Thunderstorm

• 1. Introduction

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  A thunderstorm is one of the few natural forces that can release energy comparable to the atomic bomb dropped on Hiroshima in 1945. That comparison comes not from dramatic license but from physics: a single typical storm lifts roughly 500 million kilograms of water vapor, and when that vapor condenses, it discharges around a thousand trillion joules of energy. At any given moment, approximately 2,000 thunderstorms are active somewhere on Earth. So what exactly is happening inside one of these storms? How does a column of warm, moist air become a rotating, hail-throwing, tornado-spawning engine? And why do some thunderstorms last only half an hour while others sustain themselves for hours, producing destruction on a regional scale? The answers reach from the lowest kilometers of the troposphere all the way into the stratosphere, and possibly as far as the planets of Jupiter and Saturn.

• 2. Birth of a Storm

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  Warm air rises because it is less dense than cool air. That simple fact is the engine behind every thunderstorm. Moisture lifted upward cools and condenses into cumulus clouds, and as it does, it releases latent heat. That heat warms the rising air parcel, making it less dense than the air around it, which drives the cloud higher still. In a typical thunderstorm, approximately 500 million kilograms of water vapor are lifted into the atmosphere during this developing stage.

  The trigger for this lift takes several forms. Solar heating of the ground produces thermals. Converging winds can force air upward. Terrain itself can push horizontal wind streams up slope. But meteorologists point out that some kind of cloud forcing is necessary for the air to accelerate upward rapidly enough. Weather features like a front or a shortwave trough provide that forcing.

  Measuring the atmosphere's potential for this upward development involves indices like convective available potential energy, or CAPE. Upstream CAPE values above 800 joules per kilogram are generally needed before organized convection can take hold. Precipitable water values above 31.8 mm favor organized storm complexes, and those associated with heavy rainfall commonly exceed 36.9 mm. These numbers give meteorologists the clearest early warning that a developing cumulus cloud might become something more dangerous.

• 3. How a Storm Matures

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  The average thunderstorm spans a diameter of 24 km, and each of its three stages takes roughly 30 minutes. The mature stage is the most violent. Rising air eventually strikes a layer of warmer air above and can climb no farther. Often that ceiling is the tropopause itself, and the cloud is forced to spread sideways into the distinctive anvil shape. The resulting cloud type is called cumulonimbus incus.

  Inside this anvil, water droplets coalesce into ice particles. As those particles fall and begin to melt, they become rain. If the updraft is powerful enough, it holds droplets aloft long enough that they grow too large to melt fully, falling instead as hail. The simultaneous presence of updrafts and downdrafts is the defining feature of the mature stage, and it generates the turbulence that produces severe lightning and tornadoes.

  Wind shear plays a critical role in what happens next. With little wind shear, the storm quickly rains itself out. With sufficient change in wind speed or direction across altitude, the downdraft separates from the updraft, and the storm can enter a supercell state where the mature stage sustains itself for several hours rather than dissolving.

• 4. The Four Storm Types

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  Single-cell thunderstorms, also called air-mass thunderstorms, form in environments of low vertical wind shear and typically last 20-30 minutes. They are the common summer afternoon storms across many temperate regions. When they produce a brief period of severe weather, meteorologists call them pulse severe storms. These are particularly hard to forecast because they are poorly organized and occur randomly in time and space.

  Multi-cell clusters are the most common type of thunderstorm development. Mature cells sit near the center of the cluster while dissipating cells drift downwind. Although each individual cell may last only 20 minutes, the cluster as a whole can persist for hours. They frequently arise near mountain ranges or along strong cold fronts. Hazards from these clusters include moderate hail, flash flooding, and weak tornadoes.

  Squall lines, also called multi-cell lines, are elongated bands of severe thunderstorms that can form along or ahead of a cold front. In the early 20th century, the term was actually used as a synonym for cold front. Some bow echoes within squall lines during summer are classified as derechos, fast-moving systems that cut through large sections of territory. In southern China, this kind of storm is called "Wind of the Stony Lake," or shi2 hu2 feng1.

  Supercells are the strongest of all thunderstorm types. Research has shown that at least 90 percent of supercells produce severe weather. These storms can grow 24 km wide, drive hailstones to 10 cm in diameter, and generate straight-line winds exceeding 130 km/h. Their updrafts are powerful enough to punch through the troposphere and reach the lower stratosphere. Most tornadoes on Earth originate from supercell thunderstorms.

• 5. Hail, Floods, and Downbursts

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  Hailstorms have killed people throughout recorded history. One of the earliest documented incidents occurred around the 9th century in Roopkund, Uttarakhand, India. One of the highest hail-related death tolls on record was reported in mountainous northern India in 1888. The largest hailstone ever recorded in the United States by maximum circumference fell in 2003 in Aurora, Nebraska.

  Hail is more common near mountain ranges because orographic lifting intensifies updrafts. Cheyenne, Wyoming, averages nine to ten hailstorms per season, making it North America's most hail-prone city. The region where Colorado, Nebraska, and Wyoming meet is known as Hail Alley. Wheat, corn, soybeans, and tobacco are the crops most vulnerable to hail damage, and hail is considered one of Canada's costliest weather hazards overall. When hailstones exceed 13 mm in diameter, aircraft can be seriously damaged within seconds.

  Flash flooding is most dangerous in arid regions and densely populated urban areas where vegetation and water bodies cannot absorb sudden rainfall. Back-building thunderstorms, sometimes called training thunderstorms, are especially dangerous in this respect. In Rapid City, South Dakota, in 1972, an unusual alignment of winds at different atmospheric levels caused a continuously training set of cells to drop enormous quantities of rain over the same area, resulting in devastating flash flooding. A similar event struck Boscastle, England, on the 16th of August 2004, and Chennai on the 1st of December 2015.

  Downburst winds spread outward when downdrafts hit the ground, creating straight-horizontal winds often mistaken for tornado damage. These outflow boundaries are a documented aviation hazard: a substantial change in wind speed and direction during takeoff or landing reduces airspeed and the lift available to the aircraft.

• 6. Where Storms Are Thickest

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  Kampala and Tororo in Uganda have each been cited as among the most thunderstorm-prone places on Earth. Singapore and Bogor, on the Indonesian island of Java, make the same claim. Darwin, Caracas, Manila, and Mumbai also rank among cities with the highest storm frequency.

  In the United States, some of the most powerful storms form over the Midwest and the Southern states. Florida sees air-mass thunderstorms as an almost daily occurrence during summer over its central and southern portions. Florida's high frequency of cloud-to-ground lightning leads to several fatalities per year, most commonly among people working outdoors. The Great Plains are home to VORTEX2, a large organized research effort that deploys an array of sensors including Doppler on Wheels radar, vehicles carrying automated weather stations, weather balloons, and unmanned aircraft to study storms expected to turn severe.

  Mesoscale convective systems bring the Great Plains roughly half of their annual warm-season rainfall. These large complexes of thunderstorms develop overnight and persist into the following day. They tend to form when the surface temperature varies by more than 5 degrees Celsius between day and night. In the tropics, equivalent systems develop within the Intertropical Convergence Zone or monsoon troughs.

• 7. Mythology, Energy, and Other Worlds

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  Greeks attributed thunderstorms to battles waged by Zeus, who hurled lightning bolts forged by Hephaestus. Norse tradition held that thunder and lightning were the effects of Thor striking with Mjolnir against the Jotnar. Hindu religion recognizes Indra as the god of rain and thunderstorms. Some American Indian tribes connected them to the Thunderbird, a servant of the Great Spirit. Martin Luther himself was out walking when a thunderstorm began, an experience that prompted him to pray and make a vow to become a monk. These mythologies remained mainstream as late as the 18th century.

  Beyond mythology, thunderstorms generate phenomena that scientists are still working to understand. The Fermi Gamma-ray Burst Monitor has detected that powerful thunderstorms can generate gamma rays and antimatter particles called positrons. These positrons are believed to form in terrestrial gamma-ray flashes, brief bursts inside storms associated with lightning. The streams of positrons and electrons then collide higher in the atmosphere to produce additional gamma rays. Approximately 500 of these terrestrial gamma-ray flashes may occur every day worldwide, though most go undetected.

  Thunderstorms are not confined to Earth. Jupiter hosts electrical discharges up to a thousand times more powerful than Earth lightning, detected through flashes associated with clouds where water exists as both liquid and ice. Saturn, Neptune, and probably Venus also host thunderstorms. The clouds of Venus may generate lightning at a rate at least half that of Earth.