Meteorology

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Tornadoes

Research has revealed that tornadoes usually form under certain types of atmospheric conditions. Those conditions can be predicted, but not perfectly. When forecasters see those conditions, they can predict that

tornadoes are likely to occur. However, it is not yet possible to predict in advance exactly when and where they will develop, how strong they will be, or precisely what path they will follow.

• The damage from tornadoes comes from the strong winds they contain. It is generally believed that tornadic windspeeds can be as high as 300 mph in the most violent tornadoes. Windspeeds that high can cause automobiles to become airborne, rip ordinary homes to shreds, and turn broken glass and other debris into lethal missiles. The biggest threat to living creatures (including humans) from tornadoes is from flying debris and from being tossed about in the wind.

• Tornadoes are classified according to the damage they cause. Through observational studies, T. Theodore Fujita created the following scale in the late 1960's to classify tornadoes. The scale correlates wind speeds with damage: F-0 is the weakest and F-5 the strongest.

Fujita Scale

First Tornado forecast

• Tornado Basics• What is a tornado?• A tornado is a narrow, violently rotating column of air that extends from the base of a

thunderstorm to the ground. Because wind is invisible, you can't always see a tornado. A visible sign of the tornado, a condensation funnel made up of water droplets, sometimes forms and may or may not touch the ground during the tornado lifecycle. Dust and debris in the rotating column also make a tornado visible and confirm its presence.

• What is known? • Tornadoes are the most violent of all atmospheric storms. • There are two types of tornadoes: those that come from a supercell thunderstorm, and

those that do not.• Tornadoes that form from a supercell thunderstorm are the most common, and often the

most dangerous. A supercell is a long-lived (greater than 1 hour) and highly organized storm feeding off an updraft (a rising current of air) that is tilted and rotating. This rotating updraft - as large as 10 miles in diameter and up to 50,000 feet tall - can be present as much as 20 to 60 minutes before a tornado forms. Scientists call this rotation a mesocyclone when it is detected by Doppler radar. The tornado is a very small extension of this larger rotation. Most large and violent tornadoes come from supercells.

• CONDENSATION FUNNEL - A funnel-shaped cloud associated with rotation and consisting of condensed water droplets (as opposed to smoke, dust, debris, etc.)

• SUPERCELL - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes , extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm. Storms exhibiting these characteristics often are called classic supercells; however HP storms and LP storms also are supercell varieties.

• UPDRAFT - A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.

• MESOCYCLONE - A storm-scale region of rotation, typically around 2-6 miles in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm). The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it.

Tornado

• Non-supercell tornadoes are circulations that form without a rotating updraft. One non-supercell tornado is the gustnado, a whirl of dust or debris at or near the ground with no condensation funnel, which forms along the gust front of a storm. Another non-supercell tornado is a landspout. A landspout is a tornado with a narrow, rope-like condensation funnel that forms when the thunderstorm cloud is still growing and there is no rotating updraft - the spinning motion originates near the ground. Waterspouts are similar to landspouts, except they occur over water. Damage from these types of tornadoes tends to be F2 or less.

• GUSTNADO - [Slang], gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones ); they are more likely to be associated visually with a shelf cloud than with a wall cloud.

• LANDSPOUT - [Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

• How do tornadoes form?• Scientists have learned a lot about tornadogenesis from theoretical studies,

field projects and physical models – but tornadogenesis – the way tornadoes form – has vexed researchers for decades.

• SUPERCELL TORNADOGENESIS A rotating updraft is a key to the development of a supercell, and eventually a tornado. There are many ideas about how this rotation begins. One way a column of air can begin to rotate is from wind shear – when winds at two different levels above the ground blow at different speeds or in different directions.

• An example of wind shear that can eventually create a tornado is when winds at ground level, often slowed down by friction with the earth's surface, come from the southwest at 5 mph. But higher up, at 5000 feet above the same location, the winds are blowing from the southeast at 25 mph! An invisible "tube" of air begins to rotate horizontally. Rising air within the thunderstorm tilts the rotating air from horizontal to vertical – now the area of rotation extends through much of the storm.

• Once the updraft is rotating and being fed by warm, moist air flowing in at ground level, a tornado can form. There are many ideas about this too.

• SHEAR - Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

• More Ideas About Supercell Tornadogenesis • Scientists are actively trying to prove or disprove a number of tornadogenesis hypotheses. It is

complicated science that draws on information from observations, theory, and mathematical and physical models. These are some basic ideas (basic to a scientist, that is) about the processes that might cause tornadoes to form from supercells:

• Dynamic Pipe Effect Development of a tornado begins when horizontal winds coming together from different directions are strong 3-4km above the ground and weak or absent near the ground. The result is that rotation first increases aloft. The young tornado will build downward by something called the dynamic pipe effect (DPE): air can not enter through the sides of this belt of rotating air, but can pass through its ends like a pipe. The partial vacuum created within the pipe draws weakly rotating air up into the pipe's lower end. This causes the air to spin faster and eventually become part of the pipe. New sections on the rotating pipe form at lower and lower altitudes through this same process until the pipe (tornado) is in contact with the ground.

• Another type of tornado development occurs when converging horizontal winds have the same windspeed through all levels in the thunderstorm. Rotation increases all at once and spans several kilometers along the vertical pipe. The tornado, in this case, forms nearly independent from how high it is above the ground, and develops very rapidly from the ground, up.

• Rear Flank Downdraft (RFD) The Rear Flank Downdraft (RFD) may play a role in tornadogenesis. The RFD is a region of dry air pushed towards the ground by the thunderstorm on the backside of, and wrapping around a rotating updraft, and eventually the tornado. It is often visible as a clear slot wrapping around a wall cloud (a persistent lowering from a rain-free base of the main thunderstorm). On radar, the presence of a hook or a small feature hanging from the thunderstorm may indicate the presence of an RFD. Scientists think the RFD may play a significant role in determining the development of a tornado, how long it lasts, and how intense it is. Some scientists think that the RFD, by wrapping around the low-level rotating updraft, forces the rotation to concentrate and lower to the ground.

• We still have many questions. Scientists know from field studies that perhaps as few as 20 percent of all supercell thunderstorms actually produce tornadoes. Why does one supercell thunderstorm produce a tornado and another nearby storm does not? What are some of the causes of winds moving at different speeds or directions that create the rotation? What are other circulation sources for tornadoes? What is the role of downdrafts (a sinking current of air) and the distribution of temperature and moisture (both horizontally and vertically) in tornadogenesis? Scientists hope to learn more about the processes that create wind shear and rotation, tilt it vertically, and concentrate the rotation into a tornado when they participate in a large field experiment in 2007.

• And, since not all tornadoes come from supercells, what about tornadogenesis in non-supercell thunderstorms?

• NON-SUPERCELL TORNADOGENESIS A non-supercell tornado does not form from organized storm-scale rotation. These tornadoes form from a vertically spinning parcel of air already occurring near the ground, about 1-10 km in diameter, that is caused by wind shear from a warm, cold, or sea breeze front, or a dryline. When an updraft moves over the spinning, and stretches it, a tornado can form. Eastern Colorado experiences non-supercell tornadoes when cool air rushes down off the Rocky Mountains and collides with the hot dry air of the plains. Since these types of tornadoes happen mostly over scarcely populated land, scientists are not sure how strong they are, but they tend to be small. Waterspouts and gustnadoes are formed in this way too.

• DOWNDRAFT - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft.

• DRY LINE - A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west). The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day. A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime. These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward.

• Tornado Climatology • Where and when do tornadoes occur? • Tornadoes occur in many parts of the world, including Australia,

Europe, Africa, Asia, and South America. Even New Zealand reports about 20 tornadoes each year.

• Two of the highest concentrations of tornadoes outside the U.S. are Argentina and Bangladesh. Both have similar topography with mountains helping catch low-level moisture from over Brazil (Argentina) or from the Indian Ocean (Bangladesh).

• About 1,000 tornadoes hit the U.S. yearly. Since official tornado records only date back to 1950, we do not know the actual average number of tornadoes that occur each year. Plus, tornado spotting and reporting methods have changed a lot over the last several decades.

Clip on tornadoes

A recent NSSL study, using data from 1921 to 1995, estimated the daily climatological probability of an F2 or greater tornado occurring near any location in the U.S. For this work developing highly accurate and accessible estimates of the long-term threat from thunderstorms, winds, and large

hail as well as tornadoes, an NSSL scientist was awarded a Department of Commerce Silver Medal.

Probability of Any Tornado:The map shows the average number of days per year any tornado, no matter how strong or weak, might occur within 25 miles of a point. The highest numbers indicate where at least one tornado might occur somewhere within 25 miles as often as on 1.5 days per year.

Significant Tornado (F2 or greater):Now we're looking at days per century. In other words, central Oklahomans can expect an

F2 or greater tornado within 25 miles about every 3 years.

Violent Tornado (F4 or greater):Now the scale is days per millennium, meaning that southcentral Oklahoma may have a violent

tornado within 25 miles about once every 20 years.

Annual Cycle:Residents of Norman, OK experience a distinct tornado season, beginning late February and peaking late May. Even though we are in the heart of tornado alley and can expect one- to one-and one-half

tornado days per year, our chances on any particular day peak at only about two percent.

• Tornado season usually refers to the time of year where the U.S. sees the most tornadoes. The peak “tornado season” for the southern plains -- often referred to as Tornado Alley -- is during May into early June. On the Gulf coast, it is earlier during the spring. In the northern plains and upper Midwest, tornado season is in June or July. But, remember, tornadoes can happen at any time of year. Tornadoes can also happen at any time of day, but most tornadoes occur between 4-9 p.m.

Why Tornado Alley?

• Tornado Alley is a nickname for an area that consistently experiences a high frequency of tornadoes each year. The area that has the most strong and violent tornadoes includes eastern SD, NE, KS, OK. Northern TX, and eastern Colorado. The relatively flat land in the Great Plains allows cold dry polar air from Canada to meet warm moist tropical air from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet, along a phenomenon known as a "dryline."

• The dryline is a boundary separating hot, dry air to the west from warm, moist air to the east. You can see it on a weather map by looking for sharp changes in dew point temperatures. Between adjacent weather stations the differences in dew point can vary by as much as 40 degrees or more. The dryline is usually found along the western high plains. Air moving down the eastern slopes of the Rockies warms and dries as it sinks onto the plains, creating a hot, dry, cloud-free zone. During the day, it moves eastward mixing up the warm moist air ahead of it. If there is enough moisture and instability in the warm air, severe storms can form - because the dryline is the "push" the air needs to start moving up! During the evening, the dryline "retreats" and drifts back to the west. The next day the cycle can start all over again, until a larger weather system pushes through and washes it away.

• Tornadoes kill an average of 60 people per year, mostly from flying or falling debris.

• The Tri-State Tornado of March 18, 1925 was the deadliest tornado in history, killing 695 people. It is also the longest tornado track ever known - 219 miles - across parts of Missouri, Illinois and Indiana.

• Codell, KS was struck by a tornado on May 20 three years in a row: 1916, 1917, and 1918.

• Understanding the threat posed by tornadoes in the United States - particularly the threat of strong and violent tornadoes - is valuable knowledge to everyone, but especially to weather forecasters and emergency management people. Knowledge about long-term patterns helps us be better prepared for natural disasters and could also help scientists detect shifting patterns in severe weather events caused by climate change.

Floods• Flood Basics • What is flooding?• Flooding is an overflowing of water onto land that is normally dry. It can

happen during heavy rains, when ocean waves come onshore, when snow melts too fast, or when dams or levees break. Flooding may happen with only a few inches of water, or it may cover a house to the rooftop. The most dangerous flood event, the flash flood, happens quickly with little or no warning; other flooding events occur over a long period and may last days, weeks, or longer.

• What is a river flood? • A river flood occurs when water levels rise in a river due to excessive rain

from tropical systems making landfall, persistent thunderstorms over the same area for extended periods of time, combined rainfall and snowmelt, or an ice jam.

• What is coastal flooding? • Coastal flooding occurs when a hurricane, tropical storm, or tropical depression produces a deadly

storm surge that overwhelms coastal areas as it makes landfall. Storm surge is water pushed on shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the average water level 15 feet or more. The greatest natural disaster in the United States, in terms of loss of life, was caused by a storm surge and associated coastal flooding from the great Galveston, Texas, hurricane of 1900. At least 8,000 people lost their lives.

• What is inland flooding? • When tropical cyclones move inland, they are typically accompanied by torrential rain. If the decaying

storm moves slowly over land, it can produce rainfall amounts of 20 to 40 inches over several days. Widespread flash flooding and river flooding can result.

• What is a flash flood? • A flash flood is a rapid rise of water along a stream or low-lying urban area. Flash flooding occurs

within six hours of a significant rain event and is usually caused by intense storms that produce heavy rainfall in a short amount of time. Excessive rainfall that causes rivers and streams to swell rapidly and overflow their banks is frequently associated with hurricanes and tropical storms, large clusters of thunderstorms, supercells, or squall lines. Other types of flash floods can occur from dam or levee failures, or a sudden release of water held by an ice jam. Heavy rainfall in the mountains can cause downstream canyon flooding.

• Why is a flash flood so dangerous? • Flash floods can occur with little or no warning. Flash flood damage and most fatalities tend to occur in

areas immediately adjacent to a stream or arroyo. Flash floods are very strong -- they can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Flash flood-producing rains falling on steep terrain can weaken soil and trigger catastrophic mud slides that damage homes, roads, and property.

• What areas are at risk from flash floods? • Densely populated areas are at a high risk for flash floods. The

construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. This runoff increases the flash flood potential.

• Sometimes, streams through cities and towns are routed underground into storm drains. During heavy rain, the storm drains can become overwhelmed and flood roads and buildings. Low spots, such as underpasses, underground parking garages, and basements can become death traps.

• Areas near rivers are at risk from flash floods. Embankments, known as levees, are often built along rivers and are used to prevent high water from flooding bordering land. In 1993, many levees failed along the Mississippi River, resulting in devastating flash floods. The city of New Orleans experienced massive devastating flooding days after Hurricane Katrina came onshore in 2005 due to the failure of levees designed to protect the city.

Hail• Hail Basics • What is hail?• Hail is a form of precipitation that occurs when updrafts in thunderstorms

carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice.

• How does hail form? • There are two ideas about hail formation. In the past, the prevailing thought

was that hailstones grow by colliding with supercooled water drops. Supercooled water will freeze on contact with ice crystals, frozen rain drops, dust or some other nuclei. Thunderstorms that have a strong updraft keep lifting the hailstones up to the top of the cloud where they encounter more supercooled water and continue to grow. The hail falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow.

SUPERCOOLED WATER - Liquid water that is below 0°C, or water that stays in liquid form if undisturbed even though it has been cooled to a temperature below its normal freezing

point. The smaller and purer the water droplets, the more likely they can become supercooled.

• Recent studies suggest that supercooled water may accumulate on frozen particles near the back-side of the storm as they are pushed forward across and above the updraft by the prevailing winds near the top of the storm. Eventually, the hailstones encounter downdraft air and fall to the ground.

• Hailstones grow two ways: by wet growth or dry growth processes. In wet growth, a tiny piece of ice is in an area where the air temperature is below freezing, but not super cold. When the tiny piece of ice collides with a supercooled drop, the water does not freeze on the ice immediately. 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.

• Dry growth hailstones grow when the air temperature is well below freezing and the water droplet freezes immediately as it collides with the ice particle. The air bubbles are "frozen" in place, leaving cloudy ice.

• Hailstones can have layers like an onion if they travel up and down in an updraft, or they can have few or no layers if they are "balanced" in an updraft. One can tell how many times a hailstone traveled to the top of the storm by counting the layers. Hailstones can begin to melt and then re-freeze together - forming large and very irregularly shaped hail.

Hail can cause significant damage

• What is the difference between hail, sleet, and graupel?• The different ways precipitation is formed determines what

type of precipitation it becomes. Hail is larger than sleet, and forms only in thunderstorms. Hail formation requires air moving up (thunderstorm updraft) that keep the pieces of ice from falling. Drops of supercooled water hit the ice and freeze on it, causing it to grow. When the hailstone becomes too heavy for the updraft to keep it aloft, ot it encounters downdraft air, it falls. Sleet forms from raindrops that freeze on their way down through a cloud. Snow forms mainly when water vapor turns to ice without going through the liquid stage. There is no thunderstorm updraft involved in either of these processes.

Graupel

graupel—Heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft hail except for the size convention that hail must have a diameter greater than 5 mm. Sometimes

distinguished by shape into conical, hexagonal, and lump (irregular) graupel.

• How does hail fall to the ground?• Hail falls when it becomes heavy enough to overcome the strength of the updraft

and is pulled by gravity towards the earth. How it falls is dependent on what is going on inside the thunderstorm. Hailstones bump into other raindrops and other hailstones inside the thunderstorm, and this bumping slows down their fall. Drag and friction also slow their fall, so it is a complicated question! If the winds are strong enough, they can even blow hail so that it falls at an angle. This would explain why the screens on one side of a house can be shredded by hail and the rest are unharmed!

• How fast does hail fall? • We really only have estimates about the speed hail falls. One estimate is that a 1cm

hailstone falls at 9 m/s, and an 8cm stone, weighing .7kg falls at 48 m/s (171 km/h). However, the hailstone is not likely to reach terminal velocity due to friction, collisions with other hailstones or raindrops, wind, the viscosity of the wind, and melting. Also, the formula to calculate terminal velocity is based on the assumption that you are dealing with a perfect sphere. Hail is generally not a perfect sphere!

Lightning• Lightning Basics• What is lightning?• Lightning is a gigantic electrostatic discharge (the same kind of electricity that can shock you when

you touch a doorknob) between the cloud and the ground, other clouds, or within a cloud. Scientists do not understand yet exactly how it works or how it interacts with the upper atmosphere or the earth 's electromagnetic field.

• Lightning is one of the oldest observed natural phenomena on earth. It has been seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms.

• What causes lightning? • The creation of lightning is a complicated process. We generally know what conditions are needed

to produce lightning, but there is still debate about exactly how lightning forms.The exact way a cloud builds up the electrical charges that lead to lightning is not completely understood. Precipitation and convection theories both attempt to explain the electrical structure within clouds. Precipitation theorists suppose that different size raindrops, hail, and graupel get their positive or negative charge as they collide, with heavier particles carrying negative charge to the cloud bottom. Convection theorists believe that updrafts transport positive charges near the ground upward through the cloud while downdrafts carry negative charges downward

• Thunderstorms have very turbulent environments - strong updrafts and downdrafts occur often and close together. The updrafts carry small liquid water droplets from the lower regions of the storm to heights between 35,000 and 70,000 feet - miles above the freezing level. At the same time, downdrafts are transporting hail and ice from the frozen upper parts of the storm. When these particles collide, the water droplets freeze and release heat. This heat keeps the surface of the hail and ice slightly warmer than its surrounding environment, and a soft hail, or graupel forms.

• When this graupel collides with additional water droplets and ice particles, a key process occurs involving electrical charge: negatively charged electrons are sheared off the rising particles and collect on the falling particles. The result is a storm cloud that is negatively charged at its base, and positively charged at the top.

• Opposite charges attract one another. As the positive and negative areas grow more distinct within the cloud, an electric field is created between the oppositely-charged thunderstorm base and its top. The farther apart these regions are, the stronger the field and the stronger the attraction between the charges. But we cannot forget that the atmosphere is a very good insulator that inhibits electric flow. So, a HUGE amount of charge has to build up before the strength of the electric field overpowers the atmosphere's insulating properties. A current of electricity forces a path through the air until it encounters something that makes a good connection. The current is discharged as a stroke of lightning.

• While all this is happening inside the storm, beneath the storm, positive charge begins to pool within the surface of the earth. This positive charge will shadow the storm wherever it goes, and is responsible for cloud-to-ground lightning. However, the electric field within the storm is much stronger than the one between the storm base and the earth 's surface, so about 75-80% of lightning occurs within the storm cloud.

• ELECTRICAL CHARGE - A fundamental property of matter. Protons and the nuclei of atoms have a positive charge; electrons have a negative charge; neutrons have no charge. Normally, each atom has as many protons as it has electrons and thus has no net electrical charge; in other words, it is neutral. Charged substances have an imbalance of positive and negative charges, a net charge that exerts a force on other charged substances. Charges that are both positive or both negative repel each other; charges that are different attract.

• ELECTRIC FIELD - A field or force that exists in the space between two different potentials, such as between negatively and positively charged regions of a thunderstorm

Conceptual Model of Lightning Charge Distribution Within a Thunderstorm For many years, scientists have thought thunderstorms contain three charges, called a tripole. A new conceptual model of electrical charge distribution inside deep convection (thunderstorms), developed by NSSL and university scientists, could change that thinking. In the main updraft (in

and above the red arrow), there are four main charge regions. In the convective region but outside the outdraft (in and above the blue arrow), there are more than four charge regions.

• Lightning types • GROUND FLASHES

There are two categories of ground flashes: natural (those that occur because of normal electrification in the environment), and artificially initiated or triggered. Artificially initiated lightning includes strikes to very tall structures, airplanes, rockets and towers on mountains. Triggered lightning goes from ground to cloud, while "natural" lightning is cloud to ground.

• Terms used to describe ground flashes include forked lightning, which shows branching to the ground from a nearly vertical channel; ribbon lightning, when the horizontal displacement of the channel by the wind appears as a series of ribbons; and bead lightning, when the decaying channel of a ground flash will sometimes break into a series of bright and dark spots. Ball lightning is a luminous sphere whose physics is not well understood.

Natural

Triggered

• Cloud-to-ground lightning (CG's) A channel of negative charge, called a step leader, will zigzag downward in roughly 50-yard segments in a forked pattern. This step leader is invisible to the human eye, and shoots to the ground in less time than it takes to blink. As it nears the ground, the negatively charged step leader is attracted to a channel of positive charge reaching up, a streamer, normally through something tall, such as a tree, house, or telephone pole. When the oppositely-charged leader and streamer connect, a powerful electrical current begins flowing. A return stroke of bright luminosity travels about 60,000 miles per second back towards the cloud. A flash consists of one or perhaps as many as 20 return strokes. We see lightning flicker when the process rapidly repeats itself several times along the same path. The actual diameter of a lightning channel is one-to two inches.

• STEP LEADER - A path of ionized air which extends downward from a thunderstorm during the initial stages of a lightning strike. Multiple branches, or steps, travel downward until the final step leader reaches the ground, a tall object on the ground, or a positive streamer extending upward from a ground object. The lightning strike begins when a large negative electric current flows along the path defined by the step leaders from the thundercloud to the ground.

• STREAMER - A part of a lightning bolt that rises from the ground before a lightning strike. It is a column of ionized air formed by the flow of electrons down into the ground target. The positive streamer extends vertically upward until it meets a descending step leader, at which point the air's resistance is overcome and an electric current begins to flow along the path, resulting in a lightning strike.

• A typical cloud-to-ground flash is a negative stepped leader that travels downward through the cloud, followed by an upward traveling return stroke. The net effect of this flash is to lower negative charge from the cloud to the ground. Less common, a downward traveling positive leader followed by an upward return stroke will lower positive charge to earth. These positive ground flashes now appear to be linked to certain severe storms and are the focus of intense research by scientists.

In-cloud lightning

Spider lightning

• What causes thunder?• Lightning causes thunder. Thunder is the sound caused by rapidly

expanding gases along a channel of lightning discharge. Energy from lightning heats the air to around 18,000 degrees Fahrenheit. This causes a rapid expansion of the air, creating a sound wave heard as thunder. An initial tearing sound is usually caused by the stepped leader, and the sharp click or crack heard at a very close range, just before the main crash of thunder, is caused by the ground streamer.

• Thunder is rarely heard at points farther than 15 miles from the lightning discharge, but occasionally can be heard up to 25 miles away. At these distances, thunder is heard as more of a low rumbling sound because the higher frequency pitches are more easily absorbed by the surrounding environment, and the sound waves set off by the lightning discharge have different arrival times.

Winter• Winter Weather Basics• How do winter storms form? • Just like any other storm at other times of the year, just the right combination

of ingredients is necessary for a winter storm to develop.• Three basic ingredients are necessary to make a winter storm. • Cold air – below freezing temperatures in the clouds and near the ground are

necessary to make snow and/or ice. • Lift – something to raise the moist air to form the clouds and cause

precipitation. An example of lift is warm air colliding with cold air and being forced to rise over the cold dome. The boundary between the warm and cold air masses is called a front. Another example of lift is air flowing up a mountainside.

• Moisture – to form clouds and precipitation. Air blowing across a body of water, such as a large lake or the ocean, is an excellent source of moisture.

• Snow – Most precipitation that forms in wintertime clouds starts out as snow because the top layer of the storm is usually cold enough to create snowflakes. Snowflakes are just collections of ice crystals that cling to each other as they fall toward the ground. Precipitation continues to fall as snow when the temperature remains at or below 0 degrees Celsius from the cloud base to the ground.

• Snow Flurries – Light snow falling for short durations. No accumulation or light dusting is all that is expected.

• Snow Showers – Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

• Snow Squalls – Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant. Snow squalls are best known in the Great Lakes Region.

• Blowing Snow – Wind-driven snow that reduces visibility and causes significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind.

• Blizzard – Winds over 35mph with snow and blowing snow, reducing visibility to 1/4 mile or less for at least 3 hours.

• Sleet occurs when snowflakes only partially melt when they fall through a shallow layer of warm air. These slushy drops refreeze as they next fall through a deep layer of freezing air above the surface, and eventually reach the ground as frozen rain drops that bounce on impact.

• Freezing Rain occurs when snowflakes descend into a warmer layer of air and melt completely. When these liquid water drops fall through another thin layer of freezing air just above the surface, they don't have enough time to refreeze before reaching the ground. Because they are "supercooled," they instantly refreeze upon contact with anything that that is at or below O degrees C, creating a glaze of ice on the ground, trees, power lines, or other objects. A significant accumulation of freezing rain lasting several hours or more is called an ice storm

• Freezing Rain occurs when snowflakes descend into a warmer layer of air and melt completely. When these liquid water drops fall through another thin layer of freezing air just above the surface, they don't have enough time to refreeze before reaching the ground. Because they are "supercooled," they instantly refreeze upon contact with anything that that is at or below O degrees C, creating a glaze of ice on the ground, trees, power lines, or other objects. A significant accumulation of freezing rain lasting several hours or more is called an ice storm

Crystals

• Extreme cold often accompanies a winter storm or is left in its wake. Prolonged exposure to the cold can cause frostbite or hypothermia and become life threatening. Infants and elderly people are most susceptible. What constitutes extreme cold and its effect varies across different areas of the United States. In areas unaccustomed to winter weather, near freezing temperatures are considered "extreme cold." Freezing temperatures can cause severe damage to citrus fruit crops and other vegetation. Pipes may freeze and burst in homes that are poorly insulated or without heat. In the north, below zero temperatures may be considered as extreme cold. Long cold spells can cause rivers to freeze, disrupting shipping. Ice dams may form and lead to flooding.

• Ice Storms • Heavy accumulations of ice can bring down trees, electrical wires, telephone poles and lines, and

communication towers. Communications and power can be disrupted for days while utility companies work to repair the extensive damage. Even small accumulations of ice may cause extreme hazards to motorists and pedestrians. Bridges and overpasses are particularly dangerous because they freeze before other surfaces.

• Heavy Snow Storms • Heavy snow can immobilize a region and paralyze a city, stranding commuters, stopping the flow of

supplies, and disrupting emergency and medical services. Accumulations of snow can collapse buildings and knock down trees and power lines. In rural areas, homes and farms may be isolated for days, and unprotected livestock may be lost. In the mountains, heavy snow can lead to avalanches. The cost of snow removal, repairing damages, and loss of business can have large economic impacts on cities and towns.

• Winter storms are considered deceptive killers because most deaths are indirectly related to the storm. People can die in traffic accidents on icy roads, heart attacks while shoveling snow, or of hypothermia from prolonged exposure to cold. Wind Chill is not the actual temperature but rather how wind and cold feel on exposed skin. As the wind increases, heat is carried away from the body at an accelerated rate, driving down body temperature. Animals are also affected by wind chill; however, cars, plants and other objects are not.

• Frostbite – Frostbite is damage to body tissue caused by extreme cold. A wind chill of -20 degrees F will cause frostbite in just 30 minutes. Frostbite causes a loss of feeling and a white or pale appearance in extremities, such as fingers, toes, ear lobes, or the tip of the nose. If symptoms are detected, get medical help immediately. If you must wait for help, slowly re-warm affected areas. However, if the person is also showing signs of hypothermia, warm the body core before the extremities.

• Hypothermia (low body temperature) – Warning signs of hypothermia include uncontrollable shivering, memory loss, disorientation, incoherence, slurred speech, drowsiness, and apparent exhaustion. Take the person's temperature, and if it is below 95 degrees F, immediately seek medical care. If medical care is not available, begin warming the person slowly. Warm the body core first, and use your own body heat to help, if necessary. Get the person into dry clothing and wrap them in a warm blanket covering the head and neck. Do not give the person alcohol, drugs, coffee, or any hot beverage or food; warm broth is better. Do not warm extremities (arms and legs) first! This drives the cold blood toward the heart and can lead to heart failure.

Winds• Types of damaging winds• Straight-line winds – a term used to define any thunderstorm wind that

is not associated with rotation, and is used mainly to differentiate from tornadic winds.

• Downdrafts – A small-scale column of air that rapidly sinks toward the ground. A downburst is a result of a strong downdraft.

• Downbursts – A strong downdraft with horizontal dimensions larger than 4 km (2.5 mi) resulting in an outward burst or damaging winds on or near the ground. (Imagine the way water comes out of a faucet and hits the bottom of the sink.) Downburst winds may begin as a microburst and spread out over a wider area, sometimes producing damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.

• Microbursts – A small concentrated downburst that produces an outward burst of damaging winds at the surface. Microbursts are generally small (less than 4km across) and short-lived, lasting only 5-10 minutes, with maximum windspeeds up to 168 mph. There are two kinds of microbursts: wet and dry. A wet microburst is accompanied by heavy precipitation at the surface. Dry microbursts, common in places like the high plains and the intermountain west, occur with little or no precipitation reaching the ground.

• Gust front – A gust front is the leading edge of rain-cooled air that clashes with warmer thunderstorm inflow. Gust fronts are characterized by a wind shift, temperature drop, and gusty winds out ahead of a thunderstorm. Sometimes the winds push up air above them, forming a shelf cloud or detached roll cloud.

Microburst

Gustfront

• Derecho – A derecho is a widespread thunderstorm wind event caused when new thunderstorms form along the leading edge of an outflow boundary (a surface boundary formed by the horizontal spreading of thunderstorm-cooled air). The thunderstorms feed on this boundary and continue to reproduce themselves. Derechos typically occur in the summer months when complexes of thunderstorms form over the plains and northern plains states. Usually these thunderstorms produce heavy rain and severe wind reports as they rumble across several states during the night. The word "derecho" is of Spanish origin and means "straight ahead". They are particularly dangerous because the damaging winds can last a long time and can cover such a large area.

• Bow Echo – A radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Bow echoes can be over 300km in length, last for several hours, and produce extensive swaths of wind damage at the ground.

Doppler Radar

• The type of damaging winds most dangerous to aviation, especially during landing and take-off, is the type spawned by a microburst in an isolated rain shower or thunderstorm. Downbursts or microbursts are mainly known for their ability to produce wind shears which can slow airspeed and cause aircraft to lose altitude at a very critical time for flight near the ground. A plane will encounter strong headwinds followed by strong tailwinds as it enters and flies through a microburst. Only 27 people survived the Delta Flight 191 crash caused by a microburst that occurred in 1986 in Dallas-Fort Worth. However, great strides have been made in understanding and avoiding the risk from low altitude wind shear. Part of this success has been due to the progress made in detecting and distributing real-time information regarding these hazards.

http://wxmaps.org/

• A. Why Do They Exist? • Infrared satellite image of North America • Our spinning planet is covered with moving air. Some of this air is over the ocean and some is

over the land. The ground gets warm faster than water, but also gets cold faster than water. That is why it is normally colder over land in the winter, and warmer over land in the summer. When the ocean is warmed by the sun, the water absorbs heat and warms the air above it.

• At the same time, the land can become cold and cool the air above it. Because of heating and cooling, air can move up and down. When a pocket of air gets too warm, it rises until it is the same temperature as the surrounding air. Just the opposite happens when a pocket of air gets cooled, it sinks. The combination of warm and cold areas, and rising and sinking air results in what we call weather systems.

• B. How Do They Move? • Areas of high pressure, called Highs, have air that sinks. This air tends to be cold and produces

clear skies and fair weather. Areas of low pressure, called Lows, have warm air that rises. They can cause clouds to form, rain to fall, and storms to occur. Weather systems in the United States and within the mid-latitudes, between 30° N and 60° N and between 30° S and 60° S, usually move from West to East. Weather systems in other latitudes move from East to West

Hurricanes

Hurricane Strike