(Source: NOAA National Weather Service, Southern Region HQ, http://www.srh.weather.gov/srh/jetstream/mesoscale/tstrm_intro.htm)

OVERVIEW

Text Box: Most thunderstorms occur in central Florida, northern Gulf Coast, and New Mexico Text Box: AVERAGE NUMBER OF DAYS ANNUALLY WITH THUNDERSTORMS
Learning Objectives:

-- 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.

OCCURRENCE

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:

Moisture

Instability

Lift

Text Box: unstable Sources of moisture
Typical 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.

Instability Instability
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.

Differential Heating. 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 line, 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 summer.

Outflow Boundaries. 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.

Terrain. As air encounters a mountain it is forced up the slope of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.

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The life-cycle of a Thunderstorm

1. TOWERING CUMULUS STAGE

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

2. MATURE STAGE

The 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.

3. DISSIPATING STAGE

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.

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Types of Thunderstorms

Multi-cell
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.

Squall Lines
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 Service)

Thunderstorm hazards: hail

A hailstone Hail 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. (Supercooled 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. In 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.

Thunderstorm hazards: damaging wind

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.

DERECHOS

Text Box: The figure to the left illustrates one way in which Derechos are formed. Ellipses represent expanding outflow boundaries from downdrafts in an advancing and developing squall line. Winds are enhanced at the intersection of the outflow boundaries by simple vector addition. The 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)