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Bell Ringer •Cumulonimbus clouds have a tall structure and a flat base. What are they usually associated with? Severe Storms Thunderstorms and Tornadoes What do you know about Thunderstorms? Have you ever seen a small whirlwind carry dust or leaves upward on a hot day? Thunderstorms • A storm that generates lightning and thunder. • Frequently produce gusty winds, heavy rain, and hail. • Associated with cumulonimbus clouds • Lightning: results from the build up and discharge of electric energy between positively (ground) and negatively (clouds) charged area • Thunder: the sound of rapidly expanding gases usually associated with lightning How common are thunderstorms? Occurrence of Thunderstorms • Estimated 2000 in progress on Earth at any time • About 45,000 take place each day • More than 16 million occur annually around the world • The greatest number occur in the tropics • Atmospheric Conditions needed: – Warm, moist air, uplift, and instability Development of Thunderstorms • Form when warm, humid air rises in an unstable environment • 3 Stages – Cumulus Stage – Mature Stage – Dissipating Stage Cumulus Stage • Warm, moist air is supplied to the cloud by strong updrafts • Cloud grows vertically Mature Stage • Begins within 1 hour of the initial updraft • Most active stage • Heavy precipitation, gusty winds, lightening, and sometimes hail are produced Dissipating Stage • Cloud begins to evaporate • The cooling effect of the falling precipitation and the flowing in of colder air from high above causes the storm to die down Thunderstorm Development The Thunderstorm Recipe Video What do you know about tornadoes? Tornadoes • Violent low pressure windstorms that take the form of a vortex. • Vortex- spinning/rotating, often turbulent, column of air • The vortex extends downward from a cumulonimbus cloud – produce rain and hail • Move counterclockwise Occurrence of Tornadoes • About 770 tornadoes are reported in the United States each year • Frequency is greatest between April and June • Associated with Severe Thunderstorms Which states in the U. S. do you think get the most tornado touchdowns? Development of Tornadoes • Most form in association with severe thunderstorms • The formation of the mesocyclone is an important process in tornado formation. National Geographic Tornadoes Video Mesocyclone • A vertical cylinder of rotating air that develops in the updraft of a thunderstorm Formation of a Mesocyclone How Tornadoes Form Video Tornado Intensity • Pressure within tornadoes can be as much as 10% lower than the pressure outside the storm. • Intensity is measured on the Fujita Scale Fujita Scale • • • • • First introduced in 1971 – Tetsuya Fujita Measures the strength of tornadoes Based on storm damage Ranges from F0-F5 Updated 2007 – Enhanced Fujita – Updated wind speeds match damaged caused by tornado Fujita Tornado Intensity Scale Enhanced Fujita Scale Fujita Tornado Scale Category Winds (MPH) Winds (KPH) Damage F0 <73 <116 Light damage F1 73-112 116-180 Moderate Damage F2 113-157 181-254 Considerable Damage F3 158-206 254-332 Severe Damage F4 207-260 333-419 Devastating Damage F5 >260 >419 Incredible and Speechless Damage Watch vs. Warning Tornado Watch • Alert people to the possibility of tornadoes in a specified area for a particular time • Conditions are right for a tornado to happen Tornado Warning • Issued when a tornado has actually been sighted in an area or; • Indicated by weather radar Let’s Review! Exit Ticket Severe Storms Causes Effects Thunderstorms a) b) Tornadoes c) d)