ATOC 181 Lecture Notes - Lecture 16: Roy Sullivan, Human Eye, Sound
7 Jun 2018
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LESSON 16
TORNADOES
Tornado Characteristics
• Central pressure about 900hPa
• speed of the winds and eventually the rain associated = strength of tornado
• depends in the difference in pressure of the centre and the outside of the tornado = speed of winds
• normal pressure at surface= 1000hpa
• means that have in very small difference differences in pressure of the order of 100hPa (10%) =
enormous. Type of pressure difference found in hurricanes or tropical cyclones but in hurricanes, they
spread over hundreds of km
• depression up to 100hPa
• maximum winds: <60, up to 110m/s (difficult to gather data)
• winds usually cyclonic in North Hemisphere (rotate Counter clockwise)
Tornado Winds
• when tornado approaches you and rotate in Counter clockwise, winds turn at ~25m/s. assume that come
towards you at 50m/s. add both speeds, can have speed that reaches you at 75km/h on left hand side.
Can also subtract the speed to give 25m/s
• can have several funnels or suction vortices around a tornado centre
o satellite tornado: if have main tornado
• can have cycloidal marks from multi-vortex tornado. Leave rotating damages on the surface
Formation Of A Supercell Tornado
• severe storm→ supercell
• updraft must begin to rotate. Warm air goes up, and rotate with updraft
• Three phases model:
o Mesocyclone
o Tornado cyclone
o tornado
• mesocyclone: rotating side going upwards. Inside
in it, have tornado cyclone that starts to go down
and have tornado when touches surface
• wind shear creating velocity: the air will rotate
and as goes up, will bring the rotation with it.
From horizontal vorticity to vertical vorticity
(tilting process) = reason why having rotating
clouds and updrafts. If wind is the same
everywhere (no wind shear) = not get this
• most intense tornadoes
Super Cell Circulation
• clouds grow and reach to tropopause. The
mesocyclone will also grow with it and shrink in
size (law of conservation of momentum)
• the mesocyclone starts as being spread and with
low speed, but as clouds grow, it shrinks and have
higher speed.
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• Tornado start with horizontal wind shear, creates rotation, air goes up (mesocyclone), have vertical
rotation. Start as slow rotation in core of mesocyclone, clouds go up and brings with it the mesocyclone,
shrinks as clouds grow, increases the speed for conservation of momentum and then tornado forms in
wall clouds towards the surface when fast enough and enough difference in pressure. Then, have
tornado at surface level.
Tornado signature
• Nice tool to allow to detect regions where have mesocyclone. When have mesocyclone, probability of
having tornado = high
• Use intensity of precipitation.
• Doppler radar = measures the speed in direction where it is aimed at. Green: comes towards you. Red:
goes away from you.
• Signature of mesocyclone: precise/defined zone where comes towards you and where goes away from
you
• Have this radar at different levels
Non-Supercell Tornadoes
• May occur in association with multi-cell or ordinary thunderstorms
• Any time have storm or clouds developing and have wind shear that will create rotation, may expect that
will intensify and become vertical
• May form along gust fronts. Every time have gust fronts: have regions where have winds shear, lots of
turbulences
o No water associated with them, often rotating cloud of dust or debris
o Weak and short-lived
• Sometimes associated to rapidly building cumulus congestus (landspouts)
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• Land spouts = tornado on land near lakes
• The region where find the highest probability of a tornado is either at the end of a break or middle of a
break in the squall line
Land Spouts
• mid-1980s
• no strong mid-level mesocyclones
• small vorticity centers (<2km)
• can cause up to F3 damages
• Have landspouts in Florida: because
of lakes. In Colorado because of
mountains
• sea or lake breeze (Ex: Ontario): have
lot of surface convergence created
• have convergence and have spins due to winds going in direction, have rotation, have updrafts that
bring the rotation, have clouds forming and finally the land spouts
Whirlwinds
• rotating wind, cause damages
• not tornado, not land spouts
Water Spouts
• rotating column of air over a large water body
• could be a tornado formed over land, moving over water (tornadic waterspout)
• can inflict damage to vessels
• develop from a cloud over water
• can be dangerous
• unexpected
• can cause lots of boat accidents
• Fair eather aterspouts fored oer ater eg: Florida kes
o Less intense (V<45kt)
o Move slowly and last about 10-15 min
LIGHTNING
Roy Sullivan
• living proof that this lai is false: ou a’t be hit twice by lightning
• hit 7 times, survived
What is Lightning?
• Gigantic atmospheric spark of static electricity
• Decharge of electricity in atmosphere
• Lightning can travel at speeds of 220 000 km/h and can reach temperature close to 30 000oC
• Travels very fast, practically at the speed of light
• NASA satellite research indicates Thunderstorms produce lightning flashes produced about 40x a second
worldwide
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