Chapter 14 Lecture Outline. Weather Patterns and Severe Weather

Transcription

1 Chapter 14 Lecture Outline Weather Patterns and Severe Weather

2 Air Masses Characteristics Large body of air 1600 km (1000 mi) or more across Several km thick Similar temperature at any given altitude Similar moisture at any given altitude Move and affect a large portion of a continent

3 Air Masses A Cold Canadian Air Mass

4 Air Masses Source region Where an air mass acquires its properties Classification of an air mass Two criteria used to classify air masses: 1. By the latitude of the source region Polar (P) High latitudes: cold Tropical (T) Low latitudes: warm 2. By the nature of the surface in the source region Continental (c) Form over land: dry Maritime (m) Form over water: humid

5 Air Masses Four basic types of air masses Continental polar (cp) Continental tropical (ct) Maritime polar (mp) Maritime tropical (mt)

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7 Air Masses Air masses and weather cp and mt air masses important in N. America North America (east of the Rocky Mountains) Continental polar (cp) From northern Canada and interior of Alaska Winter: Brings cold, dry air Summer: Brings cool relief

8 Air Masses Air masses and weather North America (east of the Rocky Mountains) Continental polar (cp) Responsible for lake-effect snows cp air mass crosses the Great Lakes Air picks up moisture from the lakes Snow occurs on the leeward shores

9 Air Masses

10 Air Masses Satellite image of lake-effect snow storm

11 Air Masses Air masses and weather North America (east of the Rocky Mountains) Maritime tropical (mt) From the Gulf of Mexico and the Atlantic Ocean Warm, moist, unstable air Brings precipitation Continental tropical (ct) Southwest and Mexico Hot, dry Maritime polar (mp) Brings precipitation to the western mountains Occasional influence: causes the Northeaster

12 Air Masses

13 Fronts Boundaries that separates air masses of different densities Air masses retain their identities Warmer, less dense air forced aloft Cooler, denser air acts as wedge

14 Fronts Warm front Warm air replaces cooler air Shown on a map by a line with red semicircles Shallow slope (1:200) Clouds become lower as the front nears Slow rate of advance Light-to-moderate precipitation

15 Fronts

16 Fronts Cold front Cold air replaces warm air Shown on a map by a line with blue triangles Twice as steep (1:100) as warm fronts Advances faster than a warm front Associated weather is often violent Intensity of precipitation is high Duration of precipitation is short Weather behind the front is dominated by Cold air mass Subsiding air Clearing conditions

17 Fronts

18 Fronts Stationary front Flow of air on both sides of the front is almost parallel to the line of the front Surface position of the front does not move Occluded front Active cold front overtakes a warm front Cold air wedges the warm air upward Weather is often complex Precipitation is associated with warm air being forced aloft

19 Fronts

20 Midlatitude Cyclones Primary weather producer in the middle latitudes Idealized weather Middle-latitude cyclones move eastward across the United States First signs of their approach are in the western sky Require two to four days to pass over a region Largest weather contrasts occur in the spring Changes in weather associated with the passage of a middle-latitude cyclone Changes depend on the path of the storm

21 Midlatitude Cyclones

22 Midlatitude Cyclones Weather associated with fronts Warm front Clouds become lower and thicker Light precipitation After the passage of a warm front: Winds become more southerly Temperatures warm

23 Midlatitude Cyclones Cold front Wall of dark clouds Heavy precipitation Hail and occasional tornadoes After the passage of a cold front: Winds become more northerly Skies clear Temperatures drop

24 Midlatitude Cyclones Cloud Patterns of Typical Mature Middle- Latitude Cyclone

25 Midlatitude Cyclones

26 Development

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29 Midlatitude Cyclones Role of air aloft Cyclones and anticyclones Generated by upper-level air flow Maintained by upper-level air flow Typically are found adjacent to one another

30 Midlatitude Cyclones

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32 Thunderstorms Features Cumulonimbus clouds Heavy rainfall Lightning Occasional hail Occurrence 2000 in progress at any one time! 100,000 per year in the United States Most frequent in Florida and eastern Gulf Coast region

33 Thunderstorms

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35 Thunderstorms Stages of development All thunderstorms require Warm air Moist air Instability (lifting) High surface temperatures Most common in afternoon and early evening

36 Thunderstorms

37 Thunderstorms Stages of development Continuous supply of warm air and moisture Each surge causes air to rise higher Updrafts and downdrafts form Eventually precipitation forms Gusty winds, lightning, hail Heavy precipitation Cooling effect of precipitation marks the end of thunderstorm activity

38 Thunderstorms

39 Tornadoes Local storm of short duration Features: Rotating column of air that extends down from a cumulonimbus cloud Low pressure inside Winds approach 480 km (300 mi) per hour Smaller suction vortices can form inside stronger tornadoes

40 Tornadoes

41 Tornadoes Occurrence and development Average of 770 each year in the U.S. Most frequent from April through June Associated with thunderstorms Exact cause is not known Formation of tornadoes Occur most often along a cold front May be associated with huge thunderstorms called supercells

42 Mesocyclone Development

43 Supercell thunderstorms Large, long-lasting thunderstorm with a single violent rotating updraft Strong vertical wind shear Outflow never undercuts updraft, updrafts may exceed 90kts and can cause large sized hail 3 types: Classic - CL high precipitation, HP low precipitation, LP

44 Some of the features associated with a classic tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast.

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46 Intense thunderstorms often can create flash flood conditions especially if storms are training

47 A wall cloud photographed southwest of Norman, Oklahoma.

48 Lightning and Thunder Causes of electrification of clouds graupel and hail fail into region of supercooled water, water freezes, releasing latent heat and keeping the hailstone warmer than surrounding ice crystal nuclei Net transfer.+ ions from warmer to colder, this leaves larger hail stones negatively charged and smaller ice crystals positively charged

49 When the tiny colder ice crystals come in contact with the much larger and warmer hailstone (or graupel), the ice crystal becomes positively charged and the hailstone negatively charged. Updrafts carry the tiny positively charged ice crystal into the upper reaches of the cloud, while the heavier hailstone falls through the updraft toward the lower region of the cloud

50 The generalized charge distribution in a mature thunderstorm.

51 The Lightning Stroke A discharge of static electricity Positive charge on ground, cloud to ground lightning Thunder Lightning heats air to 54,000deg F hotter than Sun s surface Explosive expansion of air - shock wave Sound travels at 330m/s or 1100 ft/s, so delay about 5 sec per mile Sound is refracted upward in unstable atm and we do not hear lightning at approximately 15km away Heat Lightning

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53 Thunder travels outward from the lightning stroke in the form of waves. If the sound waves from the lower part of the stroke reach an observer before the waves from the upper part of the stroke, the thunder appears to rumble. If the sound waves bend upward away from an observer, the lightning stroke may be seen, but the thunder will not be heard. heat lightning

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55 The four marks on the road surface represent areas where lightning, after striking a car traveling along south Florida s Sunshine State Parkway, entered the roadway through the tires. Lightning flattened three of the car s tires and slightly damaged the radio antenna. The driver and a six-year-old passenger were taken to a nearby hospital, treated for shock, and released.

56 Average lightning flash density per square kilometer per year from 1997 to Notice that in the United States, Florida is the most lightning-prone state. (Data from the North American Lightning Detection Network. Courtesy of Vaisala.)

57 Tornadoes Characteristics Diameter m ( ft) Speed 45 km (30 mi) per hour Can cut a 10 km (6 mi) long path Max winds over 500 km (310 mi) per hr Intensity measured by the Fujita intensity scale, or EF- scale

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60 Tornadoes Tornado forecasting Difficult to forecast Tornado watch To alert public to the possibility of tornadoes Issued when the conditions are favorable Covers 65,000 km 2 (25,000 mi 2 ) Tornado warning Issued when a tornado is sighted or indicated by weather radar Use of Doppler radar helps increase the accuracy by detecting the air motion

61 Tornadoes Doppler Radar image of tornado near Moore, OK..May 3, 1999!

62 May 20, 2013 Moore OK..again

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64 Hurricanes Most violent storms on Earth To be called a hurricane: Wind speed > 119 km (74 mi) per hour Rotary cyclonic circulation Form between 5º and 20º latitudes Wind speeds reach 300 kph Generate 50-foot waves at sea Typhoons in the western Pacific Cyclones in the Indian Ocean North Pacific has the greatest number per year

65 Hurricanes

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67 Hurricanes Parts of a hurricane Eyewall Near the center Rising air Intense convective activity Wall of cumulonimbus clouds Greatest wind speeds Heaviest rainfall

68 Hurricanes

69 Hurricanes Eye At the very center About 20 kilometers (12.5 miles) diameter Precipitation ceases Winds subsides Air gradually descends and heats by compression Warmest part of the storm

70 Hurricanes Hurricane formation and decay Form in all tropical waters except the South Atlantic(rare)and Eastern South Pacific Energy from condensing water vapor Develop most often in late summer Tropical depression Winds do not exceed 61 km (38 mi per hour) Tropical storm Winds km (38 74 mi per hour)

71 Hurricanes

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73 Hurricanes Diminish in intensity as: They move over cooler ocean water They move onto land The large-scale flow aloft is unfavorable

74 Hurricanes Hurricane destruction Factors that affect amount of hurricane damage Strength of storm (the most important factor) Size and population density of the area affected Shape of the ocean bottom near the shore Saffir-Simpson scale ranks the relative intensities of hurricanes

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76 Hurricanes

77 Hurricanes Hurricane destruction Categories of hurricane damage Storm surge Large dome of water 65 to 80 km (40 to 50 mi) wide sweeps across the coast where eye makes landfall Resonsible for large number of deaths Wind damage Inland flooding from torrential rains

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79 Hurricanes

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83 2012 Season In 2012, there were 19 tropical cyclones, 19 tropical storms, 10 hurricanes, and 2 major hurricanes. Damage in this season was around $77.57 billion and deaths were around 354. The majority of these damages and deaths were caused by Hurricane Sandy and Hurricane Isaac.

84 Hurricane Sandy October 22, 2012 October 31, 2012

85 2013 Season With 14 tropical storms, two hurricanes, and no major hurricanes,activity fell far below the predictions. The season's impact was minimal; although 15 tropical cyclones developed, several were weak or remained at sea. Tropical Storm Andrea killed four people after making landfall in Florida and moving up the East Coast of the United States

86 2014 Season The season's first tropical cyclone, Arthur, developed on July 1, ahead of the long-term climatological average of July 9. Early on July 3, the system intensified into a hurricane, preceding the climatological average of August 10. [16] After continuing to steadily intensify, it moved ashore between Cape Lookout and Cape Hatteras as a Category 2 hurricane, becoming the first U.S. land falling cyclone of that intensity since Hurricane Ike in 2008.Upon moving inland, Arthur became the earliest known hurricane to strike the North Carolina coastline on record.

87 2015 Season The season officially began on June 1, 2015, and ends on November 30, These dates historically describe the period each year when most tropical cyclones form in the Atlantic basin and are adopted by convention. However, the formation of tropical cyclones is possible at any time of the year. The first storm, Ana, developed a month before the official start of the season, becoming the first pre-season tropical or subtropical cyclone since 2012's Beryl, the earliest-forming cyclone since 2003's Ana, and the earliest cyclone on record to strike the United States. Despite an ongoing El Niño event, the season started unusually early, and August and September so far have featured 8 tropical cyclones. With the classification of Tropical Storm Ida, this season has featured more named storms than the previous season; however this season is still slightly below normal according to its Accumulated cyclone energy Index. In early October, Joaquin became the strongest hurricane in the Atlantic since Igor in But Wait there s MORE!