雷暴雨与极端天气课件

雷暴雨与恶劣天气雷暴雨与恶劣天气Thunderstorms and Severe WeatherAll Thunderstorms begin as Cumulus CloudsThis is runaway convection cumulonimbus over the Plains.These clouds can grow to 70,000 feet(14 miles)high!Mature thunderstorms have both downdrafts and updrafts.The precipitation forms in the updraft and falls out into the downdraft.Thunderstorm Hazards Lightning Tornadoes/downbursts Hail Flash FloodsEvery year 100-200 people die from lightning strikesThere are actually nine cloud-to-cloud strokes for each cloud-to-ground strokeIn the U.S.,the greatest number of thunderstorms and lightning fatalities(5-10 per year)occur annually in Florida.Cloud-to-Ground strokes are the most dangerous.Notice the segmented pathway to the ground.Also,some of the lighting didnt contact the ground.Lightning doesnt always hit the tallest objectbut it often does.In the middle ages,the church towers were used to store gunpowder,with predictable results.Mosques,on the other hand were struck but rarely suffered damage.Can you see why?The cloud-to-ground stroke heats the air to around 30,000 K,making it the brightest stroke.By holding the shutter open,several strokes are captured in this image.There were also lots of air discharges.An air discharge is a stroke which peters out in the air.Notice the positive streamers at a,b,and cWhen the stepped leaders charge is approaching,you might give off positive streamers!Dont be the lightnings conduit.Get inside during thunderstorms.The fellow inside the metal cage is safe.Its called a Faraday Cage.Lightning travels around him,not through him.Inside your vehicle is actually a safe place.Its a kind of Faraday Cage.Dont touch the metal parts.Out on the golf course is NOT a safe place during a thunderstorm.Lightning in the ground is so hot it will fuse sand.The resulting formations are called FulgaritesThe heating of the air causes rapid expansion.The compession is sound.We hear it as thunder.The sound of thunder travels at right angles to the lightning bolt.It travels at 1100 ft/sec so count to 5.The sound has traveled about one mile.Tornadoes,Downbursts,and HailBlue dots are high winds,Green dots are large hail,Red dots are tornadoesIn 2008,U.S.winds,hail,and tornadoes were concentrated in the summer months.Tornadoes never occur without a parent thunderstorm.There are over 1000 tornadoes in the U.S.most years.Miami,Florida tornadoA large,cone tornado.It might be a mile wide at the ground.Remarkable image of four tornadoes on the ground at the same time.This is like the first map,but just for 2008 tornadoesAll the tornadoes in 2009The number of tornadoes observed in the U.S.appears to be increasing.Or are we just getting better at finding them?Tornadoes in the Great Plains tend to be the most destructiveOn radar,often big tornadoes are seen with a“hook echo”Greensburg,KS tornado hook echoGreensburg,KS tornado Doppler velocityDamage from the May 31,1998 tornadoes in Albany,NY.On the Fujita scale,where does this fit?In 2006,the NWS introduced the Enhanced Fujita Scale,a more realistic tool for putting tornadoes into categories.House damagedHouse untouchedHouses demolishedHouses untouchedTornadoes do damage on a very small scaleIn 2008,over 2000 tornadoes were reported in the U.S.!Where dont you want to be when a tornado strikes?This is EF0 damage but from a Downburst,not a tornado.This is the State Fair in Syracuse on Sept 7,1998.A downburst is a straight line wind which comes down out of a thunderstorm.EF1 damage such as this is possible.EF2 damage has been known to occur.Most storms have gust fronts.A really strong one is a downburst.The“bow echo”is the downburst.It is also called a“Derecho”(De-ray-sho)Large hail is not a killer,but does considerable damageHailFalling from 20,000 feet or higher,large hailstones pack quite a punch!Average annual number of days with hailThis is the biggest hailstone known.It fell at a town called Aurora,NE on June 22,2003.The Aurora hailstone was 7 inches in diameter.Imagine that falling on your head from 20,000 feet up!This is the previous record-holder,the Coffeyville hailstone with some props(egg,hand)for comparison.Flooding is the#1 killer of all weather hazardsIt looks peaceful but the force of rushing water can carry away cars(with people in them).The Record June 2006 FloodI-88I-88Delaware CountyDelaware CountyPaddling down the street in downtown Walton,NYJune 2006 Flood Impacts Record flooding Delaware,Susquehanna and Mohawk River BasinsAt least 1 Billion dollars of damage.8 deaths in New YorkThe loss of life could have been much larger!-The Agnes Floods of 1972 had at least 30 deaths in NYWashington Street Bridge,Binghamton,NYWashington Street Bridge,Binghamton,NYThe Bottom Line?Thunderstorms can be dangerous.The NWS considers thunderstorms to be severe if they have large hail,powerful downbursts,or tornadoes.But flooding is also a major hazard.