Environmental Geology

Severe Thunderstorms and Tornadoes

Tornado is a small, narrow, high speed cyclonic vortex of ultra low pressure.

Rotation is counterclockwise at the surface

Extend from the base of a cumulonimbus cloud to the ground.

Funnel clouds are the same but these do not reach the ground.

Water spouts also fall into this category – tornado over water.

Dust devils are rotating updrafts of warm air – extend from the ground up – not from cloud to ground.

Tornadoes produce the strongest winds ever actually measured at the surface of the Earth, as high as 280 mph to 380 mph.

Tornadoes tend to form along the leading edge of a cold front or dry line where there is significant contrast in two air masses.

Seven basic conditions associated with tornado formation

1.    Thunderstorm (cumulonimbus cloud)
a.    Early cloud formation results from updraft of air due to convective and frontal lifting.  When updraft reaches an altitude of the dew point, the water condenses to form a cloud.  Conversion from water vapor to liquid water releases heat (latent heat of condensation) which further fuels the updraft causing the cloud to expand upward rapidly.
b.    When the cloud expands to the upper troposphere the top may become sheared due to the winds in the jet stream.  This shearing is observed as the anvil that we associate with the cumulonimbus cloud.
2.    Cold front or dry line boundary between continental polar air mass and maritime tropical air mass.
3.    Warm moist air emplaced under dry cool air aloft.
4.    mesocyclone in the middle troposphere
a.    low level jet stream pushing the front forward and surface winds from the southeast in the warm moist air ahead of the front result in shear that causes air in the middle troposphere to roll clockwise horizontally.
b.    As thunderstorm matures and updrafts become more pronounced the upward velocity becomes strong enough to cause this rolling air to flex upward.  When this horizontally clockwise rolling air is lifted into a horizontal position the rotation as observed from the ground is counterclockwise (try this by rotating your arm clockwise horizontally and continuing to rotate as you move your arm to the vertical).
5.    convergence of air at the surface beneath the cloud and divergence of air aloft I the middle to upper troposphere.
6.    Instant release of surface friction
7.    relatively flat topography (does not mean it has to be completely flat)

Damage scale: Fujita (F0-F5) and New Enhanced Fujita Scale
F0- weak, light tree branches broken, sign boards damaged
F1 – weak, moderate: trees snapped, windows broken
F2 – strong, large trees uprooted, weak structures destroyed
F3 – strong, trees leveled, cars overturned, walls removed from buildings
F4 – violent, devastating, frame houses destroyed
F5 – violent, incredible: structures the size of autos moved over 100 meters, steel-reinforced structures highly damaged.

Super Outbreaks: Example Oklahoma City F5 tornado 1999.  This system spawned numerous large and small tornados from Texas through the Great Plains within a period of about 12 to 16 hours. Another example was the April 3, 1974 outbreak that spawned 148 tornados in 12 states stretching from Alabama to Michigan.