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Earth Science - Lecture 24 Notes.docx

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Earth Sciences
Sergei Basik

October 30, 2013 THUNDERSTORMS General Information  1,800 thunderstorms occur at any moment around the world (16 million a year)  The worldwide map shows average density of annual lightning flashes per square kilometer  Most common latitudes are those near the equator  Form as unstable, warm and moist air rapidly rises into colder air and condenses  releasing heat and causing updraft  Water droplets freeze in anvil-shaped cumulonimbus clouds  Cold air pushing under warm air along cold front is a common triggering mechanism  Thunderstorms can: travel for more than 1,000km  Lightning strikes kill people  Strong winds knock down trees, power lines and buildings  Cause numerous wildfires, hail, tornadoes, flash floods Stage One  Towering cumulus cloud indicates rising air  Usually little or no rain  Lasts about 10 minutes  Occasional lightning Stage Two  Most likely time for hail, heavy rain, frequent lightning, strong winds, tornadoes  Occasionally has black or dark green appearance  Lasts about 10-20 minutes (some storms last much longer) Lightning  Results from strong separation of charge that builds up between top and bottom of cumulonimbus cloud  Charge separation increases as water droplets and ice particles are carried in droplets and collide with downward-moving ice particles or hail  Top of cloud carries positive charge  Bottom of cloud carries negative charge  Negative charge at bottom of clouds attracts positive charge toward ground surface (especially to tall objects)  Eventually electrical resistance in the air cannot keep opposite charges apart  Charge difference between ground and clouds becomes too large:  Conductive channel of air develops between cloud and ground  Small amount of charge (step leader) starts moving toward ground October 30, 2013  When it nears ground upward leader of opposite charge connects with step leader  Connection is made  Powerful discharge occurs (see discharge as flash of lightning)  Lighting instantly hears air to temperatures about 28,000 degrees Celsius  Accompanying expansion of air at supersonic speed is thunder  Causes an average of 55-60 fatalities each year in t
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