Outline for Lecture 22. Thunderstorm Review. Lightning and Thunder. Tornadoes. Tornado Forecasting. The Development and Occurrence of Tornadoes. 4/10/2003. Thunderstorm development. cumulus stage : strong updrafts act to build the storm. .
Lightning and Thunder
The Development and Occurrence of Tornadoes
cumulus stage: strong updrafts act to build the storm.
mature stage: heavy precipitation and cool downdrafts in part of the storm.
dissipating stage: warm updrafts disappear, rain stops, cloud dissipates
Strong localized downdrafts below thunderstorms are called
The colder air is, the more dense, the more dense the
faster it will fall.
In microbursts, evaporative
cooling helps to cool the air
which leads to rapid
Wind speeds can exceed
100 mph. The winds
expand out from the center
of the burst.
Lightning is the result of charge separation (part of the cloud develops an excess of negative charge, whereas another part
acquires a positive charge).
The rapid vertical motion, present in mature cumulonimbus clouds, is key to charge separation.
The formation of ice in clouds is important.
Charge separation within the freezing crystal is where it begins.
Lightning is a massive discharge of electricity. Electricity is
made up of electrons. When lightning strikes, massive amounts
of electrons shoot through the air at once. In the process, they
bump into a lot of air molecules.
When lightning strikes, intense heat (8,000°C - 33,000 °C) expands
the air in and around it so rapidly that it results in a loud noise we
know as thunder. THUNDER IS AN EXPLOSION!!
Tornadoes (twisters, cyclones) are violent windstorms that take the form of a rotating column of air
Air flows from higher to lower pressure
Wind speed is determined by the pressure gradient force
The pressure at the center of a tornado is as much as 10 percent lower than just outside it
How significant is a 10 percent drop?
is ~ 3% difference…
...OVER 1500 miles!
a 10% difference.
The distance in a
tornado is ~5 miles.
Suction vortices are 10–30 meters wide
Less than one percent of thunderstorms produce tornadoes.
Tornadoes can form in any situation that produces severe weather
including cold fronts, squall lines, and hurricanes. The most
intense tornadoes are typically from supercell thunderstorms.
For a tornado to occur, first amesocyclone (a vertical cylinder
of rotating air 2-6 miles wide) must develop. Tornadoes often
form about 30 minutes after mesocyclone formation.
Wind speed shear and strong vertical updrafts lead to the rotating
column of air.
Average annual tornado incidence per 10,000 square
miles for a 27 year period.
Interaction of cold, dry cP polar air masses with the warm, humid mT
tropical air masses results in majority of tornadoes over the U.S.
occur in the spring.
Most tornadoes move to the
northeast (recall winds in
advance of a cold front are
from the southwest).
Tornadoes are classified according to the worst damage that they did anywhere along their path.
College Park tornado was an F3:
Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown
Doppler radars and faster
computers have greatly
improved our ability to
warn the public about
Christian Johann Doppler
in 1842 explained the process.
The wavelength of the radar echo changes is such a way to
determine movement of the thunderstorm.
Lie flat in the nearest ditch or depression and use your hands to cover your head.