Announcements - NMT · 2006. 4. 5. · 1974 Super-Outbreak •Moist air breaks through to form huge...

Announcements

• No office hours tomorrow (Wed. March 30)• Homework 5 extra credit:

– Find and plot additional tracks of HurricaneIvan (after reaching its northeastmost point inU.S)

Lightning and Tornadoes

North Dakota

Outline

• Finish lightning/thunderstorms• Tornado Facts• Formation and Conditions Needed• Damage Levels• U.S. Occurrences• Examples

Electrification of Clouds

• Generally upper level of clouds - + charge• Mid level - negative charge• Lower level - mix of charge• Why? Transfer of positive ions from warm

to cold objects in collisions between icedroplets, water droplets

Upper level +

Mid level -

Lower level mix

Note positivecharge on highobjects nearcloud negativecharges

Lightning Strike

• Unlike charges attract each other– Negative charges in cloud lead to positive

charges on ground– Dense on high objects (trees, poles)

• Set up electric potential between cloud andground

• When large enough, current flows -lightning

Lightning Facts

• Several strokes occur over ~1/2 second• Travels over 6,000 miles/second• Can produce very high temperatures

(55,000 F) briefly– High temps cause air to expand, produce sound

wave (thunder)

Deaths

• ~143 deaths per year in U.S.• Ways to avoid

– Stay inside house, car, truck– Outside, move to low place

Winds

• Can be straight-line or rotating (tornadoes)• Straight-line winds can be 80-100 mph

– Damage structures, trees– Duration can be hours

What are tornadoes?

• Rapidly rotating winds usually descendingfrom thunderstorm

• Most violent storms with highestwindspeeds

• Average U.S. fatalities ~100/year• Common in Great Plains

Tornado Facts

• Typically ~100-600 m wide– Some up to 1 mile wide

• Speeds– Travel up to 62 mph (100 km/hr)– Wind speeds up to 300 mph (rare, but most

deadly)

Formation

• Some details still unknown• Tend to form with severe thunderstorms• Appear to require additional component to

cause shearing of thunderstorm cloud

Conditions

• Some same as needed for hailstorms– Northward flow of warm, moist air (Gulf of

Mexico)– Cold dry air from Canada or Western U.S.

• Jet stream - key component for spinningthe cloud– High enough that it starts top of cloud

spinning, rest of cloud pushed by mid-level airmotion

KeyComponents:

Warm moist air

Cold dry air

Jet stream

Differences between Systems• Single cell

– Updrafts of warm air, downdrafts (rain) fromcondensing, cooling air

Differences between Systems• Supercell

thunderstorm– Updrafts on leading

side: rain– Downdrafts on

trailing side:rotating air packetsthat can form funnelclouds (tornadoesafter hit ground)

– Many have centerswith rapidlyupflowing air (sucksup material)

Importance of Rotation

• Not Coriolis force issue (feature too smallto be affected)

• Once rotation starts, pulls into tight spiral– Speed increase– Smaller diameter, faster it spins

Structure of tornado

• Can be horizontal or vertical• Winds vary within cloud

– Highest a few 100 ft above ground• Objects (trees, buildings, etc) cause friction, slows

wind speed close to surface

Destruction from Tornadoes

• High speed winds– Damage to trees, buildings, etc

• Lifting force into funnel– Vehicles, people, other loosely attached objects

• Explosions due to extreme changes in airpressure– Very low pressures inside funnel, higher

pressures outside the funnel

Measuring Damage

• Fujita Scale– Developed in 1060s– Based on wind speed damage

Fujita ScaleCategory F0: Light Damage (<73 mph); Some damage to chimneys; branches broken off trees; shallow-

rooted trees pushed over; sign boards damaged.

Category F1: Moderate Damage (73-112 mph); Peels surface off roofs; mobile homes pushed offfoundations or overturned; moving autos blown off road.

Category F2: Considerable Damage (113-157 mph); Roofs torn off frame houses; mobile homesdemolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated;cars lifted off ground.

Category F3: Severe Damage (158- 206 mph); Roofs and some walls torn off well-constructed houses,trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown.

Category F4: Devastating Damage (207- 260 mph); Well-constructed houses leveled; structure withweak foundations blown off some distance; cars thrown and large missiles generated.

Category F5: Incredible Damage (261- 318 mph); Strong frame houses lifted off foundations andswept away; automobile sized missiles fly through the air in excess of 100 meters (109 yards);trees debarked; incredible phenomena will occur.

U.S. Occurrences

• For given year (1992) there were– 696 F-0– 411 F-1– 129 F-2– 43 F-3– 13 F-4– 1 F-5 ** Note that these are very rare events

Where and When are TornadoesCommon in the U.S.?

• Where: interior of U.S. is tornado capital ofthe world– Why? Conditions there are great for formation

Thunderstorms

hailstorms

Tornado map doesn’tquite match thethunderstormoccurrence map, but isclose to the hailstormmap. Why?Need cold dry air!

Deaths per state, 1950-1994

Where and When are TornadoesCommon in the U.S.?

• When: late spring, earlysummer– Varies, peak moves north

during summer– Why? Conditions right during

those months (key jet streampositioning)

Fatalities• Common locations

– Mobile homes• No interior rooms for protection• Weaker coupling to ground

– Exterior rooms with windows• Not as much protection as solid walls

Best Places to Ride out aTornado

• Basement• Interior closet, bathroom, hallway on

lowest floor of a building• If outside, find low point (ditch, streambed)

Predictions and Warnings• # of deaths/decade is decreasing

Why?

Prediction and Warning• Less fatalities partially

from building design• Partially from

development of warningsystems– Tornado watch: conditions

are right for events– Spotters, weather radar

look for tornadoes– Warning issued after

touchdown spotted• Sirens• Radio/TV broadcasts• Can provide minutes of

warning

Tri-State Tornado

• March 1925• Largest known single tornado (F-5)• Lasted over 3.5 hours• Traveled 219 miles through MO, IL, IN• Swath of ~1 mile wide (widest recorded)• 689 fatalities

Murphysboro:most fatalities

Why somanyfatalities?Nowarningsystem

1974 Super-Outbreak

• Weather conditions collide– Cold front from Rockies– Low pressure moving east– Humid air over Gulf of Mexico– Polar jet stream down into TX– Dry air mass from SW moving towards low pressure

• Form inversion layer: dry air above moist air

1974 Super-Outbreak• Moist air breaks through to form huge thunderclouds• Set spinning by jet stream, converging air masses• 16 hours, 147 tornadoes in 13 states

– 6 were F-5 tornadoes– 24 were F-4

F-5 Ohio

F-4 Indiana

Ohio subdivision after 1974 Outbreak

Recent• Tornado Activity in 2005 (from National Weather

Service)• Jan 13, 2005 The 2005 tornado season got off to an early

start. A destructive F3 tornado moved across 20 miles ofUnion County, Arkansas from Junction City to Lawson.Two people were killed, both 83 years old, in separatemobile homes about a mile apart. About 30 homes weredestroyed.

• January 13, 2005 In a different storm, a married coupledied at Arlington, Early County, Georgia, when theirmobile home was destroyed.

Only in rural areas?

• NO!• Can form in cities

– Although they are small targets• Recent (late 1990’s-2000) events include

– F-5 in Oklahoma City (42 fatalities), F-4 inWichita, KS; Cincinnati, OH (6-7 fatalitieseach)

Outbreak inOklahoma had ~59tornadoes

Remote sensingsatellite data showsswath hit bytornado near OKCity

Waterspout

• Rotating column of air over large body ofwater– Can be a tornado that formed over land and

then traveled over water– Some form over water: “fair weather”

waterspouts• Generally smaller diameter than average

tornado, move more slowly, shorterduration

Next Time

• Hurricanes Part 1