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EESA09H3 (185)
Lecture 5

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Department
Environmental Science
Course
EESA09H3
Professor
Tanzina Mohsin
Semester
Fall

Description
Lecture 5 What is a Thunderstorm?  Convective storm caused by surface heating  Common in Southern Ontario in summer  Only storm that has thunder and lightning  Large thunderstorms can spawn tornadoes Dynamics of Thunderstorms  Ordinary o Often develop within large air masses that are vertical and unstable o Not necessarily near a frontal system o Little vertical wind shear (change of horizontal speed with height) o Three Stages  Differential surface heating induces upward flow in unstable air, updraft, cumulus cloud formation  Mature phase – development of a downdraft with precipitation  Gust front develops as downdraft air spreads along horizontal surface  Gust front forces more air up into the updraft  Updraft and downdraft form a convective cell (key to storm; storm will sustain as long as cell is alive)  When the gust front moves past the updraft, the updraft weakens  Rain starts to fall into the updraft, cutting off the rising humid air  Final Stage – downdraft cuts off updraft and storm loses energy source and dissipates o Relatively short-lived ( < 1hr, diameter is 1km or less)  Multi-cell Thunderstorm o Very similar to ordinary thunderstorms except there are a moderate vertical wind shear. This shear causes the storm to tilt and the downdraft is formed downwind of the updraft; thus the storm can last longer. The gust front of the downdraft is more likely to induce another thunderstorm and a string of thunderstorms occur, often at different stages of development  Supercell Thunderstorm o Forms with strong vertical wind shear o Along cold front of a midlatitude cyclone o 100-600m in diameter o Tornadoes can spawn o Downdraft does not cutoff updraft so storm can last for several hours o Hail can form o Microbursts can also form  Microbursts o Radial burst of surface wind o Aviation hazard  MCC o Mesoscale convective complexes o Multiple Tunderstorms  Circular fashion  Covers over 100000 sq km  12 hours of more, self-sustaining  Heavy precipitation o Can form only with weak vertical wind shear o Squall line  String of thunderstorms along a cold front  Hail and tornadoes formed Lightning  Special characteristic of thunderstorms  Charge separation occurs in the cloud  Smaller particles tend to go to top of storm with positive charge, larger ones with negative charge to the bottom  Acts like a magnet to attract positive charge at the surface  Most (90%) lightning starts at cloud base and goes to the surface  First step- a path of 50m or so is ionized by 3M volts of electricity  Further steps of 50 to 100m until surface is reached  As surface is neared, positive ions from the surface move upward  When the two connect, the luminous return stroke is seen  Several cm in diameter  Process can repeatoleading to forked lightning  Air heats to 30000 C  Generates shock wave- thunder!  Sound travels more slowly than light  Radio waves, sferics, are produced allowing for lightning detection worldwide Hail  Largest form of solid precipitation and most damaging  2 costliest disaster in Canada (after 1998 Ice storm)  Largest: 757 grams (1.67 lbs) found in Coffeyville Kansas – 1
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