-Small intense vortex of spinning air often at increased speeds. -Associated with storms that develop in association with mid latitude cyclones or tropical cyclones.
-Dark funnel cloud hanging from the base of a dense cumulonimbus cloud. -Appears dark because of moisture, debris, and dust inside. -Wind speeds exceed speeds known in any other storm, ranging from 40 mph- 300 mph - General tornado is about 110 mph
-Intense wind speed of a tornado is caused by extreme pressure differences over very short horizontal distances. -Large pressure gradient force, resulting in intense wind speeds as the air rushes into the low-pressure center of the
-99% of tornadoes in the Northern Hemisphere rotate counterclockwise. -99% of tornadoes in the Southern Hemisphere rotate clockwise.
-Associated with severe thunderstorms. -Severe thunderstorms provide a key ingredient for tornadoes- strong vertical circulation. -The other key ingredient is the presence of wind shear.
Change of wind speed or direction with height.
Spinning circulation aligned with the ground that occurs in regions where there is significant wind shear.
-Vertical tower of slowly rotating air resulting from strong convection that lifts portions of the horizontal vortex. -Start out broad and as vertical convection extends the top of the mesocyclone into the atmosphere it stretches and narrows. -20% of the time, a tornado forms.
Measuring Tornado Intensity
-Most common measure is the Fujita intensity scale (F-scale). -Has been recently replaced with the Enhanced Fujita scale (EH- scale). -Ranks tornadoes 0-5, weakest to strongest.
Measuring Tornado Intensity
-F and EF scales are damage based. -EF scale incorporates 28 types of structures, with up to 12 different degree of damage ratings. -5% of tornadoes reach category EF4 or EF5.
-The 5% of tornadoes that are EF4 or EF5 are responsible for 70% of tornado deaths. -An average of 75 people die each year in the US from tornadoes, more than in any other natural phenomena (except floods and lightning).
Tornadoes in the U.S.
-U.S. experiences the most tornadoes in the world. -Due to: -the warm Gulf of Mexico and cold Arctic air -flat land in the eastern 2/3 of the country -lack of east to west mountains
-Occur in almost all states -Majority occur in 'tornado alley' (Texas to Nebraska) -Ohio and Florida are other tornado hot spots. -Texas has the highest number of tornadoes (over 100 annually), but ranks 9th in its number of tornadoes per square mile.
-Oklahoma has the highest tornado density, the most tornado dense area in the world. -OK has a large amount of moderate to violent tornadoes, F2 or higher, making it also the leader in strong tornadoes. -FL has the 2nd highest tornado density.
-Can occur at anytime of the year but are most common in spring. -Spring is when the air mass contrasts are especially strong. -May has the greatest number of tornadoes with June in second. -Peak tornado season is not uniform throughout the US.
-The South of the US has its peak tornado season in April. -Moving north and slightly west, most of tornado alley's tornadoes are in May. -Far north, from NE- WI, the peak is usually in June. -Tornadoes are most likely to occur between 3 and 8 PM.
US Tornado Occurence Trends
-Tornado occurrence doubled from the 1950's to the 1990's. -Can most likely be attributed to an increase in the reporting of tornadoes with the advancement of radar and Doppler and an increase in storm chasers.
-Most structural damage from tornadoes results from extreme winds. -Flying debris is the main cause of injury and death. -The most violent tornadoes have several small zones of intense rotations called multiple vortex tornadoes.
-Tornadoes have such a short lifespan that most do not cause damage to property or human health. -1950-1994, there was an average of 91 people killed in an average of 760 tornadoes. -40% of fatalities occur in mobile homes, 31% in permanent homes
-Stay indoors, go to basement and hide in the center of a windowless room under something heavy. -Do not open windows. -Go into a ditch if driving.
Issued by the National Weather Service when conditions are favorable for the development of severe weather (thunderstorms or tornadoes)
Issued when severe criteria has been met or a tornado has been seen or detected by radar.
-March 18th, 1925 -Travelled from eastern Missouri through Illinois and into Indiana -Killed 700 total, 600 in Illinois -Leveled several towns -Single largest killer in history
1974 Super Outbreak
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