In the unstable single-cell environment, oftentimes pulse severe storms form. Single-cell storms are difficult to forecast. Like a single-cell storm, each cell of a multicell cluster storm lasts usually only about 20 minutes; however, the cluster itself can last for several hours. The multicell cluster, the most common type of thunderstorm, comprises a number. The most mature cells are found at the center of the cluster, new cells form at the upwind usually the west or southwest edge, while the dissipating cells are found at the downwind usually east or northeast edge.
Multicell clusters are stronger than single-cell storms, and produce heavy rainfall, down-bursts wind speeds reaching 80 mph , medium-sized hail, and periodic tornadoes. A long line of storms with a leading edge of strong wind gusts is called a multicell line storm, or squall line. Moving forward, the wind gusts of cold air force unstable warm air into the updraft at the stormfront's edge; heavy rain and large hail immediately follow.
A large area behind this produces lighter rain. Squall lines produce golf-ball-size hail, heavy rains, tornadoes, and most notably, weak to strong downbursts. The most severe and rare type of thunderstorm is the supercell.
It is a highly organized storm consisting of one main updraft that can reach — mph. This rotating updraft is called a mesocyclone and works to produce extremely large hail 2 in , major downbursts 80 mph , and fierce tornadoes. Microbursts are small-scale, hard-hitting downdrafts that result in both vertical and horizontal wind shears that can be extremely hazardous to low-altitude aircraft.
Microbursts most commonly occur during convective activity. They can appear at the point of heaviest rain during a thunderstorm or they can occur within weaker convective cells with far less precipitation.
The downdraft's cold air usually 1 mi in diameter accelerates as it descends from the cloud base about 1,—3, ft above ground , reaching its highest speeds about five minutes after initially hitting the ground. The resulting "curl" air moving away from the impact point accelerates further, and can extend to approximately 2.
A downdraft can reach top speeds of 6, A supercell thunderstorm. Gale Group. Microbursts are not easily detectable by conventional radar due to their size, duration no longer than 15 minutes , and because they can appear in areas without surface precipitation. Visual clues, however, provide proof of their existence. These clues include rings of blowing dust that often mark the impact point of a microburst; a rain foot—the "unfinished," outward distortion of the edge of an area of precipitation, suggesting the presence of a wet microburst; and a dust foot—the resulting plume of dust after the microburst hits the ground and moves away from its impact point.
Multiple occurrences of downdrafts of this nature can continue for up to an hour; it is not uncommon for more than one microburst to occur in one area. Dust storms, so common to the southwestern United States, can wreak havoc for motorists traveling in the area.
The sudden dark-brown cloud with strong winds and debris is often to blame for tragic, chain-reaction accidents. Dust storms associated with summer thunderstorms are common in the southwestern United States and are found generally in the desert areas of western New Mexico, southern Arizona, and in the southeastern deserts of California.
Dust storms develop due to the cool downdrafts of a thundershower that reach the ground and spread out in all directions, picking up dust along the way. Dust storms associated with late winter and early spring storm systems are common during March and April. Courtesy of United States Air Force. Lightning strikes Earth an estimated times per second. The average annual death toll for lightning is greater than for tornadoes or hurricanes.
Lightning is a secondary effect of electrification within a thunderstorm cloud system. Updrafts of warm, moist air rising into cold air can cause small cumulus clouds to grow into the large cumulonimbus cloud systems we associate with thunderstorms.
These turbulent cloud systems tower about their companions, and dominate the atmospheric circulation and electrical field over a wide area. The transition from a small cloud to a turbulent, electrified giant can occur in as little as 30 minutes.
As a thunderstorm cumulonimbus develops, interactions of charged particles, external and internal electrical fields, and complex energy exchanges produce a large electrical field within the cloud. No completely acceptable theory explaining the complex processes of thunderstorm electrification has yet been advanced. But it is believed that electrical charge is important to formation of raindrops and ice crystals, and that thunderstorm electrification closely follows precipitation.
The distribution of electricity in a thunderstorm cloud is usually a concentration of positive charge in the frozen upper layers, and a large negative charge around a positive area in the lower portions of the cloud.
Earth is normally negatively charged with respect to the atmosphere. As the thunderstorm passes over the ground, the negative charge in the base of the cloud induces a positive charge on the ground below and several miles around the storm. The ground charge follows the storm like an electrical shadow, growing stronger as the negative cloud charge increases. The attraction between positive and negative charges makes the positive ground current flow up buildings, trees, and other elevated objects in an effort to establish a flow of current.
But air, which is a poor conductor of. Lightning occurs when the difference between the positive and negative charges—the electrical potential— becomes great enough to overcome the resistance of the insulating air, and to force a conductive path for current to flow between the two charges. Potential in these cases can be as much as million volts. Lightning strokes typically represent a flow of current from negative to positive, intra-cloud, and may proceed from cloud to cloud, cloud to air, cloud to ground, or, where high structures are involved, from ground to cloud.
The typical cloud-to-ground stroke we see most frequently begins as a pilot leader too faint to be visible, advances downward from the cloud, and sets up the initial portion of the stroke path. A surge of current called a step leader follows the pilot, moving ft or more at a time toward the ground, pausing, then repeating the sequence until the conductive path of electrified ionized particles is near the ground.
There, discharge streamers extending from the ground intercept the leader path and complete the conductive channel between ground and cloud charges. When this path is complete, a return stroke leaps upward at speeds approaching that of light, illuminating the branches of the descending leader track. Because these tracks point downward, the stroke appears to come from the cloud.
The bright light of the return stroke is the result of glowing atoms and molecules of air energized by the stroke. Once the channel has been established and the return stroke has ended, dart leaders from the cloud initiate secondary returns, until the opposing charges are dissipated or the channel is gradually broken up by air movement. Even when luminous lightning is not visible, current may continue to flow along the ionized channel set up by the initial step leader.
Ground-to-cloud discharges are less frequently observed than the familiar cloud-to-ground stroke. In these cases, step leaders generally proceed from a tall conductive or semi-conductive structure to the clouds; the initial leader stroke is not followed by a return stroke from the cloud, possibly because charges are less mobile in the cloud than in the highly conducting Earth.
Once the conductive path is established, however, current flow may set up cloud-to-ground sequences of dart leaders and returns. Lightning comes in many forms. Streak lightning , a single or multiple line from cloud to ground, is the form seen.
Forked lightning shows the conductive channel. Sheet lightning is a shapeless flash covering a broad area, often seen in cloud-to-cloud discharges. Heat lightning is seen along the horizon during hot weather, and is believed to be the reflection of lightning occurring beyond the horizon.
Ribbon lightning is streak lightning whose conductive channel is moved by high winds, making successive strokes seem to parallel one another. Beaded lightning appears as an interrupted stroke. Ball lightning is in some ways the most interesting—and most controversial—form. Ball lightning has been reported in various shapes—from a luminous globe to a doughnut-shaped toroid to an ellipsoid. It hisses as it hurtles from cloud to Earth, maneuvers at high speeds, rolls along structures, or hangs suspended in the air.
The dual character of lightning—it carries high currents and produces destructive thermal effects—makes it doubly dangerous. The current peaks, which may reach magnitudes of , amperes or more, produce forces that have a crushing effect upon conductors, and which can build to explosive levels in non-conducting or semi-conducting materials like wood or brick.
The continuous current produces heat, and is responsible for the numerous fires attributed to lightning. At the NOAA, lightning is the subject of considerable scientific interest. Department of Commerce scientists at NOAA's Environmental Research Laboratories are experimenting with lightning suppression techniques, measuring atmospheric electricity over the open ocean, and studying the apparent but elusive connections between lightning and other events in the atmosphere, ionosphere, Earth, and geomagnetic field.
Thunder is the crash and rumble associated with lightning and is caused by an explosive expansion of air heated by the stroke. When lightning is close by, its thunder makes a sharp explosive sound. More distant strokes produce the familiar growl and rumble of thunder, a result of sound being. Persons struck by lightning receive a severe electrical shock and may be burned, but they carry no electrical charge and can be handled safely.
A person struck by lightning who shows no vital signs can often be revived by prompt mouth-to-mouth resuscitation, cardiac massage, and prolonged artificial respiration.
In a group struck by lightning, the apparently dead should be treated first; those who show vital signs will probably recover spontaneously, although burns and other injuries may require treatment. Recovery from lightning strikes is usually complete except for possible impairment or loss of sight or hearing. Because the speed of light is about a million times that of sound, the distance in miles to a lightning's stroke can be estimated by counting the number of seconds between lightning and thunder, and dividing by five.
The electromagnetic impulses of a lightning stroke produce whistlers—gliding tones that travel along lines of force in Earth's magnetic field from their lightning source in one hemisphere to a similar point in the opposite hemisphere, often echoing back and forth several times. Their sound is something like the whistle of World War II bombs, occasionally modified in a way that produces musical variations.
Hailstones are precipitation in the form of lumps of ice that form during some thunderstorms. Hail can range in size from that of a pea to a softball. Hailstones are usually round, but may also be conical or irregular in shape, some with pointed projections.
While it takes about one million cloud droplets to form a single raindrop, it takes about 10 billion cloud droplets to form a golf-ball-size hailstone.
Hail is formed as ice pellets which were initially snowflakes or frozen raindrops strike supercooled water droplets within a storm cloud. The supercooled water flows over the ice particles and part of it freezes instantly.
Some of the unfrozen water remains attached to the growing hailstone until it freezes, and part of it slips away. This continues until the weight of the hailstone can no longer be supported by the updrafts, and it falls to the ground. What is a severe thunderstorm? How many thunderstorms are there? Worldwide, there are an estimated 16 million thunderstorms each year, and at any given moment, there are roughly 2, thunderstorms in progress.
There are about , thunderstorms each year in the U. When are thunderstorms most likely? Thunderstorms are most likely in the spring and summer months and during the afternoon and evening hours, but they can occur year-round and at all hours. Along the Gulf Coast and across the southeastern and western states, most thunderstorms occur during the afternoon. Thunderstorms frequently occur in the late afternoon and at night in the Plains states. What kinds of damage can thunderstorms cause?
Many hazardous weather events are associated with thunderstorms. Under the right conditions, rainfall from thunderstorms causes flash flooding, killing more people each year than hurricanes, tornadoes or lightning.
Lightning is responsible for many fires around the world each year, and causes fatalities. Hail up to the size of softballs damages cars and windows, and kills livestock caught out in the open. Strong up to more than mph straight-line winds associated with thunderstorms knock down trees, power lines and mobile homes. Tornadoes with winds up to about mph can destroy all but the best-built man-made structures.
It incorporates 28 different damage indicators , based on damage to a wide variety of structures ranging from trees to shopping malls. The U. There were 14 deaths in , seven in Alabama, three in North Carolina, and one each in Louisiana, Mississippi, Pennsylvania and Texas.
In , 76 people perished in tornadoes compared with 42 in On March 2 and 3, 25 people were killed in tornadoes in central Tennessee, including the city of Nashville. Tornado deaths in were the highest since , when people were killed in 1, tornadoes. While scientists cannot say that these storms are increasing, it is clear that the losses are increasing, as a result of population growth and economic development. In addition, the geography, frequency and intensity of these storms also may be changing.
Aon reports that there were 14 separate billion-dollar economic, or total loss severe convective events in The most expensive included the August 10 Midwest derecho, or straight lines winds. The United States experiences more tornadoes than any other country. The National Oceanic and Atmospheric Administration notes that tornadoes can happen any time of year. The costliest U. See chart below.
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