“They call the wind Maria” or so a classic tune from a 1950s musical informs us. A few years earlier, the forecaster-protagonist of the 1941 novel Storm did just that (and perhaps inspired the song) by dubbing a particularly mean cyclone Maria. The fixedness of our earth's landscape helps to shape the wind into familiar patterns that come and go. Across the world, people can't seem to resist naming the wind something – especially the types of wind that regularly visit the same place.
Blowing hot and cold
Some of our most recognizable winds come into existence as a result of mountains and hills that channel air streams. As wind blows toward and up a mountain range, the space between ground level and the top of the atmosphere narrows, so the air accelerates in response, like water rushing through a squeezed garden hose. Upsloping winds can produce lots of rain and snow, but after they pass a crest and head downslope, they take on a wholly different cast. Downsloping wind picks up momentum as it descends, which helps it to blow even more vigorously.
The most famous downslope winds are either markedly hot or cold, depending on which of two factors governs. Descending air is normally compressed as it sinks, which helps to warm it up. This is the mechanism that dominates in the hot, dry downslope winds epitomized by the Santa Ana of the Los Angeles Basin or the brickfielders of Sydney. The equivalent in the Alps is the fohn, a term often used as a general label for warming downslope winds. In winter, a fohn can bring delightfully mild temperatures, but at the cost of howling gusts that can set teeth on edge and wreak havoc. In Boulder, Colorado, a 1982 chinook – the term means “snow-eater” – peaked at 220kph/137mph.
When a strong cold front reaches a mountain range, the cool, dense air may flow down the slope with vigour. Even though it warms through compression, it's still colder than the air it's replacing. These biting winds are known as the bora along the Adriatic Sea, canyon flow in the Salt Lake Valley of Utah and the mistral in France.
Both warm and cold downsloping winds typically last only a few hours at their strongest, although a persistent large-scale regime might bring several bouts of wind over a couple of days. Downslope winds do tend to occur in conjunction with plenty of sunshine, thanks to the evaporation of whatever cloud droplets may be in the air as it expands and descends. A thick, dramatic mass of cloud called a fohn wall might be seen banked against the ridge line. Such a cloud takes shape just over the ridge, where the air's
ascending, and can hug the mountains for hours. If a cold bora-type wind is strong enough, a few drops of rain or snow can be flung over areas a few kilometres downstream, even where the sky directly overhead may be clear. When moisture is especially scant, the only cloud visible above a downsloping wind might be the smooth, graceful arc of a lenticular or cap cloud above the highest peaks.
Downbursts and derechoes
Another kind of windstorm – a bane to farmers and aviators alike – is a downburst, marked by cool, moist air descending from a thunderstorm. It was long recognized that thunderstorms could cause severe damage on the ground without a tornado being evident, yet nobody could explain what was going on. Meanwhile, as air travel grew in the 1960s and 1970s, more and more planes were crashing to the ground in and near storms. Tornado guru Theodore Fujita made a connection as he flew over damaged cornfields in the US Midwest following the worlds biggest outbreak of twisters on April 3,1974. Fujita noticed that some of the downed cornstalks weren't arranged in tornado-induced spirals, but were instead flayed outward in a starburst pattern. It reminded Fujita of bombing damage he'd surveyed in World War II. As it turns out, this kind of meteorological bomb was not only ravaging crops, but causing aircraft mishaps such as a 1975 crash at New Yorks Kennedy Airport that killed 113 passengers.
Fujita and colleagues later verified that the tons of rain and hail in a cloud can trigger brief, yet destructive, rain-cooled downbursts of air blowing at up to 240kph/150mph. The smallest of these – less than 4km/2.5 miles across – are labelled microbursts. Even a weak storm positioned over dry surface air can produce a surprisingly strong downburst as rain evaporates and cools the air below. A plane entering microburst first encounters a headwind that slows its descent and raises its nose. The pilot's logical response – tipping the nose downward – can send the plane hurtling to the ground when it reaches the tailwind on the other side of the microburst.
A blend of surface instruments, radar and software now tracks this dangerous type of wind shear (wind difference across space) near airports in the US and many other storm-prone countries. Downbursts are a hazard away from airports, too. Nine New York schoolchildren were killed in November 1989 when their school's cafeteria wall collapsed. The event, New York's deadliest – ever windstorm, was first tagged as a tornado, but Fujita and other experts later concluded it was most likely a downburst.
The king of downbursts is the derecho, a long-lasting windstorm that can leave a swath of damage a few miles or kilometres wide and more than 800km/500 miles long. It's especially common from the US Midwest into central Canada, where severe thunderstorms can take on a particular temperature and wind structure that lends itself to long-lived jets of descending wind. Once called prairie hurricanes, and sometimes referred to in Canada as blowdowns, derechoes may pack winds of well over 160kph/lOOmph that blow for upwards of an hour at any given spot. Derechoes shred crops and can even deroof buildings – damage that's often mistaken for tornadic mayhem until specialists take a closer look.
There are plenty of small-scale atmospheric vortices that keep our air spinnings Most of the world's warmer oceans and lakes, including the Mediterranean – and even some colder ones, like the Great Lakes of North America – see waterspouts, also known in Italy as trombe marine (marine trumpets). Like other tornadoes, these too emerge from showers and thunderstorms, but a waterspout is considered a different animal unless and until it moves over land A typical waterspout forms along the boundary that separates standard marine air from the cooler air mass beneath a rain (or snow) shower. It's a 30- to 60-minute-long process that begins with the appearance of a dark spot on the water surface. Some of these spots develop spiral bands that reflect inflowing winds. Next comes a ring of spray just above the water surface. A few minutes later, a slender funnel may descend from the cloud base; some-times the visible funnel stops short of the sea and leaves a transparent section in between. The typical waterspout spans no more than 75m/250ft (often as little as 9m/30ft) and lasts 20-30 minutes before it pulls rain-cooled air into its updraught and dissipates.
Graceful as they may look, waterspouts can be deadly. Most develop winds between gale and hurricane force, and a few produce gusts above 240kph/150mph. A number of boaters have been killed in such US haunts as North Carolina's Outer Banks and the Florida Keys. If the conditions are right, a pack of three or more waterspouts may appear along a line of convergent winds. Even the heat from island volcanoes can induce swarms of waterspouts, such as the dozens that formed down-wind of Surtsey in 1963. Over land, some weaker tornadoes are classified as landspouts because they form through a similar process as waterspouts. One Texan captured a row of six simultaneous landspouts on camera. Like their marine cousins, landspouts often resemble transparent tubes with invisible mid-sections.
Anyone who's driven across parched fields on a hot summer's day knows about dust devils. Unlike the other spin-ups already discussed, dust devils aren't a product of rain clouds. Indeed, they thrive on clear skies, which allow the sun to produce intense heat near the ground and induce bubbles of rising air called thermals. All that's needed then is a slight breeze or a small variation in surface heating (as between a ploughed and unploughed field) to help create eddies that might serve as dust-devil nuclei. Because they're too small and short-lived to be affected by the turning of the Earth, dust devils have no preferred direction of rotation. They occur most commonly around noon, when solar heating is at its strongest, and just afterward. American researchers have counted as many as a hundred dust devils on a single day. Land devils or hay devils have been observed in Great Britain and other areas.
Most dust devils are puny by tornadic standards. In one Arizona survey, more than 80 percent of them spanned less than 15m/50ft, although they can extend upward for more than 0.8km/0.5 miles; their winds rarely exceed 80kph/50mph. Dust devils move with the surrounding wind, and they tend to be larger when that wind is stronger, as long as it's not too strong. Some dust devils can last for hours if buildings, hills or even vehicles happen to be arranged to channel the wind in a favourable way. Power lines have been downed and trailers knocked over by fierce dust devils in Arizona.
When a raging fire produces ground-based heat, fire whirls (pictured opposite) can result These can rival weak tornadoes in their strength and size, and they can help advance a forest fire with violent intensity. Spin-ups have been seen near the heat vents from active volcanoes. Even fires in manmade settings, such as the horrific firestorm caused by the World War II bombing of Dresden, Germany, can produce vortices.