Thunderstorms Flashcards
Measuring tornados
fujita Scale
The atmosphere composition
Dry air contains about 78%
nitrogen, 21% oxygen, 1% argon,
0.04% carbon dioxide, and small
amounts of other gases. Air also
contains water vapor. The amount
of water vapor varies from near 0%
under some conditions to as much
as 5% in hot, humid air masses.
Dew point
Dew point is the temperature to which air must be cooled to become saturated with water
vapor. If the air is cooled further, the water vapor will condense to form liquid water
Default condition of atmosphere
The ‘default’ condition of the atmosphere is stable. Generally, air masses stay in place and don’t have a
tendency to move vertically (creating clouds and weather
Unstable air masses
Unstable air masses are buoyant and tend to have vertical movement. Vertical movement of air masses is the
key ingredient of weather (including severe weather).
Unstable air masses
Unstable air masses are buoyant and tend to have vertical movement. Vertical movement of air masses is the
key ingredient of weather (including severe weather).
What leads to a thunderstorm
Instability, amount of water vapor, and latent heat release led to self-sustaining growth of a
thunderstorm. There is ample moisture to generate significant rainfall. Note the anvil-shaped cloud top. This
indicates that the storm is so large and tall that its top touches the stratosphere (the next layer above the
troposphere) where strong winds def
Moisture
A thunderstorm needs a moisture supply to generate the heavy rains associated with these events. Water
vapor (when it condenses) also supplies the latent heat which drives development of the storm.
Instabilty
Instability is a measure of how buoyant (likely to rise) an air mass is. An unstable air mass is easily triggered
into rising - creating clouds, latent heat release, and growth of a storm system
Lift
A necessary ingredient of thunderstorm formation which is less common is lift. Some process or property is
needed to start vertical movement of an unstable air mass, and initiate formation of a thunderstorm
Three ways lift is created
-convective heating
-passage of a weather front
-topography
Convective heating for lift
In some circumstances, the solar radiation (which heats the ground, which in turn
heats the near-surface air mass) may be sufficient to create a highly unstable air mass - enough for the air
mass to begin rising due to its buoyancy alone
passage of a weather front: lift
A weather front is the boundary between two dissimilar
air masses. If one air mass is displacing the other, one will be forced upward. A cold air mass is heavier than
the warm air mass it replaces, so it is the warm air mass that is forced upward. Cold front boundaries are
steeper than other weather fronts, so the lift is more abrupt. Hence, the strongest lift and the strongest storms
are associated with cold fronts.
topography lift
Orographic lift occurs when an air mass is forced upward as it moves
over rising terrain. This commonly produces clouds and storms on the windward side (and top of) mountains.
Severe thunderstorms
Severe thunderstorms are different from common(‘pop-up’) thunderstorms not only in their intensity but in their
fundamental physical properties.
In a severe thunderstorm, the updrafts and downdrafts coexist for an extended period of time (hours) without
destructive interference.
The United States National Weather Service classifies a thunderstorm as SEVERE if any of the following
occur:
*Hail greater than 1-inch diameter
*Winds over 58 mph
*Tornadoes
Severe thunderstorms are derived from three distinct settings - squall lines, mesoscale convective systems,
and supercells.
Squall line
A squall line is a line of
thunderstorms that typically
forms along a strong cold
front. Under some
conditions, squall lines may
contain very strong
thunderstorms, derechos
(powerful straight-line winds)
and possibly tornadoes
Mesoscale Convective Complexes
MCCs commonly develop from the merging of thunderstorms into a larger system, often where
conditions for thunderstorm development are in place over a relatively large area.
An MCC usually starts with a few scattered, common-type thunderstorms. If these individual cells are
able to interact, the whole system may grow rapidly.
One method of interaction is a process by which deflected downdrafts from one mature cell initiates lift in
surrounding air masses and spawns several new storms nearby (each of which may spawn several
more, etc.).
Supercells
Thunderstorms that persist for
a long time (several hours or
more) may develop into
massive storms called
supercells. Supercell
formation requires a wind
shear that rotates the storm in
a way that keeps the updrafts
separated from the
downdrafts. This prevents the
storm from entering the
dissipation stage and allows it
to continue to grow. The wind
shear can also create rotating
wall clouds that may evolve
into tornadoes. Note that
even a tornado is only a small
component of an enormous
supercell storm.
Strong Winds -
Derechos
The downdrafts of
a thunderstorm
can be translated
into powerful
straight-line winds
on the Earth’s
surface. If these
winds merge and
reinforce along a
squall line, they
can form a
derecho. A
derecho can cause
significant damage
over a wide area.
