P1246990944BmSIH

1
SEVERE WEATHER!
2
Severe Thunderstorms Tornadoes Blizzards Hail H
urricanes Water Spouts Floods Downbursts Al
l the product of atmospheric instability!
3
Instability is the measure of potential buoyancy
of air in the lower atmosphere. It is related
directly to the how the atmosphere cools with
height -or- the lapse rate.
4
Weather balloons are sent up twice a day to
measure winds, moisture, pressure and the lapse
rate (change in temperature with height) of the
lower atmosphere.
5
Instability (lift) can be generated in several
ways 1. Heating the near surface
atmosphere 2. Cooling the air aloft 3.
Divergence aloft 4. Orographic effect. Each
can generate enough instability to produce severe
weather, although orographic effects usually
produce severe winter weather only!
6
In Florida, heating of the lower atmosphere is
the most common way to produced severe
thunderstorms. These are called air-mass
thunderstorms because the develop in the air
mass, not along a front.
7
In the mid-latitudes, surface heating is not
usually enough to create severe weather.
However, the jet stream can increase instability
well beyond the severe weather threshold!
8
Divergence aloft creates surface low pressure
systems which force air masses to collide. The
rising air (instability) can be evacuated at
tremendous speeds allowing more air to rise from
below, keeping storms alive for hours!
9
A strong jet stream over a thunderstorm is like
sucking on a straw! Tremendous amounts of air
can be pulled into the base of the storm,
producing a tornado!
10
Once a funnel cloud touches down, it is by
definition a tornado. The funnel does not have
to be visible all the way to the surface. If
there is a debris cloud on the ground, it is a
tornado. A funnel cloud can not cause damage.
11
The smooth surface of water is an ideal location
for twisters to form. With less disruptive
surface flow, twisters can spin up with less
energy over the water. That is why most water
spots are weaker than tornadoes, and usually
dissipate when they move on shore!
12
Tornadoes often form over the Central United
States, where all the ingredients for tornado
development can be found in abundance!
13
In the summer, there is no shortage of unstable
air in Florida, however, the jet stream is
usually up in Canada. Without a jet stream to
evacuate the air flowing into the base of the
storm, a tornado can only develop when the storm
is young and growing. That is why most Florida
tornadoes are small and short-lived. However,
Florida thunderstorms have no trouble producing
strong winds called downbursts or microbursts!
14
Raindrops begin to drag air down from the top of
the storm. As some drops evaporate they cool the
air even more. The falling air becomes heavier
and falls faster, smashing into the ground at 100
mph or more and splashing out in a straight-line
wind.
15
If a thunderstorms has strong enough updrafts, it
can lift rain above the freezing level. The
drops will freeze forming hail. A hail stone can
make several trips up and down, growing bigger
each time. A severe thunderstorms is defined as
a storm producing winds in excess of 55 mph or
hail 3/4 of an inch in diameter.
16
Hail stones can stick together producing monster
stones. This one is 6 inches in diameter, the
size of a large grape fruit. Individual stones
can grow to an inch or more!
17
Severe weather warnings are not usually issued
for lightning, but lightning is often the
deadliest form of weather in Florida. Surprisingl
y, there is still much to learn about how
lightning forms and propagates!
18
While cloud to ground lightning generally occurs
under the base of the storm, powerful bolts can
strike up to ten miles away. These bolts are
often the most deadly, because people believe
they are far enough away from the storm and do
not have to seek shelter.
19
(No Transcript)
20
(No Transcript)