Meteorology 2014- Thunderstorms

Cyclone

"Cyclone" usually refers to an extra-tropical (non-tropical) cyclone, which is a low pressure area that forms near the boundary between warm and cool air masses. (It alternatively refers to a hurricane that forms in the Indian Ocean). Cyclones are huge revolving storms caused by winds blowing around a central area of low atmospheric pressure. In the northern hemisphere, cyclones are called hurricanes or typhoons and their winds blow in an anti-clockwise circle. In the southern hemisphere, these tropical storms are known as cyclones, whose winds blow in a clockwise circle.

Cold Front

A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour.

Occluded Fronts

A developing cyclone typically has a preceding warm front (the leading edge of a warm moist air mass) and a faster moving cold front (the leading edge of a colder drier air mass wrapping around the storm). North of the warm front is a mass of cooler air that was in place before the storm even entered the region.

Downdraft

A downward current or draft of air, esp. one down a chimney into a room.

Dry Line

A dry line is a boundary that separates a moist air mass from a dry air mass. Also called a "Dew Point Front", sharp changes in dew point temperature can be observed across a dry line. Dry lines are most commonly found just east of the Rocky

Fronts

A front is defined as the transition zone between two air masses of different density. Fronts extend not only in the horizontal direction, but in the vertical as well. Therefore, when referring to the frontal surface (or frontal zone), we referring to both the horizontal and vertical components of the front.

Mesocyclones

A mesocyclone is a large rotating vortex of air. They rotate in the same direction as a low air pressure system would in the same hemisphere as the mesocyclone. They are formed when wind shear starts a portion of air in the lower atmospher spinning in a tube like formation around a horizontal axis. The updraft found in a supercell can cause the "tube" to angle upwards untill the air is rotating around a vertical axis.

Mesoscale Convective Complexes

A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. They are long-lived, nocturnal in formation and commonly contain heavy rainfall, wind, hail, lightning and possibly tornadoes.

Squall Lines

A squall line is a line of severe thunderstorms that can form along and/or ahead of a cold front. It contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts. Squall lines typically form in unstable atmospheric environments where low-level air can rise unaided after being initially lifted (e.g., by a front) to the point where condensation of water vapor occurs. Heat is released during condensation, resulting in the rising air becoming lighter than nearby air at the same height. This leads to an increase in the speed of the rising air which sometimes reaches speeds above 30 mph. In models this initial lifting is specified through an idealization of the flow associated with the front or other lifting mechanism or through the use of observational flow information. The gust front is located along the line where these winds meet -- which extends from the surface well up into the the storm.

Causes of Lightning

A storm is only classified a a thunderstorm when there is lightning. Thus, its important to discuss the causes of lightning. Some cloud physicists believe that charge separation occurs during the formation of ice pellets. Experimentation shows that as droplets begin to freeze, positively charged ions are concentrated in the colder regions of the droplets, whereas negatively charged ions are concentrated in the warmer regions. Thus, as the droplets freeze from the outside in, they develop a positively charged ice shell and a negatively charged interior. As the interior begins to freeze, it expands and shatters the outside shell. The small positively charged ice fragments are carried upward by turbulence, and the relatively heavy droplets eventually carry their negative charge toward the cloud base. As a result, the upper part of the cloud is left with a positive charge, and the lower portion of the cloud maintains an overall negative charge with small positively charged pockets. As the cloud moves, the negatively charged cloud base alters the charge at the surface directly below by repelling negatively charged particles. Thus, the surface beneath the cloud acquires a net positive charge. These charge differences build to millions and even hundreds of millions of bolts before a lightning stroke acts to discharge the negative region of the cloud by striking the positive area of the ground below, or, more frequently, the positively charged portion of that cloud, or a nearby cloud.

Supercells

A supercell is a large rotating thunderstorm with a mesocyclone. They can last longer than normal thunderstorms and can produce tornadoes and baseball size hail.

Warm Fronts

A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and more humid than it was before.

Effects of Thunderstorms

According to the National Weather Service, only 10% of people that are struck by lightning are killed, leaving the remaining 90% with various injuries If you get hit by lightning, it usually damages the nervous system. When the brain is affected, the person may have difficulty with short-term memory, coding new information and accessing old information, multitasking, and being easily distracted. Lightning victims may also suffer personality changes because of frontal lobe damage and become irritable and easy to anger. In addition, some survivors complain of becoming more easily exhausted than before being struck.

Air Masses

An air mass is a large body of air that has similar temperature and moisture properties throughout. The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below. Maritime tropical air masses (mT), for example, develop over the subtropical oceans and transport heat and moisture northward into the U.S.. In contrast, continental polar air masses (cP), which originate over the northern plains of Canada, transport colder and drier air southward.

Anticyclone

An anti-cyclone -- also known as a high pressure area -- is a large atmospheric circulation system with the wind flowing clockwise around it in the Northern Hemisphere, and counter-clockwise in the Southern Hemisphere.

Updrafts

An upward current or draft of air

Freezing Rain

Freezing rain can be extremely dangerous. It occurs when there is a layer of warm air aloft freezing cold air near the ground. Rain will fall, then freeze as it nears the ground. The result is a layer of ice instead of snow. Even for places that are acoustom to snow storms, as little as 1 cm can completely paralyze a city. Dangers including driving, telephone and electrical wire damage, and entire crops can be destroyed.

Heat lightning

Heat lightning appears to produce no thunder. In fact, it does, but it happends so far away that the observer does not hear it, because the sound dissipate through the air.

Formation of Tornadoes

If conditions are right, the rotation of winds within a mesocyclone allows a vortex to form underneath it, and a funnel cloud usually forms with this. It gains energy as it descends and it becomes a tornado once it touches down.

Lightning 30/30 Rule

Lightning 30/30 rule: If it takes less than 30 seconds to hear thunder after seeing the flash, lightning is near enough to pose a threat; after the storm ends, wait 30 minutes before resuming outdoor activities

Cloud lightning

Lightning does not always strike the ground. It can either occur between two seperate clouds, or within the same cloud, which is the most common. When it occurs with in the same cloud, it will usualy start in the lower portion of the anvil, and move downward.

The Dissipating Stage

Marking the end of the storm, the dissipating stage is dominated by downdrafts and entrainment. Without a supply of moisture from updrafts, the cloud soon evaporates. It should be noted that within a single air-mass thunderstorm there may be several individual cells—that is, zones of adjacent updrafts and downdrafts

Tornado Hazards

Much of the damage caused by a tornado can be related to the high winds, as this is the essence of a tornado. However, a lot of damage is also caused by the flying debris resulting from the destruction of some structures. Their impact can destroy other buildings more easily. Other hazards include downed power lines, broken gas lines and pumps, and fires.

Maturity of Tornadoes

Once the funnel cloud becomes a tornado, it enters its mature stage. This is where all the destruction comes in.

How Lightning Strikes

Pop quiz: Does lightning start from the cloud and move down, or does it start from the ground and move up? The answer: Neither. This is because a lightning strike is not a single brilliant bolt, but actually several strokes. First, there is a stream of electrons that moves downwards from the cloud. This is called the initial leader(or Step leader). As it nears the ground, electrons are pulled from the surrounding air, resulting in a ionized path from the cloud to ground. Then, electrons pour from this channel of charge. This is the main stroke and is what we think of "lightning".

Positive Lightning

Positive lightning occurs when there are little to no clouds. These lightning bolts originate from the top of a cloud, usualy the anvil, and travels horizontally for several miles before turning and moving downward to meet the initial leader.

Severe Thunderstorms

Severe Thunderstorms are capable of producing heavy downpours and flash flooding as well as strong, gusty straight-line winds, large hail, frequent lightning, and perhaps tornadoes. For a thunderstorm to be officially classified as severe by the National Weather Service, it must have winds in excess of 93 kilometers (58 miles) per hour or produce hailstones with diameters larger than 1.9 centimeters (0.75 inch) or generate a tornado. Of the estimated 100,000 thunderstorms that occur annually in the United States, about 10 percent (10,000 storms) reach severe status. Regular air-mass thunderstorms are localized, relatively short lived phenomena that dissipate after a brief, well-defined life cycle(above). The key factor for a severe thunderstorm is a strong vertical wind shear. That way the cold downdraft does not cut off the updrafts, which are the thunderstorm's "fuel".

Snow Storms

Snow is less dense than liquid by a factor of a approximately ten when in temperatures just under freezing. This means that 1 inch of rain would be about 10 inches of snow. This can make snow storms very problematic, especially in areas that are not used to getting heavy snow; however more than 6 inches of snow will be a problem anywhere. Some of the key dangers of snow storms include hypothermia, frostbite, car wrecks, or even avalanches if near or on a mountain. In order for a snow storm to be classified as a "blizzard" it must have the following characteristics: Visibility reduced to less than 1/4 mile Winds greater than 35 miles per hour Last for a long period of time such as three hours.

5 Types of Fronts

Stationary, Cold, Warm, Occluded, Dry Line.

Straight line winds

Straight-line wind is wind that comes out of a thunderstorm. If these winds meet or exceed 58 miles per hours then the storm is classified as severe by the National Weather Service. These winds are produced by the downward momentum in the downdraft region of a thunderstorm.

Supercell Thunderstorms

Supercells are the most powerful thunderstorms. By their definition, supercells are always severe. Supercells are responsible for a disproportionate amount of damage and casualties. The most significant difference arises from the presence of a rotating updraft, or mesocyclone. These features insure the longevity of a thunderstorm by allowing the flanking line to enhance the inflow into one main updraft, rather than helping create new updraft centers. The combination of rotation and longevity increase the chances for the development of strong or violent tornadoes

The Cumulus Stage

The Cumulus stage is dominated by rising currents of air (updrafts) and the formation of a towering cumulonimbus cloud. Falling precipitation within the cloud causes drag on the air and initiates a downdraft that is further aided by the influx of cool, dry air surrounding the cloud, a process termed entrainment. This stage then progresses to the mature stage

Geographical and Seasonal distribution

The United States are home to the largest amount of tornadoes. Most of them occur in a central region known as Tornado Alley, which contains the states of Texas, Oklahoma, Kansas, Nebraska, and the edges of other states, depending on the definition. However, tornadoes have been observed on every continent excluding Antarctica, and every state in the United States

Ball Lightning

The entire exsitance of ball lightning can be disputed because of it's lack of observation. Ball lightning has been spotted hundrends of times around the world, but very rarely by meteorologists. Observers say that ball lightning appears as a sphere, differing in size from between a few inches in diameter to several meters, and veries in color between red, orange, yellow, even green or white. It can appear after a large thunderstorm. It travels mostly horizontally, from about waist high to severl meters off the ground. Usually ball lightning comes with a bad smell. It can come in through open doors or windows, including closed screens, and sometimes chimneys. No ball lightning stays for more than a few seconds, and it moves at a brisk pace- several meters per second. Somtimes observers report that it will "bounce" between puddles. Because even the existence of ball lightning can't be proven, not very much is known about it other than its appearance. As of right now, no theories have been suggested that can explain the strange movement, appearance, and how it can produce a constant stream of light and energy. It is thought that UFO sightings after a large storm can actually be ball lightning. So the next time you see a ball of light high in the sky after a large storm, you may not be seeing a UFO, but instead a rare example of ball lightning.

The Mature Stage

The mature stage is marked by the downdraft leaving the base of the cloud and the release of precipitation. With gusty winds, lightning, heavy precipitation, and sometimes hail, the mature stage is the most active period of a thunderstorm

Maritime Tropical

The principal type of tropical air, produced over the tropical and subtropical seas; it is very warm and humid, and is frequently carried poleward on the western flanks of the subtropical highs. Frequently causes thunderstorms, from Gulf of Mexico.

Life Cycles of Tornadoes

Three stages usually categorize a tornado's life. Formation, maturity, and Dissipation. ( not the most common )

Thunderstorms

Thunderstorms can occur anywhere that warm, moist air meets cooler air. The warm air moves rapidly upward and condenses, which forms cumulonimbus clouds. Lightning, thunder, and rain are associated with thunderstorms, and severe storms may be accompanied by heavy rain, strong winds, hail, and on occasion, tornadoes.

Tornado Characteristics

Tornadoes are large clouds mostly characterized by extremely high winds. They are usually found in the most intense supercells and are caused by winds traveling in different directions, or wind shear. They usully look like large funnels touching down from the main cloud. Note that although most tornadoes look like funnel clouds, they do not necessarily need to have one, as long as the winds touch both the ground and the cloud. Consequently, a funnel cloud may occur but not a tornado if the funnel does not touch down.

The Fujita Scale

Two major scales measure tornadoes: the Fujita scale and the Enhanced Fujita Scale. Both measure from 0 to 5, but the characteristics of both are different.

Waterspouts

Waterspouts are similar vortexes that occur over water. They are usually less violent than regular tornadoes, although they can be rather powerful given a strong storm.

Stationary Front

When a warm or cold front stops moving, it becomes a stationary front. Once this boundary resumes its forward motion, it once again becomes a warm front or cold front. A stationary front is represented by alternating blue and red lines with blue triangles pointing towards the warmer air and red semicircles pointing towards the colder air.

Aviation conventions

When describing weather radar returns, pilots, dispatchers, and air traffic controllers will typically refer to three return levels:[18] level 1 corresponds to a green radar return, indicating usually light precipitation and little to no turbulence, leading to a possibility of reduced visibility. level 2 corresponds to a yellow radar return, indicating moderate precipitation, leading to the possibility of very low visibility, moderate turbulence and an uncomfortable ride for aircraft passengers. level 3 corresponds to a red radar return, indicating heavy precipitation, leading to the possibility of thunderstorms and severe turbulence and structural damage to the aircraft. Aircraft will try to avoid level 2 returns when possible, and will always avoid level 3 unless they are specially-designed research aircraft.

Dissapation of Tornadoes

When the mesocyclone loses its rotation and/or conditions are no longer right for a tornado, it begins to dissipate. The shape of the tornado can be altered into a rope-like form or some other shape, depending on the characteristics of the storm it is in.

Weather Stations

a facility, either on land or sea, with instruments and equipment for observing atmospheric conditions to provide information for weather forecasts and to study the weather and climate

mesoscale convective complexes

a grouping of storms that is defined by characteristics on infrared satellite imagery

Downbursts

a localized area of damaging winds caused by air rapidly flowing down and out of a thunderstorm.

Cumulonimbus Cloud

cumulonimbus: a dark cloud of great vertical extent charged with electricity; associated with thunderstorms.

The three stages of thunderstorms

cumulus, mature stage, and dissipating stage

microbursts

downdraft (sinking air) in a thunderstorm that is less than 2.5 miles in scale. Some microbursts can pose a threat to life and property, but all microbursts pose a significant threat to aviation. Although microbursts are not as widely recognized as tornadoes, they can cause comparable, and in some cases, worse damage than some tornadoes produce. In fact, wind speeds as high as 150 mph are possible in extreme microburst cases.

Leading Cause of Death from Thunderstorms

flashfloods

lightning direction finders

is a device that detects lightning produced by thunderstorms

derechos

is a widespread, long-lived, straight-line wind storm that is associated with a land-based, fast-moving band of severe thunderstorms.

downdrafts

is the vertical movement of air as a weather related phenomenon

How to Read Reflectivity Radars

magenta: 65 dBZ (extremely heavy precipitation, possible hail) red: 52 dBZ yellow: 36 dBZ green: 20 dBZ (light precipitation)

Semi-permanent highs and lows

persistent pressure systems that appear over an area during the year and affect how storms and weather systems enter the United States

gust fronts

the leading edge of cool air rushing down and out from a thunderstorm. There are two main reasons why the air flows out of some thunderstoms so rapidly