Wind
Wind is the natural movement of air or other gases relative to a planet's surface. On the 10th of April 1996, on Australia's Barrow Island, a gust during tropical Cyclone Olivia reached 408 km/h, the strongest ever recorded on Earth. That single number hints at a force that ranges from a thunderstorm flow lasting tens of minutes to global currents driven by the Sun. The study of this force has a name of its own: anemology. Why does air bother to move at all? What turns a quiet sea breeze into something that shatters windows and sandblasts paint from cars? And why do directions seem backwards, so that a western wind blows from the west toward the east? The answers stretch from the equator to the poles, from Vedic gods to black holes, and from a windmill grinding corn to a furnace in ancient Sri Lanka.
Differences in atmospheric pressure set air in motion, and those differences come mostly from temperature. Air flows from higher pressure toward lower pressure, producing winds of various speeds. On a rotating planet, that moving air gets deflected by the Coriolis effect, except exactly on the equator. Two factors drive the large-scale patterns of atmospheric circulation. One is the differential heating between the equator and the poles, where unequal absorption of solar energy creates buoyancy forces. The other is the rotation of the planet itself. Near the Earth's surface, friction slows the wind and bends it more directly inward toward low-pressure areas. Meteorologists break wind into idealized components to simplify the equations of motion. The geostrophic wind balances the Coriolis force against the pressure gradient force, flowing parallel to isobars above the boundary layer in the midlatitudes. The thermal wind is the difference in geostrophic wind between two atmospheric levels, and it exists only where horizontal temperature gradients do. That same thermal wind concept explains why a jet stream exists at all.
Easterly winds dominate the flow across the poles and again across the tropics, while westerly winds rule the mid-latitudes poleward of the subtropical ridge. Directly under that ridge sit the doldrums, also called the horse latitudes, where the winds go light. Many of Earth's deserts lie near this average latitude, because descending air reduces the relative humidity of the air mass. The trade winds blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. They steer tropical cyclones forming over the oceans, and they carry African dust westward across the Atlantic into the Caribbean. The Westerlies prevail between 35 and 65 degrees latitude, blowing from the southwest in the Northern Hemisphere and the northwest in the Southern. Together with the trade winds, they once enabled round-trip sailing routes across the Atlantic and Pacific. Their strongest band, between 40 and 50 degrees south, earned the name the Roaring Forties, fierce because so little land interrupts the southern ocean. The polar easterlies finish the picture: dry, cold, weak and irregular winds that spill from the polar highs and bend westward under the Coriolis effect.
Wind direction is named for where the wind comes from, so a northerly wind blows from north to south. Weather vanes pivot to show that direction, while windsocks at airports also estimate speed by the angle of their hang. Anemometers measure speed, most commonly with rotating cups or propellers. Research applications can read the wind from the propagation speed of ultrasound signals, from a heated wire's changing resistance, or from pitot tubes that sense a dynamic pressure differential. Sustained wind speeds are reported globally at a 10 m height, averaged over 10 minutes. The United States uses a 1-minute average for tropical cyclones and a 2-minute average for weather observations, while India typically reports a 3-minute average. The sampling window matters, because a one-minute sustained wind runs about 14% higher than a ten-minute one. To probe the winds aloft, radiosondes fix their position by GPS, radio navigation, or radar, and theodolites can track a weather balloon visually from the ground. Remote sensing fills in the rest, with SODAR, Doppler lidars and radars reading the Doppler shift off aerosols, and satellite imagery clocking how far cloud tops drift between frames.
Francis Beaufort created an empirical scale that reads wind speed from observed sea conditions. It began as a 13-level scale numbered 0 through 12, then expanded during the 1940s to 18 levels running 0 through 17. Within that scale, gale-force winds fall between 28 kn and 55 kn, with adjectives like moderate, fresh, strong, and whole marking the gradations. A storm sits at 56 kn to 63 kn. Tropical cyclone terminology shifts from one ocean basin to another, with Regional Specialized Meteorological Centers worldwide using 10-minute average winds to set categories. The Enhanced Fujita Scale takes a different approach, rating tornadoes by the damage they leave to estimate the wind speed behind it. It runs six levels, from visible damage to complete destruction, and is used in the United States along with Canada and France. On surface weather maps, a wind barb carries both direction and speed: each half-flag marks 5 kn, each full flag 10 kn, and each filled pennant 50 kn. Plot enough of them and you can draw isotachs, lines of equal wind speed that pinpoint the jet stream at or above the 300 hPa level.
In coastal regions, the slow heating of the sea against the quicker heating of the land sets up the sea breeze. Water's greater specific heat keeps it cooler, so warm air rises over the land and cooler air flows inland to fill the gap. At night the land cools faster than the ocean, the pattern reverses, and a land breeze forms. Rugged terrain rewrites the local flow entirely. Hills and valleys deflect the wind parallel to a range as a barrier jet, which can boost the low-level wind by 45%. Where a pass cuts through, Bernoulli's principle sends air rushing through with considerable speed, leaving turbulence dangerous to climbing and descending airplanes. Cool winds funneling through gaps have collected regional names: the Papagayo, Panama, and Tehuano in Central America, and the Bora, Tramontane, and Mistral in Europe. Winds flowing down into lower elevations turn warm and dry, called foehn in the Alps, halny wiatr in Poland, zonda in Argentina, koembang in Java, the Nor'west arch in New Zealand, and chinook in the Great Plains. In California, the Santa Ana and sundowner winds are funneled through passes, and downslope wind speeds can exceed 160 km/h. On the windward side, orographic lift wrings out the moisture and leaves a rain shadow on the leeward slope.
Vayu is the Vedic and Hindu god of wind, one figure among many the wind became across cultures. The Greeks named Boreas, Notus, Eurus, and Zephyrus, with Aeolus as keeper of the four winds, and they built the Tower of the Winds in Athens to mark the seasons. Fujin, one of the eldest Shinto gods, was said to be present at the creation of the world, freeing the winds from his bag to clear away the mist. Norse myth gives Njordr and four dwarves named Nordri, Sudri, Austri and Vestri, while the Slavs named Stribog, the Maori Tawhirimatea, and the Yoruba Oya. Wind also bent the course of history. Kamikaze, the divine wind, names the typhoons said to have saved Japan from Mongol fleets under Kublai Khan in 1274 and again in 1281. The Protestant Wind deterred the Spanish Armada from invading England in 1588, and favorable winds later carried William of Orange across in 1688. During Napoleon's Egyptian Campaign, French soldiers choked in the khamsin while the Ottomans took cover, and in the North African campaign of World War II, sandstorms from the khamsin halted troops mid-battle and rendered compasses useless. Wind has done patient work as well. The ancient Sinhalese of Anuradhapura aimed monsoon winds at furnaces as early as 300 BCE, reaching 1200 C, and vertical-axle windmills appeared in Sistan, Afghanistan from the 7th century CE.
The fastest wind ever recorded did not come from any planet's surface. It poured from the accretion disc of the black hole IGR J17091-3624 at 20,000,000 mph, which is 3% of the speed of light. The solar wind is a stream of charged particles, a plasma ejected from the Sun's upper atmosphere at 400 km/s, mostly electrons and protons with energies near 1 keV. That stream inflates the Heliosphere, a vast bubble in the interstellar medium, and it drives geomagnetic storms that can knock out power grids, the aurorae, and the plasma tails of comets that always point away from the Sun. A second kind, the planetary wind, lets light gases like hydrogen drift up to the exobase and reach escape velocity into space. Over geologic time this process can turn a water-rich planet like Earth into one like Venus. Across the Solar System the numbers climb. Venus's cloud-top winds of 300 km/h circle the planet every four to five Earth days, and Saturn shows peak easterly winds of 375 m/s in Cassini-Huygens data. Neptune's equatorial winds reach 400 m/s. Yet the record for any known planet belongs to HD 80606 b, 190 light years away, where the wind blows at more than 11,000 mph, or 5 km/s.
Common questions
What is wind and what causes it?
Wind is the natural movement of air or other gases relative to a planet's surface. It is caused by differences in atmospheric pressure, which are primarily due to temperature differences, with air moving from higher to lower pressure. On a rotating planet, that moving air is also deflected by the Coriolis effect.
What is the strongest wind gust ever recorded on Earth?
The strongest wind gust on Earth reached 408 km/h on Australia's Barrow Island during tropical Cyclone Olivia on the 10th of April 1996. It surpassed the previous record of 372 km/h set on Mount Washington, New Hampshire, on the 12th of April 1934.
Why does a westerly wind blow from the west and not toward it?
Wind direction is named for the direction the wind comes from, not where it is heading. A western or westerly wind blows from the west to the east, and a northern wind blows toward the south. This convention is sometimes counter-intuitive.
What is the Beaufort wind force scale?
The Beaufort wind force scale was created by Francis Beaufort to describe wind speed from observed sea conditions. It began as a 13-level scale numbered 0 through 12 and was expanded during the 1940s to 18 levels running 0 through 17. Gale-force winds within it lie between 28 kn and 55 kn.
How is wind speed measured?
Wind speed is measured by anemometers, most commonly using rotating cups or propellers. Sustained wind speeds are reported globally at a 10 m height and averaged over a 10-minute time frame. The United States uses a 1-minute average for tropical cyclones, while India typically reports a 3-minute average.
What is the solar wind and how fast does it travel?
The solar wind is a stream of charged particles, a plasma ejected from the Sun's upper atmosphere at a rate of 400 km/s, consisting mostly of electrons and protons with energies of about 1 keV. It creates the Heliosphere and causes geomagnetic storms, the aurorae, and the plasma tails of comets.
What is the fastest wind ever recorded in space?
The fastest wind ever recorded came from the accretion disc of the black hole IGR J17091-3624, blowing at 20,000,000 mph, which is 3% of the speed of light. The fastest wind on any known planet is on HD 80606 b, 190 light years away, where it blows at more than 11,000 mph or 5 km/s.