Todays Topics Chapter 11 Thunderstorms and Tornadoes Distribution

Today’s Topics • Chapter 11 – Thunderstorms and Tornadoes – – – – Distribution and types Factors for Growth Air Mass Thunderstorm development Super. Cells Thunderstorm Damage Lightning Tornadoes

Thunderstorm Distribution

Basic Types • Ordinary Single Cell / Air Mass Thunderstorms – Usually the result of ground heating and convection. These are short lived (an hour or so) and are rarely severe. – Entrainment of air is a important process to initiate collision-coalescence of the cloud droplets • Multicell Thunderstorms • Supercell Thunderstorms

Factors affecting Growth • Need a lifting Mechanism (chapter 4) , the dryline (chapter 9) can cause lifting. • Unstable Atmosphere – Lifted index is one method for gauging the amount of instablity • Vertical wind shear – The greater the shear the more tilt. Tilted storms can begin to rotate leading to severe weather

Factors Affecting Growth (2) • Low Level Jet Stream – Transports warm moist air into a developing storm. This mechanism is largely responsible for Thunderstorms that form at night • Capping Inversions – Can lead to concentration of energy and “explosive” development

Air mass thunderstorms are the most common and least destructive usually lasting for less than an hour. The cumulus stage begins when unstable air begins to rise and cool adiabatically to form fair weather cumulus clouds. The mature stage begins when precipitation starts to fall dragging air toward the surface as downdrafts form in the areas of intense precipitation. As the cloud yields heavy precipitation, downdrafts occupy an increasing portion of the cloud base, the supply of additional water vapor is cut off, and the storm enters its dissipative stage.

Multicell Thunderstorms • Called Mesoscale Convective Systems. (MCSs) There are two varieties: – Squall Lines – Mesoscale Convective Complexes (MCCs) • These are much longer lived and single cell storms, because they partially self propogating. • Furthermore, they are tilted, so they avoid squelching updrafts due to rainout.

Squall line thunderstorms consist of a large number of individual storm cells arranged in a linear band, about 500 km in length. They tend to form parallel to and about 300 to 500 km ahead of cold fronts. Wind velocities in the direction of storm movement typically increase with height. The strong winds aloft push the updrafts ahead of the downdrafts and allow the rising air to feed additional moisture into the storm. As the downdrafts reach the ground, they surge forward as a wedge of cold, dense air, called a gust front.

Mesoscale convective complexes (MCCs) appear as oval or roughly circular organized systems containing several thunderstorms and are self-propagating in that their individual cells often create downdrafts, leading to the formation of new, powerful cells nearby.

Mesoscale Convective Complex

A supercell storm consists of a single, extremely powerful cell. Despite their single-cell structure, supercell storms are remarkably complex, with the updraft and downdraft bending and wrapping around each other due to strong wind shear. The downdrafts serve to amplify the adjacent updrafts.

Doppler radar can reveal a feature of a supercell called a hook, which looks like a small appendage attached to the main body of the storm whose appearance usually means tornado formation is imminent. When displayed on a radar screen a large portion of the storm seems to be missing. This zone, known as a vault, is where the inflow of warm surface air enters the supercell.

Supercell Surface Structure

A severe thunderstorm watch means that the situation is conducive to the formation of such activity. If a severe thunderstorm has already developed, a severe thunderstorm warning is issued. Likewise, tornado warnings alert the public to the observation of an actual tornado or the detection of tornado precursors on Doppler radar.

Damage from Severe Thunderstorms • Flash Flooding • Hail – Ice crystals that grow to large size. They are kept aloft in clouds by powerful updrafts • Microbursts • Lightning • Tornadoes

Strong downdrafts may also create downbursts, potentially deadly gusts of wind that can reach speeds in excess of 270 km/hr. When strong downdrafts reach the surface, they can spread outward in all directions to form intense horizontal winds capable of causing severe damage at the surface. Generated by sudden evaporational cooling of the air just below the cloud. Downbursts with diameters of less than 4 km are called microbursts, and can produce a particularly dangerous problem when they occur near airports.

Today’s Topics • Test #2 is on Thursday (Nov. 8 th) • Chapter 11 – Thunderstorms and Tornadoes – – – – Distribution and types Factors for Growth Air Mass Thunderstorm development Super. Cells Thunderstorm Damage Lightning Tornadoes

Concert • “Attendance at a Classical Concert does not have to ruin your entire day” -- Findlay Cockrell • Sunday 3 pm – PAC Main Theatre

Lightning About 80 percent of all lightning is cloud-to-cloud lightning, or sheet lightning, which occurs when the voltage gradient within a cloud, or between clouds, overcomes the electrical resistance of the air. The result is a large and powerful spark that partially equalizes the charge separation.

Cloud-to-ground lightning occurs when negative charges accumulate in the lower portions of the cloud. Positive charges are attracted to a relatively small area in the ground directly beneath the cloud establishing a large voltage difference between the ground and the cloud base. The positive charge at the surface is a local phenomenon; it arises because the negative charge at the base of the cloud repels electrons on the ground below. Farther away, the surface maintains its normal negative charge relative to the atmosphere.

All lightning requires the initial separation of positive and negative charges into different regions of a cloud. Most often the positive charges accumulate in the upper reaches of the cloud, negative charges in lower portions. Small pockets of positive charges may also gather near the cloud base.

The actual lightning event is preceded by the rapid and staggered advance of a shaft of negatively charged air, called a stepped leader.

When the leader approaches the ground, a spark surges upward from the ground toward the leader (top). When the leader and the spark connect, they create a pathway for the flow of electrons that initiates the first in a sequence of brightly illuminated strokes, or return strokes (bottom).

Another leader (the dart leader) forms within about a tenth of a second, and a subsequent stroke emerges from it. This sequence of dart leaders and strokes may repeat itself four or five times. Because the individual strokes occur in such rapid succession, they appear to be a single stroke that flickers and dances about. We call the combination of strokes a lightning flash, the net effect of which is to transfer electrons from the cloud to the ground.

Lightning

The tremendous increase in temperature during a lightning stroke causes the air to expand explosively and produce the familiar sound of thunder. The decrease in the density of air with height causes sound waves from lightning strokes over 20 km away be to be bent upward. As a result, the lightning seems to occur without thunder and is sometimes called heat lightning.

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Tornadoes are zones of extremely rapid, rotating winds beneath the base of cumulonimbus clouds. Though the majority of tornadoes rotate cyclonically a few spin in the opposite direction. Strong tornadic winds result from extraordinarily large differences in atmospheric pressure over short distances.

Tornado Life Cycle

The first observable step in tornado formation is the slow, horizontal rotation (a) of a large segment of the cloud which begins deep within the cloud interior. The resulting large vortices are called mesocyclones. Under the right conditions, strong updrafts cause the horizontal vortex of air to be tilted upward (b).

The narrowing column of rotating air stretches downward, and a portion of the cloud base protrudes downward to form a wall cloud. Wall clouds form where cool, humid air from zones of precipitation is drawn into the updraft feeding the main cloud. The cool, humid air condenses at a lower height than does the air feeding into the rest of the cloud. Wall clouds most often occur on the southern or southwestern portions of supercells, near areas of large hail and heavy rainfall.

Funnel clouds form when a narrow, rapidly rotating vortex emerges from the base of the wall cloud. A funnel cloud has all the characteristics and intensity of a true tornado; the only difference between the two is that a funnel cloud has yet to touch the ground.

Wall Cloud & Funnel Cloud

No other country in the world has nearly as many as the U. S. The continent covers a wide range of latitudes; its southeastern portion borders the warm Gulf of Mexico, while the northernmost portion extends into the Arctic. Much of the eastern portion of the continent is flat and no major mountain range extends in an east–west direction. These features allow for a collision of maritime tropical air with continental polar air along the polar front.

Tornadoes around the globe.

Although most tornadoes rotate around a single, central core, some of the most violent ones have relatively small zones of intense rotations (about 10 m in diameter) called suction vortices. It is these small vortices that probably cause the familiar phenomenon of one home being destroyed while the one next door remains unscathed.

The Fujita scale provides a widely used system for ranking tornado intensity. Documented tornadoes fall into seven levels of intensity, with each assigned a particular F-value ranging from 0 to 5. In the U. S. , the majority (69 percent) fall into the weak category, which includes F 0 and F 1 tornadoes.

Waterspouts occur over warm water bodies and are typically smaller than tornadoes, having diameters between about 5 and 100 m. Though they are generally weaker than tornadoes, they can have wind speeds of up to 150 km/hr. Some waterspouts originate when land-based tornadoes move offshore. The majority are formed over the water itself.