Thunderstorms -Basics
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Thunderstorms are characterized by rain, thunder, and lightning, resulting from convection and requiring moisture, unstable air, and a lifting mechanism. They commonly occur in spring and summer but can happen year-round, often manifesting in various types like single-cell, multi-cell, and supercell storms. Severe thunderstorms can cause high winds, hail, tornadoes, and flash flooding, contributing to significant hazards and damage.
Thunderstorms -Basics
- 1. 1 THUNDERSTORM -BASICS By Prof A. Balasubramanian CENTRE FOR ADVANCED STUDIES IN EARTH SCIENCE UNIVERSITY OF MYSORE MYSORE-6
- 2. 2 Introduction: A thunderstorm is a rain shower during which you hear thunder. Since, the thunder comes from lightning, all thunderstorms have lightning. A thunderstorm is the result of convection. A thunderstorm is classified as “severe” when it contains one or more of the following: hail one inch or greater, winds gusting in excess of 50 knots (57.5 mph), or a tornado.
- 3. 3 Occurrence: Thunderstorms are most likely to occur in the spring and summer months and during the afternoon and evening hours, but they can occur year-round and at all hours. Thunderstorms frequently occur in the late afternoon and at night in the Plains states. Locations of severe thunderstorms : The greatest severe weather threat in the U.S. extends from Texas to southern Minnesota.
- 4. 4 Origin of 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.
- 5. 5 If this warm surface air is forced to rise , 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, releases the heat, condenses and forms a cloud. The cloud eventually grows upward into areas where the temperature is below freezing.
- 6. 6 As a storm rises into freezing air, different types of ice particles can be created from freezing liquid drops. The ice particles can grow by condensing vapor (like frost) and by collecting smaller liquid drops that haven't frozen yet (a state called "supercooled"). When two ice particles collide, they usually bounce off each other, but one particle can rip off a little bit of ice from the other one and grab some electric charge.
- 7. 7 Lots of these collisions build up big regions of electric charges to cause a bolt of lightning, which creates the sound waves we hear as thunder. Thunderstorms are a great way for the atmosphere to release energy. When warm moist air meets colder drier air, the warm air rises, the water vapor condenses in the air, and forms a cloud.
- 8. 8 As the water vapor condenses it releases heat, which is a form of energy. A large amount of the thunderstorm's energy comes from the condensation process that forms the thunderstorm clouds. As the thunderstorm progresses, eventually the rain cools the entire process down and the energy is gone. Thunderstorms also help keep the Earth in electrical balance.
- 9. 9 The Earth's surface and the atmosphere conduct electricity easily – the Earth is charged negatively and the atmosphere, positively. There is always a steady current of electrons flowing upwards from the entire surface of the earth. Thunderstorms help transfer the negative charges back to earth (lightning is generally negatively charged).
- 10. 10 Without thunderstorms and lightning, the earth- atmosphere electrical balance would disappear in five minutes. Sources of moisture: Typical source of moisture for thunderstorms are the oceans. Instability: Air is considered unstable if it continues to rise when given a nudge upward (or continues to sink if given a nudge downward).
- 11. 11 An unstable air mass is characterized by warm moistair near the surface and cold dry air aloft. In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As this parcel rises it cools and some of the water vapor will condense forming the familiar tall cumulonimbus cloud that is the thunderstorm.
- 12. 12 Sources of Lift (upward): Typically, for a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. This upward nudge is a direct result of air density. Some of the sun's heating of the earth's surface is transferred to the air which, in turn, creates different air densities.
- 13. 13 The propensity for air to rise increases with decreasing density. This is difference in air density is the main source for lift and is accomplished by several methods. Differential Heating: The sun's heating of the earth's surface is not uniform. For example, a grassy field will heat at a slower rate than a paved street.
- 14. 14 A body of water will heat slower than the nearby landmass. This will create two adjacent areas where the air is of different densities. The cooler air sinks, pulled toward the surface by gravity, forcing up the warmer, less dense air, creating thermals.
- 15. 15 Fronts, Drylines and Outflow Boundaries: Fronts are the boundary between two air masses of different temperatures and therefore different air densities. The colder, more dense air behind the front lift warmer, less dense air abruptly. If the air is moist thunderstorms will often form along the cold front.
- 16. 16 Drylines are the boundary between two air masses of different moisture content and divides warm, moist air from hot, dry air. Moist air is less dense than dry air. Drylines therefore act similarly to fronts in that the moist, less dense air is lifted up and over the drier, more dense air. The air temperature behind a dryline is often much higher due to the lack of moisture.
- 17. 17 That alone will make the air less dense but the moist air ahead of the dryline has an even lower density making it more buoyant. T he end result is air lifted along the dryline forming thunderstorms. This is common over the plains in the spring and early summer. Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead.
- 18. 18 The rain-cooled, more dense, air acts as a "mini cold front", called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form. Terrain: As air encounters a mountain it is forced up because of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.
- 19. 19 The Thunderstorm Life Cycle: Thunderstorms have three stages in their life cycle: 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.
- 20. 20 There is little to no rain during this stage but occasional lightning. The thunderstorm enters the mature stage when the updraft continues to feed the storm, but precipitation begins to fall out of the storm, creating a downdraft (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.
- 21. 21 The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft beginning the dissipating stage. Rainfall decreases in intensity, but lightning remains a danger.
- 22. 22 Shape of thunderstorms: Thunderstorms may look like tall 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.
- 23. 23 The cloud growth abruptly stops and flattens out to take the shape of an anvil. Thunderstorm Types : 1. Single-Cell Thunderstorms 2. Multi-Cell Storm 3. Squall Line 4. Supercell 5. Bow Echo 6. Derecho
- 24. 24 Often called “popcorn” convection, single-cell thunderstorms are small, brief, weak storms that grow and die within an hour or so. They are typically driven by heating on a summer afternoon. Single-cell storms may produce brief heavy rain and lightning. A multi-cell storm is a common, garden- variety thunderstorm in which new updrafts form along the leading edge of rain-cooled air (the gust front).
- 25. 25 Individual cells usually last 30 to 60 minutes, while the system as a whole may last for many hours. Multicell storms may produce hail, strong winds, brief tornadoes, and/or flooding. A squall line is a group of storms arranged in a line, often accompanied by “squalls” of high wind and heavy rain. Squall lines tend to pass quickly and are less prone to produce tornadoes than are supercells.
- 26. 26 They can be hundreds of miles long but are typically only 10 or 20 miles wide. 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.
- 27. 27 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. A supercell is an often-dangerous thunderstorm with a very organized internal structure including a rotating updraft that allows it to keep going for up to several hours.
- 28. 28 Supercells are capable of producing severe weather including high winds, large hail, and strong tornadoes. They are most frequently isolated and often develop in the warm air ahead of a squall line. A supercell also usually forms in an environment with strong vertical wind shear that causes the updraft to begin rotating. A “bow echo” is a radar signature of a squall line.
- 29. 29 How many thunderstorms are there? Worldwide, there are an estimated 16 million thunderstorms each year, and at any given moment, there are roughly 2,000 thunderstorms in progress. There are about 100,000 thunderstorms each year in the U.S. alone. About 10% of these reach severe levels.
- 30. 30 SCALE OF THUNDERSTORMS: A Mesoscale Convective System (MCS) is a collection of thunderstorms that act as a system. An MCS can spread across an entire state and last more than 12 hours. On radar one of these monsters might appear as a solid line, a broken line, or a cluster of cells.
- 31. 31 A derecho (pronounced similar to “deh-REY- cho” in English) is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms. Although a derecho can produce destruction similar to that of tornadoes, the damage typically is directed in one direction along a relatively straight swath.
- 32. 32 As a result, the term “straight-line wind damage” sometimes is used to describe derecho damage. By definition, if the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho.
- 33. 33 Thunderstorm Detection-Weather Satellites: We can see thunderstorms with a variety of tools. Most areas of Earth can be seen by weather satellites. Satellites take pictures of Earth at regular intervals from space, telling us where clouds are located. Meteorologists watch these pictures over time to watch for rapidly growing clouds, a clue to a possible thunderstorm.
- 34. 34 Satellites also can tell us the temperature of the clouds. Clouds with cold tops are usually very high up in the atmosphere, and could mean the cloud is tall enough to be a thunderstorm. Meteorologists also track how these clouds move to see what areas will be affected by the storm next.
- 35. 35 RADARS: Weather radar is very important to meteorologists because it can detect rain and severe weather even when it is cloudy or dark. Doppler radar sends out electromagnetic wave fields that can be reflected back to the radar by things in the air like precipitation. The amount of energy that is reflected back can tell us how heavy the rain might be or tell us there is hail.
- 36. 36 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.
- 37. 37 Thunderstorm Forecasting: The National meteorology Department monitors and forecasts the potential for severe weather over the nation and will give critical information concerning the threat of severe weather at your location.
- 38. 38 COMPUTER FORECAST MODELS Meteorologists 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. These 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.
- 39. 39 Data is gathered from weather balloons launched around the globe twice each day, in addition to measurements from satellites, aircraft, ships, temperature profilers and surface weather stations. The models start with these current weather observations and attempt to predict future weather using physics and dynamics to mathematically describe the atmosphere's behavior.
- 40. 40 The predictions are usually output in text and graphics (mostly maps). ENSEMBLE FORECASTING: Another technique is to run the same model several times with varying starting weather conditions. This approach results in a number of predictions that produce a range of possible future weather conditions.
- 41. 41 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. SATELLITES: 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.
- 42. 42 Cumulus clouds grow rapidly into cumulonimbus clouds if conditions are right, and you can track their growth using satellite images. Satellites also give clues about what type of thunderstorm may be developing. What direction does the wind come from during a thunderstorm? There is no one direction the wind comes from when thunderstorms or tornadoes occur.
- 43. 43 Kinds of damage thunderstorms may 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.
- 44. 44 Hail up to the size of softballs damages cars and windows, and kills livestock caught out in the open. Strong (up to more than 120 mph) straight-line winds associated with thunderstorms knock down trees, power lines and mobile homes. Tornadoes (with winds up to about 300 mph) can destroy all but the best-built man-made structures.
- 45. 45 Why are severe thunderstorms so dangerous? 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 cause fatalities. Hail up to the size of softballs damages cars and windows, and kills wildlife caught out in the open.
- 46. 46 Strong (up to more than 120 mph) straight-line winds associated with thunderstorms knock down trees, power lines and mobile homes. Tornadoes (with winds up to about 300 mph) can destroy all but the best-built man-made structures.