Thunderstorms

It s with very good reason that lightning evolved as a symbol of divinity in both the Bible and the Koran, as well as in Greek mythology and countless other religious and secular traditions. Few natural events are as familiar yet as awe-inspiring as a thunderstorm. The best estimate is that some 2000 thunderstorms are raging across the planet's surface at any one moment. That's a lot of soaked picnics and sleepless nights.

The world couldn't do without thunderstorms, of course. They furnish some areas with more than half of their annual rainfall. Lightning even converts nitrogen into a form that fertilizes the soil. Even so, thunderstorms exact a heavy toll for the good they achieve. Hundreds of people around the world are killed each year by lightning. Many hundreds more fall victim to tornadoes, flash flooding and hailstones.

What makes a thunderstorm?

Few places on Earth are free of thunderstorms. Wherever there is enough heat and moisture, they are likely to form Mountains help trigger them by forcing air upward. The only spots that are virtually free of thunder are those where it hardly ever rains, chiefly the parched Sahara and the bitter reaches of the Arctic and Antarctic. Even here, however, surprises are possible. The northernmost settlement in the US – Barrow, Alaska, perched on the Arctic Ocean – got its first thunderstorm in living memory on June 19, 2000. It terrified animals and children who didn't know what to make of the flashing light and deafening sound.

In order to produce thunder and lightning, you need a cloud tall enough to pull moisture up into the sub-freezing realm over 5km/3 miles high. These clouds are cumulus, the bubbly, cauliflower-like formations that typify summer. Cumulus can form any time of year as long as there's warmth below and coolness above, the same set-up – meteorologists call it convection – by which a hot burner makes water boil and bubble. Data from a weather balloon can tell forecasters how much fuel there is for thunderstorms by showing how much contrast, or instability, exists between the layers of air. Once cumulus clouds reach a height where ice crystals form, they become cumulonimbus. Here the upward motion gradually slows. With the help of stronger upper-level winds, the cloud spreads out in a sheet-like formation called the anvil (for its resemblance to the top of a blacksmith's anvil).

As ice crystals form, they collide with each other and with still-unfrozen water droplets. Current theory has it that each bump and jostle results in a tiny bit of electrical charge being generated, just as scraping your shoes against carpet on a dry day gives your body a static charge. All this takes place in a larger, storm-wide electric field that's enhanced by the collisions. If the field becomes intense enough, lightning soon results.

The many faces of lightning

Experts refer to lightning by pairs of initials. IC means intracloud; it's the kind of lightning that stays aloft, dancing through and around the thunderhead itself. Each IC flash lasts, on average, about a quarter of a second. CGs are doud-to-ground flashes, produced when a tentative leader of electrons descends from a storm and is met by a return stroke of positive charge from the ground. Although common sense tells us that lightning comes down from above, it's actually the return stroke heading upward that produces the visible flash. Each leader pauses for a tiny fraction of a second a few times along its downward path – sometimes splitting along the way – as it hunts for the most efficient route. The resulting twists and turns give lightning its crooked, forked appearance. Each CG includes a series of anywhere from one to two dozen pulses of electricity (the average is four). These produce the characteristic flicker of a CG, which can last up to several seconds.

About 10 percent of CGs have leaders that bring positive instead of negative charge to the ground. The long, continuous currents from these positive CGs trigger more than their share of fires, as many of them strike well away from the rainy part of a thunderstorm. Overall, of course, lightning is a beneficial part of Earth's ecosystem. Some species, such as Jack pine, rely on fire in order to reproduce. In the days before campers and cigarettes, only nature could provide the needed spark. The human desire to limit forest fires has added to the potential damage from lightning. In some places, a century's worth of dead and living vegetation now waits to burn, as it did across the western US in the disastrous, smoky summer of 2002.

The intense heat of lightning, estimated at 30,000°C/54,000°F, can cause instant havoc for people as well. Entire football teams have been felled by a single flash. Only a minority of people struck by lightning are killed, but the injuries – physical, neurological and psychological – can be severe and long-lasting.

To help track lightning, ground-based sensors have been used since the 1980s by utility companies, fire control agencies and others in North America and more recently in Europe, Asia and South America. These sensors calculate the locations of CGs and plot them on up-to-the-minute maps. Satellites have collected long-term data on all lightning (ICs and CGs alike) since the mid-1990s. They've shown that the global average is about 30 to 40 flashes per second, less than half of what we once thought it was.

As the heat expands air in and near a lightning flash, the world's most familiar shock wave – thunder – radiates outward. Thunder can resemble a sharp crack or a low, sustained roll, depending on the density of the air, the orientation of the bolt, and the bolt's distance from the observer. Thunder travels about 1km every 3 seconds (or 1 mile every 5 seconds), so by counting the seconds between flash and rumble, you can estimate how far lightning is from you. A CG can strike as far as 32km/20 miles away from a storm's core, far enough so that haze or obstacles may prevent any visual awareness of a storm before the bolt from the blue arrives. Indeed, most lightning injuries and deaths occur at a storm's outset, before the hazard is obvious. Guidelines now recommend taking shelter if thunder can be heard within 30 seconds of a lightning flash.

Perhaps the biggest weather find of the late 1990s was a whole new class of lightning that surges upward from storm tops. Pilots and amateur observers had reported these strange, colourful flashes for decades, but their reports were largely dismissed. Low-light video taken from ground and space finally proved them right. The fancifully-named discoveries include sprites (red pillars that extend up to 90km/55 miles), elves (broad disks of light, often on top of sprites) and blue jets (beams that shoot out from storm tops). Their impacts on aircraft, air chemistry and radio waves are only beginning to be explored.

Thunderstorm types

Most thunderstorms spit out only a few lightning flashes and a modest amount of rain before dissipating. These so-called single-cell storms (meaning a single updraught) pop up and die down so quickly on a summer afternoon that meteorologists refer to them, a tad scornfully, as “popcorn convection” They're doomed from the start by lackadaisical upper winds that keep them becalmed. Once rain begins, it cools the air below and cuts off the storm's energy in less than an hour. Sometimes new single-cell storms can form along the cool air outflowing from old ones, or several cells may coalesce to form a multicell, possibly dropping small hail and somewhat heavier rain. When a strong cold front is marching through, a squall line may form. This band of connected cells moves through quickly with strong wind, heavy rain and perhaps hail or even a brief tornado.

The biggest movers and shakers of the storm world are supercells. These behemoths only form when instability is quite strong and, typically, when upper-level winds strengthen with height. This keeps the storm moving and keeps its top ventilated, so that warm, moist air is pulled in from below as in a chimney. Many of the jaw-dropping elements of the strongest thunderstorms – torrential rain, large hail, hurricane-force wind and violent tornadoes – occur with supercells. Because supercells are so well differentiated, they can cause a variety of trouble at different points beneath them during their lifespan of six hours or more. Like the tornadoes they spawn, supercells are most common in the mid-latitudes, especially the central and eastern US; they occur less often in parts of northern and western Europe, eastern and southern Asia, Argentina, South Africa and Australia.

The largest thunderstorm groups on earth have a suitably big name: mesoscale convective systems (MCSs). Think of an MCS as a storm on steroids. Actually a collection of storms – typically organized as a cluster or a squall line – it can span 160-320km/100-200 miles and last for more than twelve hours. Partly due to their sheer size, MCSs can generate buckets of rain (over 250mm/10in can fall) and vast amounts of lightning (upwards of 10,000 strikes in an hour, or about three per second). They are known for batches of spidery lightning that stretch through the cloud almost from horizon to horizon. Record-breaking floods of 1993 along the US Mississippi and Missouri rivers were fed by more than 70 MCSs and smaller, related systems that traversed the plains through the summer.

MCSs favour the moist heat of the warm season across the mid-latitudes and tropics, from the Americas to Africa and Asia. In many places, they peak during the overnight hours, as smaller storms merge and low-level jet streams intensify. Once rolling, they often prowl well into the next day. If an MCS forms or moves over the ocean, it can serve as the nucleus of a hurricane or typhoon.