(Source: NOAA National Weather Service, Southern Region
-- Where to most thunderstorms occur in the
contiguous 48 states?
-- What are the three necessary ingredients for thunderstorm formation?
-- What is unstable air?
-- List three sources of lift.
-- Describe the life-cycle stages of a thunderstorm.
It is estimated that there are as many as
40,000 thunderstorm occurrences each day world-wide. This translates into an
astounding 14.6 million occurrences annually! The United States certainly
experiences its share of thunderstorm occurrences.
This figure shows the average number of thunderstorm days each year
throughout the U.S. The frequency of occurrence is greatest in the
southeastern states, with the northern Gulf coast (>70) and central Florida
(>100) having the highest annual incidence of thunderstorms. Florida, being
a subtropical peninsula, experiences very strong solar heating and a nearby
source of warm humid air from both the Gulf of Mexico and the Atlantic
Ocean. Iowa, for example, experiences only about half the number of
thunderstorms the northern Gulf coast and Florida get. However, even
located more than 1000 miles to the south, the Gulf of Mexico is still its main
moisture source there.
The Necessary Ingredients for Thunderstorms
All thunderstorms require three
ingredients for their formation:
Sources of moisture
sources of moisture are large bodies of water
such as the Atlantic and
Pacific oceans as well as the Gulf of Mexico. The southeastern U.S. and
especially Florida has access to two moisture sources in the Atlantic
ocean and the Gulf of Mexico which helps explain why there are so many
thunderstorms in that region.
An unstable air mass is characterized by warm moist air near the surface
and cold dry air aloft. Cold air is more dense (heavier) than warm
air. Dry air is more dense (heavier) than moist air at the same
temperature. That means that, without any other outside
influences, cold dry air aloft will always have a tendency to sink into
and replace underlying warm moist air forcing it upward.
Furthermore, as the moist air decompresses with altitude, it cools
forcing the moisture in it to condense. The condensation process
releases latent heat.
This additional heat source further warms the rising moist air making it
even more unstable than it was before at the beginning of the process.
I refer to this process as the internal combustion engine of
thunderstorms and hurricanes.
Sources of Lift
Typically, 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. This is done by several processes.
The heating of the ground and lower atmosphere is not uniform.
For example, a grassy field will heat at a slower rate than a paved
street. The warmest air, called thermals, tends to rise while the
surrounding surface air gets pulled into the warming spot to replace
the lifted parcels of air.
Fronts. Fronts are the
boundaries between two air masses of different temperatures. In an
advancing cold front, dense cold air wedges under warmer moist air and lifts
it abruptly. If the air is
moist and unstable thunderstorms will readily form along the cold front.
Drylines. Drylines are the boundaries between two air
masses of different moisture content and separate warm moist air from
hot dry air. While the temperature may not be very different across the dry
the main difference is the rapid decrease in moisture behind the
dry line. It is the lack of moisture which allows the temperatures to
occasionally be higher than ahead of the dry line. However, the result
is the same as the warm moist air is lifted along the dry line forming
thunderstorms. This is common over the plains in the spring and early
Outflow Boundaries are a result of the rush of
cold air as a thunderstorm moves overhead. The rain-cooled air acts as
a "mini cold front", called an outflow boundary. Like fronts, this
boundary lifts warm moist air and can cause new thunderstorms to form.
encounters a mountain it is forced up the slope of the terrain.
Upslope thunderstorms are common in the Rocky Mountain west during the
The life-cycle of a Thunderstorm
1. TOWERING CUMULUS
The building block of all thunderstorms is
the thunderstorm cell. The thunderstorm cell has a distinct life-cycle that
lasts about 30 minutes. The
first stage is the towering cumulus stage. A cumulus
cloud begins to grow vertically, perhaps to a height of 20,000 feet (6 km).
Air within the cloud is dominated by updraft with some turbulent eddies
around the edges
second stage is the mature stage. The storm has
considerable depth, often reaching 40,000 to 60,000 feet (12 to 18 km).
Strong updrafts and downdrafts coexist. This is the most dangerous stage
when large hail, damaging winds, and flash flooding may occur.
The final stage is the dissipating stage.
The downdraft cuts off the updraft. The storm no longer has a supply of warm
moist air to maintain itself and therefore it dissipates. Light rain and
weak outflow winds may remain for a while during this stage, before leaving
behind just a remnant anvil top.
Types of Thunderstorms
Although there are times when a thunderstorm consists of just one ordinary
cell that transitions through its life cycle and dissipates without
additional new cell formation, thunderstorms often form in clusters with
numerous cells in various stages of development merging together. Unlike
ordinary single cells, cluster storms can last for several hours producing
large hail, damaging winds, flash flooding, and isolated tornadoes.
Sometimes thunderstorms will form in a line which can extend laterally for
hundreds of miles. These "squall lines" can persist for many hours and
produce damaging winds and hail. The rain cooled air or "gust front"
spreading out from underneath the squall line acts as a mini cold front,
continually lifting warm moist air to fuel the storms.
Schematic of a squall line (top) and accompanying photograph (right). Often
along the leading edge of the line a low hanging arc of cloudiness will form
called the shelf cloud. Gusty, sometimes damaging outflow winds will spread
out horizontally along the ground behind the shelf cloud.
Super cell Thunderstorms
Super cell thunderstorms are a special kind of single cell thunderstorm that
can persist for many hours. They are responsible for nearly all of the
significant tornados produced in the U.S. and for most of the hailstones
larger than golf ball size. Super cells are also known to produce extreme
winds and flash flooding.
They are characterized by a rotating updraft (usually cyclonic - above left)
which results from a storm growing in an environment of significant vertical
wind shear. Wind shear occurs when the winds are changing direction and
increasing with height. The most ideal conditions for super cells occurs when
the winds are veering or turning clockwise with height. For example, in a
veering wind situation the winds may be from the south at the surface and
from the west at 15,000 feet (4500 m). Beneath the super cell, the rotation
of the storm is often visible as well (above right).
The lowering, in the photograph (bottom), represents the wall cloud. The wall
cloud is sometimes a precursor to a tornado. If a tornado were to form, it
would usually do so within the wall cloud.
(Courtesy NOAA National Weather
is precipitation that is formed when updrafts in thunderstorms carry
raindrops upward into extremely cold areas of the atmosphere. Hail can
damage aircraft, homes and cars, and can be deadly to livestock and people.
One of the people killed during the March 28, 2000 tornado in Fort Worth was
killed when struck by grapefruit-size hail.
While Florida has the most thunderstorms,
New Mexico, Colorado, and Wyoming
usually have the most hail storms. Why? The freezing level in the Florida
thunderstorms is so high, the hail often melts before reaching the ground.
Hailstones grow by collision with supercooled water drops.
drops are liquid drops surrounded by air that is below freezing which is a
common occurrence in thunderstorms.) There are two methods by which the
hailstone grows, wet growth and dry growth, and which produce the "layered
look" of hail. In
wet growth, the hailstone nucleus (a tiny piece of ice) is in a region where
the air temperature is below freezing, but not super cold. Upon colliding
with a supercooled drop the water does not immediately freeze around the
nucleus. Instead liquid water spreads across tumbling hailstones and slowly
freezes. Since the process is slow, air bubbles can escape resulting in a
layer of clear ice.
With dry growth, the air temperature is well below freezing and the water
droplet immediately freezes as it collides with the nucleus. The air bubbles
are "frozen" in place, leaving cloudy ice.
Multi-cell thunderstorms produce many hail storms but usually not the larges
hailstones. The reason is that the mature stage in the life cycle of the
multi-cell is relatively short which decreases the time for growth. However,
the sustained updraft in super cell thunderstorms support large hail
formation by repeatedly lifting the hailstones into the very cold air at the
top of the thunderstorm cloud.
all cases, the hail falls when the thunderstorm's updraft can no longer
support the weight of the ice. The stronger the updraft the larger the
hailstone can grow.
Damaging wind from thunderstorms is much more
common than damage from tornadoes. In fact, many confuse damage produced by
"straight-line" winds and often erroneously attribute it to tornadoes. Wind
speeds can reach up to 100 mph with a damage path extending from
hundreds of miles.
Several factors contribute to damaging winds at the surface. As precipitation
begins to fall, it drags some of the air with it. This "precipitation drag"
initiates a downdraft. The downdraft is intensified by evaporative cooling as
drier air from the edges of the storm mix with the cloudy air within the storm.
This can be very hazardous to aircraft in flight.
Also some of the strong winds aloft are carried down with the downdraft by a
process called "momentum transfer". These processes lead to a rapid downward
rush of air. As the air impacts the ground it is forced to spread out laterally
causing the gusty and sometimes damaging winds associated with thunderstorms.
word "derecho" was coined by Dr. Gustavus Hinrichs, a physics professor at the
University of Iowa, in a paper published in the American Meteorological Journal
in 1888. Dr. Hinrichs chose this terminology for thunderstorm induced
straight-line winds as an analog to the word tornado. Derecho is a Spanish word
which can be defined as
or "straight ahead" while
tornado is thought by some, including Dr. Hinrichs, to have been derived from
the Spanish word "tornar" which means "to turn". Derechos are the result of the combination of several outflow boundaries, which
are set up by the precipitation downdraft. Derechos result from particularly strong
downdrafts and can travel many hundreds of miles before dissipating.
There are two dangers associated with Derechos:
- The duration of the damaging winds and
- Widespread coverage of such winds,
Damaging winds are classified as those winds
exceeding 50-60 mph. In derecho situations it is not uncommon for
winds of this magnitude to last for over 30 minutes, with occasional gusts of
100 mph and cover up to 1250 square miles or about the
size of Rhode Island.
(Source: NOAA National
Weather Service, Southern Region HQ, http://www.srh.weather.gov/srh/jetstream/mesoscale/tstrm_intro.htm)