Transcript Thunderstorms and Tornadoes

Thunderstorms (Tormenta) and
Tornadoes
After completing this section, students will discuss the formation of
violent weather patterns such as thunderstorms and tornadoes
(Standard PI – 060)
Storms come in various sizes.
Hurricanes can cover several states,
but thunderstorms and tornadoes are
small-scale phenomenon
Thunderstorms (tormenta) are
storms that produce lightning
(rayo), thunder, rain and
sometimes hail
Tornadoes sometimes form in thunderstorms
All thunderstorms require three
ingredients for their formation:
• Moisture
• Instability
• a lifting mechanism
Typical sources of moisture are large
bodies of water such as the Atlantic
and Pacific oceans as well as the Gulf
of Mexico.
An unstable air mass is characterized
by warm moist air near the surface
and cold dry air aloft.
Air that is forced upward will continue to rise, and
air that is forced downward will continue to sink.
For a thunderstorm to develop, there needs
to be a mechanism which initiates the
upward motion, something that will give the
air a nudge upward.
Fronts lift warm moist air.
Life Cycle of a Thunderstorms
The thunderstorm cell has a distinct life-cycle that
lasts about 30 minutes.
Cumulus Stage - A cumulus cloud begins
to grow vertically, to a height of about
20,000 feet (6 km).
Air within the cloud is dominated by updraft with
some violent air currents around the edges
Mature Stage - The storm has
considerable depth, often reaching
40,000 to 60,000 feet (12 to 18 km).
This is the most dangerous stage when large hail,
damaging winds, and flash flooding may occur
Dissipating stage – The storm no
longer has a supply of warm moist air
to maintain itself
Light rain and weak outflow winds may remain for a while
during this stage
Thunderstorms usually form
along cold fronts
These storms are associated with low pressure
systems
Sometimes thunderstorms form
before the cold front arrives –
squall lines
Thunderstorms also produce lightning
(rayo) – discharge of electricity from a
cloud to the ground, to another cloud or
some other spot
Tornadoes
A violently rotating column of air that usually
touches the ground
The most destructive tornadoes occur
from supercells (celula super) rotating thunderstorms with a welldefined radar circulation called a
mesocyclone
Tornado season is generally March
through August, although tornadoes
can occur at any time of year.
Over 80 percent of all tornadoes strike between
noon and midnight.
Tornado intensity is measured by
using the Fujita Scale
F-0
F-1
F-3
F-4
F-2
F-5
The National Weather Service issues
watches and warnings to give people
time to find shelter
Tornado watch – conditions are right for tornadoes to form
Tornado warning – a tornado is actually spotted, by humans
or on radar