Transcript Thunderstorms - De Anza College

Thunderstorms/Lightning and
Tornadoes
Jeff Gawrych
Met 10
Thunderstorms
Thunderstorms are generally classified into one
of two groups:

Air Mass Thunderstorms
 These are not-so-intense storms that are
short-lived and localized.

Mesoscale Convective Systems
 These systems lead to the generation of
numerous thunderstorms.
Air Mass Thunderstorms
These thunderstorms form within a single air mass
and are not tied to fronts or mid-latitude cyclones.
Daily solar heating is primarily responsible for rising
air motion,

producing rising cumulus clouds.
These storms may go through growing, mature and
dissipation stage

In just an hour or two
Often associated with summer storms.
Air Mass Thunderstorms
•Air mass thunderstorms are “self extinguishing” –
their natural evolution forces their dissipation
An updraft is required to release the latent heat that
drives the thunderstorms. In the later stages, rainfall
will lead to air cooling and a downdraft. This largely
kills the updraft and thus the thunderstorm
Severe Thunderstorms
As the name suggests, these thunderstorms
are part of a mesoscale system. The
horizontal scale of up to a few hundred
kilometers.
The actual structure of an these storms can
vary considerably.



Severe Thunderstorms
As the name suggests, these thunderstorms
are part of a mesoscale system. The
horizontal scale of up to a few hundred
kilometers.
The actual structure of an these storms can
vary considerably.
–mesoscale convective complexes
–squall line thunderstorms
–supercell thunderstorms
Severe Thunderstorms
Change in wind with
altitude important to
formation:
“Wind Shear”
Squall-line Thunderstorms
Severe Thunderstorms
Characteristics
–Capable of producing large hail
–Strong gusty surface winds
–Flash floods
–Tornadoes
Definition of Severe Thunderstorm:
–¾ inch hail or
–Surface wind gusts of 50 knots
Supercell Thunderstorms
Average # of days thunderstorms
observed
Average number of days hail observed
Lightning and Thunder
Lightning is the discharge of electricity that
occurs within a thunderstorm.
The extreme heating associated with
lightning causes air to expand rapidly, and
produces sound waves we recognize as
thunder.
If you want to estimate the distance of an
approaching thunderstorm:
–Count the number of seconds between a lightning
strike and the sound of thunder
–For every five seconds, the storm is 1 mile away
Normal charge
separation in a mature
thunderstorm
 Charge separation not
fully understood, but
 rapid convection
(vertical motion) is
certainly important
Separation of Charge in
Clouds
One theory suggests that
the separation of charge
is due to collisions
between ice particles.
After exchanging charge,
the heavier ice crystals
settle toward the cloud
base.
++
Development of lightning stroke
Cloud to Ground Lightning Strike
Positive charge is drawn up to the
stepped leader. Usually through
the highest conducting object.
The return stroke can travel at 1 
108 ms-1 (roughly 1/3 the speed
of light.) This return stroke is
visible to the human eye.
There are commonly three or four
strokes along a common path,
which is why it is sometimes
appears like the lightning
flickers.
Why one shouldn’t
shelter under a tree
during a
thunderstorm…
Lightning Facts
It is estimated that globally 5000 people are killed by lightning
annually.
At any given moment there are ~ 1000 thunderstorms
occurring over the globe. Two thirds of all lightning strikes
occur within the tropics.
Most lightning strikes are NOT cloud to ground strikes (20%) rather cloud to cloud strikes are most common.
Aircraft are usually not damaged by lightning strikes.
Tornadoes …
… are also called twisters or cyclones.
… are rapidly rotating winds that blow around a small
area of intense low pressure.
… come in many shapes, but mostly look like funnels
or tubes.
… often descend from large cumulonimbus clouds.
A funnel cloud is a tornado that doesn’t hit the
ground.
A waterspout is a tornado-like storm that occurs over
the ocean
Tornado Characteristics
Majority of tornadoes rotate counter-clockwise (cyclonic)
Most tornadoes only last a few minutes
Most tornadoes are ~ 100 – 600 m (300-2000 ft) in diameter
Fujita Tornado Damage Scale
F0......Gale tornado...... winds of 40-70 mph
Some damage to chimneys; branches broken off trees, pushes
over shallow-rooted trees…
Fl...... Moderate tornado.....winds of 73-112 mph
Peels surface off roofs; mobile homes destroyed.
F2......Significant tornado......winds of 113-157 mph
Considerable damage. Roofs torn off frame houses- mobile
homes demolished; boxcars pushed over; larger trees snapped
or uprooted-, light object projected like missiles.
Fujita Tornado Damage Scale
F3......Severe tornado......winds of 158-206 mph
Roof and some wall torn off, well constructed houses, trains overturned;
most trees in forest uprooted
F4......Devastating tornado......winds of 207-260 mph
Well-constructed houses leveled, structures with weak foundations blown
some distance; cars thrown…
F5......Incredible tornado......winds of 261-318 mph
Strong frame houses lifted off foundations and carried considerable
distances to disintegrate- automobile sized missiles fly in excess of 100
meters; trees debarked; steel reinforced concrete badly damaged.
Tornado Formation
Formed in association with severe thunderstorms
Conditionally unstable atmosphere is important
Multiple tornadoes can come from a single storm (like a
supercell storm)
•Example: May 4-5, 2003, during a 24 hour period, there
were over 80 reported tornadoes.
Recipe for a tornado
Strong wind shear
- Can enhance rotation
- Caused mainly by jet stream
Warm moist air below dry colder
air
- Large instability
- Explosive growth due to
latent heat release
Supercell storms good candidate
for tornadoes (they already have
rotation).
Tornado occurrence?
Tornadoes possible everywhere in the world, but
most are in the U.S. (tornado alley Texas –
Nebraska
3/4 of the tornadoes occur from March to July,
with the maximum in _____.
May
 Jet stream is still a large influence
late afternoon (4-6pm)
Most often occur in the _______________
before sunrise
Least frequent ______________
Tornado incidence by state
25 year total
Why is Tornado Alley the most
likely place to get tornadoes?
Perfect location for the mixing of air masses



Warm, moist gulf air to the south
Cold, dry to the north/northeast
Rockies mountains to west/northwest
 Downslope flow is cool and dry

Right latitude for the polar jet stream
On Radar, the presence
of a hook echo indicates
a mesocyclone; a region
in a thunderstorm very
likely to spawn a tornado