Thunderstorm Basics
What is a thunderstorm?
A thunderstorm is a rain shower during which you hear thunder. Since thunder comes from lightning, all thunderstorms have lightning. A thunderstorm is classified as "severe" when it contains one or more of the following: hail three-quarter inch
What is known?
An average thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. At any given moment, there are roughly 2,000 thunderstorms in progress around the world. It is estimated that there are 100,000 thunderstorms each year. About 10% of these reach severe levels.
How does a thunderstorm form?
Three basic ingredients are required for a thunderstorm to form: moisture, rising unstable air (air that keeps rising when given a nudge), and a lifting mechanism to provide the "nudge."
The sun heats the surface of the earth, which warms the air above it. If this warm surface air is forced to rise -- hills or mountains, or areas where warm/cold or wet/dry air bump together can cause rising motion -- it will continue to rise as long as it weighs less and stays warmer than the air around it. As the air rises, it transfers heat from the surface of the earth to the upper levels of the atmosphere (the process of convection). The water vapor it contains begins to cool, releasing the heat, and it condenses into a cloud. The cloud eventually grows upward into areas where the temperature is below freezing. Some of the water vapor turns to ice and some of it turns into water droplets. Both have electrical charges. Ice particles usually have positive charges, and rain droplets usually have negative charges. When the charges build up enough, they are discharged in a bolt of lightning, which causes the sound waves we hear as thunder.
The Thunderstorm Life Cycle
Thunderstorms have a life cycle of three stages: The developing stage, the mature stage, and the dissipating stage.
The developing stage of a thunderstorm is marked by a cumulus cloud that is being pushed upward by a rising column of air (updraft). The cumulus cloud soon looks like a tower (called towering cumulus) as the updraft continues to develop. There is little to no rain during this stage but occasional lightning. The developing stage lasts about 10 minutes.
The thunderstorm enters the mature stage when the updraft continues to feed the storm, but precipitation begins to fall out of the storm, and a downdraft begins (a column of air pushing downward). When the downdraft and rain-cooled air spreads out along the ground it forms a gust front, or a line of gusty winds. The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. The storm occasionally has a black or dark green appearance.
Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft beginning the dissipating stage. At the ground, the gust front moves out a long distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. Rainfall decreases in intensity, but lightning remains a danger.
THE SINGLE CELL STORM
Single cell thunderstorms usually last between 20-30 minutes. A true single cell storm is actually quite rare because often the gust front of one cell triggers the growth of another.
Most single cell storms are not usually severe. However, it is possible for a single cell storm to produce a brief severe weather event. When this happens, it is called a pulse severe storm. Their updrafts and downdrafts are slightly stronger, and typically produce hail that barely reaches severe limits and/or brief microbursts (a strong downdraft of air that hits the ground and spreads out). Brief heavy rainfall and occasionally a weak tornado are possible. Though pulse severe storms tend to form in more unstable environments than a non-severe single cell storm, they are usually poorly organized and seem to occur at random times and locations, making them difficult to forecast.
THE MULTICELL CLUSTER STORM
The multicell cluster is the most common type of thunderstorm. The multicell cluster consists of a group of cells, moving along as one unit, with each cell in a different phase of the thunderstorm life cycle. Mature cells are usually found at the center of the cluster with dissipating cells at the downwind edge of the cluster.
Multicell Cluster storms can produce moderate size hail, flash floods and weak tornadoes.
Each cell in a multicell cluster lasts only about 20 minutes; the multicell cluster itself may persist for several hours. This type of storm is usually more intense than a single cell storm, but is much weaker than a supercell storm.
THE MULTICELL LINE STORM (SQUALL LINE)
The multicell line storm, or squall line, consists of a long line of storms with a continuous well-developed gust front at the leading edge of the line. The line of storms can be solid, or there can be gaps and breaks in the line.
Squall lines can produce hail up to golf-ball size, heavy rainfall, and weak tornadoes, but they are best known as the producers of strong downdrafts. Occasionally, a strong downburst will accelerate a portion of the squall line ahead of the rest of the line. This produces what is called a bow echo. Bow echoes can develop with isolated cells as well as squall lines. Bow echoes are easily detected on radar but are difficult to observe visually.
THE SUPERCELL STORM
The supercell is a highly organized thunderstorm. Supercells are rare, but pose a high threat to life and property. A supercell is similar to the single-cell storm because they both have one main updraft. The difference in the updraft of a supercell is that the updraft is extremely strong, reaching estimated speeds of 150-175 miles per hour. The main characteristic which sets the supercell apart from the other thunderstorm types is the presence of rotation. The rotating updraft of a supercell (called a mesocyclone when visible on radar) helps the supercell to produce extreme severe weather events, such as giant hail (more than 2 inches in diameter, strong downbursts of 80 miles an hour or more, and strong to violent tornadoes.
The surrounding environment is a big factor in the organization of a supercell. Winds are coming from different directions to cause the rotation. And, as precipitation is produced in the updraft, the strong upper-level winds blow the precipitation downwind. Hardly any precipitation falls back down through the updraft, so the storm can survive for long periods of time.
The leading edge of the precipitation from a supercell is usually light rain. Heavier rain falls closer to the updraft with torrential rain and/or large hail immediately north and east of the main updraft. The area near the main updraft (typically towards the rear of the storm) is the preferred area for severe weather formation.
Thunderstorm Climatology
Where are thunderstorms most common?
In the United States, thunderstorms are most common over the Florida Peninsula and the southeast plains of Colorado have the highest frequency of thunderstorms. Small thunderstorms occur about once a year in Alaska and 2-3 times a year in the Pacific Northwest.
Where are severe thunderstorms most common?
The greatest severe weather threat in the U.S. extends from Texas to southern Minnesota. But, no place in the United States is completely immune to the threats of severe weather.
When are thunderstorms most likely?
Thunderstorms are most likely to happen in the spring and summer months and during the afternoon and evening hours, but 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
• Thunder and lightning can occasionally accompany snow or freezing rain!
What does a thunderstorm look like?
VISUAL EVIDENCE
Upper level features:
Thunderstorms can look like heads of cauliflower or they can have "anvils". An anvil is the flat cloud formation at the top of the storm. An anvil forms when the updraft (warm air rising) has reached a point where the surrounding air is about the same temperature or even warmer. The cloud growth abruptly stops and flattens out to take the shape of an anvil.
If the thunderstorm has a very strong updraft, a small portion of the updraft air will poke through the flat part of the anvil – looking like a bubble of cloud above the rest of the anvil. This bubble is called an overshooting top. Most thunderstorms will have an overshooting top for a short time, but if you see a storm with a large, dome-like overshooting top that lasts for more than 10 minutes, chances are good that the thunderstorm updraft is strong enough hand persistent enough to produce severe weather.
The anvil can provide other clues to the strength of the storm and how long it might last. If the anvil is thick, smooth-edged, and cumuliform (puffy, like the lower part of the storm), then the storm likely has a strong updraft and is a good candidate to produce severe weather. If the anvil is thin, fuzzy, and wispy like cirrus clouds, then the updraft is probably not as strong, and the storm is less likely to produce severe weather. If the anvil is large and seems to be streaming away from the storm in one particular direction, then there are probably strong upper-level winds in the storm's environment and the precipitation will be blown away from the updraft rather than fall through it.
Mid-level features:
Things you might notice in the middle levels of the storm are usually associated with the storm's main updraft tower. If the clouds in the main updraft area are sharply outlined and look like a cauliflower, then the clouds are probably associated with a strong updraft that could produce severe weather. If the clouds in the updraft area have a fuzzy, mushy appearance, the updraft is probably not as strong. If the updraft tower is almost perfectly upright, the storm probably has an updraft strong enough to resist the upper-level winds blowing against it. If the updraft leans downwind, then the updraft is usually weaker.
Thunderstorms with good storm-scale organization usually have a series of smaller cloud towers to the south or southwest of the main storm tower. These smaller towers are called a flanking line and usually have a stair-step appearance as they build toward the main storm tower.
Some supercells during their development will show signs of rotation in the updraft tower. You may see streaks of cloud material that give the storm tower a "corkscrew" or "barber pole" appearance (called striations) and strongly suggest rotation. A mid-level cloud band may also be visible encircling the tower like a ring around a planet. This is another sign of possible rotation within the storm.
As a storm grows in size and intensity, it will begin to dominate its local environment (within about 20 miles). If cumulus clouds and other storms 5-15 miles away from the storm dissipate, it may be a sign that the storm is taking control in the local area. Sinking motion on the edges of the storm may be suppressing any nearby storms. All of the instability and energy available locally may focus on that one storm which could result in its continued development.
Low-level features:
Some of the most critical cloud features to determine if a thunderstorm is severe and whether it could produce a tornado are found at or below the level of the cloud base. These features can be confusing and frustrating.
An easy feature to identify is the rain-free cloud base. It is an area of smooth, flat cloud beneath the main storm tower with little or no falling precipitation. The rain-free base is usually just to the rear of the precipitation area, and marks the main area of inflow where warm, moist air at low levels enters the storm. The rain-free base is sometimes called the "intake area."
Inflow bands are ragged bands of low cumulus clouds extending from the main storm tower to the southeast or south (usually). The presence of inflow bands suggests that the storm is pulling in low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of a rotating updraft.
The beaver's tail is another significant type of cloud band. The beaver's tail is a smooth, flat cloud band extending from the eastern edge of the rain-free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. The beaver's tail is usually seen with high-precipitation supercells and suggests rotation in the storm.
A wall cloud is an isolated cloud lowering attached to the rain-free base. A wall cloud forms as the storm intensifies, and the updraft draws in low-level air from several miles around. Some of the low-level air is pulled into the updraft from the rain area. The rain-cooled air is very humid, and the moisture in the rain-cooled air quickly condenses at a lower altitude than the rain-free base to form a wall cloud. The wall cloud is usually to the rear of the visible precipitation area. Wall clouds are usually about two miles in diameter and mark the area of strongest updraft in the storm. To determine if the wall cloud may be tornadic, it will have four basic characteristics. First, the wall cloud will be persistent – lasting for 10-20 minutes (it may change shape) before a tornado appears. Second, the wall cloud will rotate consistently, and often violently before a tornado develops. Third, strong surface winds will blow in toward the wall cloud from the east or south-east (inflow). Surface winds of up to 25-35 miles an hour are often found near tornadic wall clouds. Fourth, the wall cloud will show rapid vertical motion in the form of small clouds in or near the wall cloud and will quickly rise up into the rain-free base. However, not all tornadic wall clouds will have these characteristics, and some tornadoes do not form from wall clouds!
Shelf clouds or roll clouds are examples of other clouds that you may see beneath the cloud base of a storm. Shelf clouds are long, wedge-shaped clouds associated with the gust front. Roll clouds are tube-shaped clouds and are also found near the gust front. Shelf and roll clouds can form anywhere where there is outflow. Shelf clouds typically form near the leading edge of a storm or squall line. A shelf cloud can form under the rain-free base, and look like a wall cloud. A shelf cloud may also appear to the southwest of a wall cloud and is associated with phenomena called the rear-flank downdraft.
To tell the difference between wall clouds and shelf or roll clouds, remember a wall cloud 1) suggests inflow and an updraft, 2) maintains its position with respect to rain, and 3) slopes upward away from the precipitation area. In contrast, shelf clouds 1) suggest downdraft and outflow, 2) move away from rain, 3) slope downward away from the precipitation area.
SATELLITE EVIDENCE
Satellites show us pictures of the clouds before they become big enough to be thunderstorms. We can watch these pictures over an hour and notice that the clouds are growing rapidly. Satellites also can tell us the temperature of the clouds – and we can tell if a cloud has grown tall enough to be a thunderstorm. We can even see the thunderstorm anvil from satellites.
RADAR EVIDENCE
Doppler radar sends out pieces of energy that can be reflected back to the radar by things like rain and hail. The amount of energy that is reflected back can tell us how heavy the rain might be or give us an indication of hail. Doppler radar can also show us how the wind is blowing near and inside the storm. This is helpful in understanding what kinds of hazards the thunderstorm might have (tornado, microburst, gust fronts, etc) associated with it. It also helps us understand how the thunderstorm is feeding itself.
Can thunderstorms be forecast or predicted?
MODELS
Forecasters often rely on massive computer programs called numerical weather prediction models to help them decide if conditions will be right for the development of thunderstorms. The models start with current weather observations and attempt to predict future weather using physics and dynamics to mathematically describe the atmosphere's behavior.
Numerical weather prediction models have long been used to guide forecasters as they produce forecast products. These models are computer programs that ingest observations from around the world and use complicated mathematical equations to predict the weather – something that can only be done by huge computers. The predictions are usually output in text and graphics (mostly maps).
Numerical weather prediction models are designed to calculate what the atmosphere will do at certain points over a large area, from the Earth's surface to the top of the atmosphere. Accurate observations about what the weather is doing now is key to help predict what it will do in the future. Data is gathered from weather balloons launched around the globe twice each day, in addition to measurements from satellites, aircraft, and temperature profilers and surface weather stations. The more "grid points," the better the model will predict.
Ensemble forecasting
Computer models work great if the weather follows the rules we have set. When the weather breaks the rules, the predictions have trouble too. Another technique being developed is the concept of "ensemble forecasting." Instead of using just one model, a supercomputer runs several models at one time – an ensemble. If each run looks similar, then we can assume the weather will likely follow the rules. If the runs look different in different places, then we understand that something in the atmosphere is causing the weather to misbehave.
Interpreting the model output is key, and takes a lot of practice. Forecasters use their experience, knowledge, persistence (what makes us think the weather is going to change from what it is now?) and eyes (looking out the window!) to fine-tune their forecasts. An important advancement has been made in model displays – the output used to be on black and white maps. Now forecasters can look at the output on their computer workstations and use different colors to understand more clearly what is happening.
SATELLITE
Satellites are critical in short-term forecasting. Satellite images can give an early indication of a developing thunderstorm by showing where cumulus clouds are forming. Cumulus clouds grow rapidly into cumulonimbus clouds if conditions are right, and you can track their growth using satellite images.
Since the satellites are positioned over the equator, they are viewing the northern hemisphere at an angle so you can get a sense of the vertical development of the clouds. Also taller clouds will cast shadows onto lower ones so visible imagery is an excellent tool for locating developing thunderstorms.
There are three types of satellite images:
- Visible imagery
Visible imagery is just like the name suggests; an image of the earth in visible light. This is a similar manner to that of a person taking a picture with a camera. The satellite senses sunlight reflected from objects within the viewfinder. In the case of the satellite, the objects are the upper surfaces of clouds. Thick clouds do a much better job of reflecting light and therefore appear brighter in visible photos. When the satellites are positioned over the equator, they view the northern hemisphere at an angle so you can get a sense of the vertical development of the clouds. Also taller clouds will cast shadows onto lower ones so visible imagery is an excellent tool for locating developing thunderstorms.
Example of visible satellite image - Infrared imagery
The obvious problem with visible imagery is that it is only available during the day. To combat this problem, the infrared (IR) sensor was devised. It senses radiant (heat) energy given off by the clouds. Warmer (lower in the atmosphere) clouds give off more energy than cold (higher) clouds. The IR sensor measures the heat and produces several images based upon different wavelengths in the IR range of the electromagnetic spectrum. Often these images are color enhanced to help better distinguish the taller (coldest, usually from thunderstorms) cloud tops. - Water vapor imagery
Water vapor imagery is unique in that it can detect water vapor (water in a gas state) in addition to clouds. However, due to absorption of energy by the atmosphere this view only "sees" of the top third of the troposphere. While the low level moisture is hidden from the satellite sensor, the upper level moist/dry areas are plainly observable. Moist areas show up as white, dry areas as black.
The GOES weather satellites also have equipment that will acquire profiles of temperature and moisture for clear or partly clear fields of view. These profiles are further processed to produce several derived meteorological parameters. In addition, cloud tracking allows for the measurement of wind in the atmosphere. This information is used for input to the weather models which result in improved weather analysis and forecasting.
What kinds of damage can they cause?
Many hazardous weather events are associated with thunderstorms. Lightning is responsible for many fires around the world each year, as well as causing deaths when people are struck. Under the right conditions, rainfall from thunderstorms causes flash flooding, which can change small creeks into raging torrents in a matter of minutes, washing away large boulders and most man-made structures. Hail up to the size of softballs damages cars and windows, and kills wildlife caught out in the open. Strong (up to more than 120 mph) straight-line winds associated with thunderstorms knock down trees and power lines. Tornadoes (with winds up to about 300 mph) can destroy all but the best-built man-made structures.
From wreaking havoc on airline schedules to threatening outdoor sporting events, thunderstorms have a big effect on our society.
Understanding Thunderstorm Risks
Thunderstorms pose risks from lightning, flooding or tornadoes
From lightning:
people at risk are those who are outdoors (especially under or near tall trees, in or on water, or on or near hilltops.)
From flooding:
people who are in automobiles when flash flooding occurs near them are at risk.
From tornadoes:
the greatest risk is for those people who are in mobile homes and automobiles.
What to listen for:
Severe Thunderstorm Watch:
tells you when and where severe thunderstorms are more likely to occur. Watch the sky and stay tuned to know when warnings are issued. Watches are intended to heighten public awareness and should not be confused with warnings.
Severe Thunderstorm Warning:
issued when severe weather has been reported by spotters or indicated by radar. Warnings indicate imminent danger to life and property to those in the path of the storm.
