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Thunderstorms & Tornados. Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida. Go to Visualization. Thunderstorm Life Cycle. Thunderstorm Life Cycle.
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Thunderstorms & Tornados Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida
Go to Visualization Thunderstorm Life Cycle
Thunderstorm Life Cycle • Cumulus Stage is the growth stage with the transformation of water vapor into liquid particles as warm air feeds in from below • Mature Stage starts with the appearance of a downdraft as precipitation & cold air from evaporation entrain into the cloud while the top of the cumulonimbus can reach 40,000 feet and the downdraft reaches the surface to form a gust front • Dissipating Stage occurs when updrafts weaken due to the overwhelming downdraft and the supply of warm/moist air being cut-off
Severe Thunderstorms • Long-lived T-Storms have a greater chance of becoming severe • Hail to at least ¾ inch in diameter and winds gusts ≥ 50 knots (58 mph) & tops to 60,000 feet with violent updrafts • Need strong vertical wind shear to produce supercell storms lasting over 1 hour that are tilted so that the outflow of cold air from the downdraft never undercuts the updraft
Downbursts & Microbursts • Localized downdraft that spreads like a radial burst of wind • If its small winds extending 4 km or less it is called a microburst, which can have winds up to150 mph
Thunderstorm Lines & Clusters • Squall lines are usually observed near or ahead of a cold front and may extend over 600 miles • Mesoscale Convective Complexes (MCCs) are clusters that may over an entire state moving slowly and often lasting over 12 hours
Thunderstorm Distribution • 50,000 T-Storms occur daily throughout the world, especially over equatorial land masses • In the U.S. thunderstorms form often in the Southeast, but also in the Midwest
Lightning and Thunder • A giant spark that can take place within a cloud, from cloud to cloud, or from cloud to ground (20%) • Extreme heating causes the air to expand explosively, initiating a shock wave that becomes a booming sound wave called thunder • Sound travels 1 mile in 5 seconds, so we can count seconds to determine how far lightning has occurred
Go to Visualization The Lightning Stroke • For lightning to occur, separate regions containing opposite electrical charges must exist within a cumulonimbus cloud • Negative charge at the bottom of the cloud causes the ground to have a positive charge and an electric potential between them (voltage)
The Lightning Stroke • Air is not a good conductor, but as the voltage becomes sufficiently large the insulating properties of the air break down, current flows, and lightning occurs
The Lightning Stroke • First a stepped leader (invisible to the human eye) of negative electrons rushes from cloud to ground • A current of positive charge starts up from the ground (from elevated objects) to meet it • After they meet, a luminous return stroke surges upward to the cloud along the path of the stepped leader • Hence, the downward flow of electrons establishes the bright channel of upward propagating current • Dart leaders allow for repeat strokes along the same established channel, making lightning appear to flicker
Tornado • A rapidly rotating column of air that blows around a small circulation that reaches the ground • Most are 100 to 600 meters in diameter (extreme tornadoes can be 1 mile wide) • Most move 25-50 mph, some up to 70 mph • Most last just a few minutes with a path on the ground averaging 4 miles (extreme tornadoes last hours and travel hundreds of miles) • A funnel cloud is a tornado whose circulation has not reached the ground
Tornado Occurrence • No country experiences more tornadoes than the U.S., with more than 1000 annually (most from March to July) • Tornado Alley is located in the Central Plains from Texas to Nebraska, where warm humid air is overlain by cooler drier air aloft, making for an unstable atmosphere
Tornado Winds • Fujita scale classifies tornadoes based on wind damage done by the storm • Most are F0 and F1 with winds of less than 100 knots, and even the most powerful seldom exceed 220 knots • Violent tornadoes (F3 and higher) comprise only a few percent each year, but cause most deaths
Tornado Formation • Speed and directional wind shear produces rotation near the surface (vortex tube) • Very strong updraft tilts the tube and draws it into the storm, forming a mesocyclone
Tornado Appearance • Updraft is so strong that rain and hail is absent, depicted as a weak echo region or hook echo on radar • Rotating supercell mesocyclone shows up as rotating clouds lowering at the base of the storm, or wall cloud
Waterspouts • May be tornadic • Those seen in Caribbean and Florida Keys are almost always “fair weather” • Unstable air mass • Converging winds start spinning • Developing cumulus helps “lift” spinning air up into the cloud • Slow moving and less intense, with winds under50 mph