward (updrafts), condensing and creating a cumulus cloud. The cloud will continue to grow as long as warm air below it continues to rise.

7.  2. mature stage - Raindrops start to fall through the cloud when the rising air can no longer hold them up. Cool air sinks rapidly to the ground along with the precipitation creating downdrafts. Updrafts and downdrafts form a convection cell that produces gusty surface winds.

8.  3. dissipation stage – after 30 minutes, the thunderstorm begins to dissipate. This occurs when the downdrafts in the cloud begins to dominate over the updraft. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dies out with light rain as the cloud disappears from bottom to top.

10.  A line of thunderstorms can last for hours or even days as they continually regenerate themselves with new, warm air that is introduced into the updrafts.

11.  Severe thunderstorms can produce some of the most violent weather conditions on Earth.  They can develop into supercells, which are characterized by intense, rotating updrafts.  Supercells often produce: ▪ damaging wind ▪ large hail ▪ tornadoes

12. These can last for several hours and have updrafts as strong as 240 km/hr. Only about 10% of the ~100,000 thunderstorms per year in the U.S. are considered severe.

16.  Lightning is electricity caused by the rapid rush of air in a cumulonimbus cloud.  1. Separation of the positive and negative charges of the cloud into two levels: ▪ the + charge at the top ▪ the - charge at the bottom

17.  2. A flow of negative charge rushes toward the Earth. This is known as a stepped leader. ▪ The + charges of the Earth are attracted to this stepped leader, so a flow of + charge moves into the air.  3. When the stepped leader and the + charge from the earth meet, a strong electric current carries + charge up into the cloud. ▪ This illuminated electric current is known as the return stroke ▪  LIGHTNING

20.  Return stroke channel is 100 million V of electricity!  A lightning bolt heats the surrounding air to ~ 30,000°C  Thunder is produced as this superheated air rapidly expands & contracts.  Lightning accounts for ~7,500 forest fires each year the U.S.  Lightning strikes in the U.S. cause an average of 300 injuries & 93 deaths per year.

22.  Violent downdrafts that are concentrated in a local area are called downbursts.  Wind speeds exceeding 160 km/hr ▪ Macrobursts = cause of path of destruction up to 5km wide, have wind speeds of more than 200 km/hr and can last up to 30 minutes. ▪ Microbursts = affect areas of less than 3km but can have winds exceeding 250 km/hr & last less than 10 minutes.

23.  Each year $1 billion in damage is caused by hail.  Hail is caused when raindrops are lifted up into the atmosphere by updrafts during a thunderstorm and then supercooled by temperatures below freezing, turning them into ice balls.

24. Hail

25.  If winds are weak, weather systems & storms move slowly causing them do dump their rain over a limited location.  If the rain falls faster than the ground can absorb it, or faster than the streams & rivers can transport it out of the area, flooding can occur. ▪ *Fact: floods are the main cause of thunderstorm- related deaths each year.

27.  A tornado is a violent, whirling column of air in contact with the ground.  *before it reaches the ground it’s called a funnel cloud.

28.  1. Winds of different speeds from different directions (wind shear) creates a horizontal rotation in the lower atmosphere parallel to the ground.  2. Strong updrafts tilt the rotating the column upward into the thunderstorm creating a funnel cloud.  3. A tornado forms within the rotating winds.

30.  Tornadoes are classified according to the Fujita tornado intensity scale: ranks tornadoes according to their  (1) path of destruction  (2) wind speed  (3) duration

32.  Most tornadoes form in the spring during the late afternoon & evening, when temperature contrasts between polar air & tropical air is the greatest.  Occur most frequently in the Central U.S., where continental polar air collides with maritime tropical air moving north from the Gulf of Mexico. ▪ These occur in a region known as “Tornado Alley” – extends from northern Texas through Oklahoma, Kansas & Missouri ▪ *Fact: more than 700 tornadoes touch down each year in the U.S.

34.  In the U.S. an average of 80 deaths and 1,500 injuries result from tornadoes annually.  The National Weather Service issues tornado warnings & watches before a tornado actually strikes.  During a severe thunderstorm, the presence of dark, greenish skies, a towering wall of clouds, large hailstones, and a loud, roaring noise are signs of an approaching or developing tornado.

37.  Large, rotating, low-pressure storms called tropical cyclones form during summer and fall in the topics.

38.  Warm air at tropics rises and condenses into clouds and rain.  Rising air creates an area of L pressure @ ocean surface.  The Coriolis effect causes the moving air to turn counterclockwise in the N. hemisphere.   this produces the cyclonic rotation of a tropical cyclone.

39.  Tropical cyclones require 2 conditions to form:  1. abundant supply of warm ocean water  2. some sort of disturbance to lift warm air & keep it rising. ▪ They occur in all tropical oceans except the S. Atlantic & the Pacific Ocean west of the S. American Coast.

40.  Western Pacific Ocean = “Typhoons”  Indian Ocean = “Cyclones”  Atlantic Ocean, Caribbean Sea, Gulf of Mexico, & west coast of Mexico = “Hurricanes”  They occur most frequently in late summer and early fall, when the Earth’s oceans contain their greatest amount of stored heat energy.

41.  Tropical cyclones move according to the wind currents that steer them.  In the tropics, tropical cyclones are caught up in the circulation of high-pressure systems. ▪ They move to the west then turn poleward when they reach the edges of the H pressure system. ▪ Then they are guided by the prevailing Westerlies.

43.  1. Tropical disturbance  area of thunderstorms continues for 24 hours or more.  2. Tropical Depression  when the winds increase to (23 - 39 mph), organized circulation of wind in the center of the thunderstorms is detected.  3. Tropical Storm  forms when the maximum sustained winds have intensified to between (39-73 mph). It becomes better organized and begins to look like a hurricane with a circular shape.  4. Hurricane  if air pressure continues to fall & winds reach sustained winds of 74 mph.

44. Katrina

45.  Development of calm center = eye  Strongest winds concentrated in a band immediately surrounding the eye = eyewall

46.  Saffir-Simpson hurricane scale classifies hurricanes according to:  1.wind speed  2. air pressure in the center  3. potential for property damage. ▪ Ranges from Category 1  Category 5

48.  A hurricane will last until it can no longer produce enough energy to sustain itself.  When the storm moves over land and no longer has access to the warm ocean surface.  Or the storm moves over colder waters.

49.  Strong winds moving onshore in coastal areas are partly responsible for another major hurricane threat: storm surges.  Occurs when hurricane force-winds drive a mound of ocean water toward coastal areas, where it washes over the land.

52.  The National Hurricane Center – issues regular advisories that indicate a storm’s position, strength and movement.

53.  Droughts are extended periods of well-below normal rainfall.  The result of shifts in global wind patterns that allow H pressure systems to persist for weeks or months over continental areas. ▪ Sinking air will resist any lifting moisture and condensation cannot occur.

54. Folsom Lake, Northern CA – Drought

55.  Heat waves are extended periods of above- normal temperatures.  Formed by the same H pressure that cause droughts. ▪ Air sinks and warms by compression ▪ Blocks cooler air masses from moving into the area ▪ Difficult for condensation to occur = little cloud cover ▪ Jet stream is farther north and weak so the system does not move very much

57.  Cold wave is an extended period of below- normal temperatures.  Caused by systems of continental polar or arctic origin. ▪ During winter, little sunlight available & snow covered surface radiates heat back to space. ▪  development of large pools of extremely cold air ▪ Cold air sinks, pressure increases creating H pressure system.

59.  Wind chill factor is a phenomenon measured by the wind-chill index, which estimates the heat loss from human skin caused by a combination of wind and cold air.

60. Study, Study, Study!!! Test: Wednesday, April 8th