What is wind? Where does it come from, and why does it blow? Why are storm winds so strong? Are winds on other planets like they are on Earth? Read on for answers to these "blue-sky" questions and more!
Earth, Wind, and Water
The simplest definition of wind is air in motion. Wind is generated by uneven pressure in the atmosphere, which is caused by uneven heating by the Sun, land, and oceans.
The air closest to the ground is also the warmest, but warmer air rises, and cooler air sinks. At the same time, Earth's rotation makes air masses move toward and away from the poles because of the Coriolis effect. Atmospheric friction from our planet's rotation makes air masses and storm systems twist counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Between these two powerful drivers of circulation, Earth's atmosphere is constantly cycling and changing.
Earth has five major wind zones, organized by latitude. From the poles to the equator, we have the polar easterlies, the westerlies, the horse latitudes, the trade winds, and the doldrums.
Prevailing winds are masses of air that travel in a single direction over a specific area of the Earth. Generally, prevailing winds blow east-west rather than north-south. Trade winds are permanent east-to-west prevailing winds circulating between the horse latitudes and the equator. The horse latitudes are a region of high pressure that roughly overlaps with the tropics, which (because high pressure pushes outward) tends to "shed" air masses. Winds toward the south of it are pushed toward the equator, while those on its northern side become the westerly winds, which veer off toward the poles—dragging tropical and extratropical cyclones along with them.
The term 'horse latitudes' comes from a bit of grim nautical history. During the period when the Spanish were colonizing the Americas and West Indies, many ships carried horses as part of their cargo. Within about thirty degrees of the equator, winds tend to be calm, and pressure tends to be high, which means weather systems don't linger. (This is one factor that pushes hurricanes away from the poles and toward temperate latitudes.) Clear weather is nice, but if the winds don't blow, a sailing ship could stall for days or weeks. Becalmed crews often ran out of drinking water. Faced with dying of thirst, according to legend, sailors on these stricken ships would sometimes throw the horses they were transporting overboard.
Storm Winds
What causes hurricanes? Why are hurricane and tornado wind speeds so high? If a hurricane is a tropical cyclone, is there such a thing as a temperate cyclone? The answer to all these questions lies in how the Coriolis force imposes itself on the atmosphere. Spiraling wind currents become convection cells and then storms that can explode in intensity in a matter of hours.
Hurricanes
Hurricanes, also known as tropical cyclones, form only in the tropics, where the top 50 meters of ocean is at least 80 degrees Fahrenheit. For hurricanes that form in the Atlantic Ocean, westerly winds blow warm air laden with dust from the Sahara out into the Atlantic. As the wind passes over the surface of the ocean, some water evaporates (turns into water vapor) and rises. As it rises, it cools and creates a low-pressure zone.
Some water droplets condense and nucleate around particles of desert dust, creating towering cumulonimbus storm clouds. As more and more droplets collide, they fall as rain. Meanwhile, friction between droplets and dust granules produces lightning and thunder. Sometimes, this just creates powerful storms—but when the Coriolis effect takes over, a vortex can develop that turns a storm front into a hurricane.
As air is sucked into the low-pressure center of the storm, momentum builds; the vortex tightens, and wind speeds increase. The Coriolis effect shapes those winds into a spiral, creating a hurricane's characteristic shape. In fact, hurricanes depend so much on the Coriolis effect that within five degrees of the equator, where the effect is much reduced, hurricanes struggle to form. Eventually, once a hurricane has exhausted its energy over land, the storm will dissipate, its remnants blowing back out to sea.
Temperate Cyclones and the Jet Stream
The very same Coriolis forces that create vortices in Earth's atmosphere in the tropics also create vortices at temperate latitudes, often over land, called temperate cyclones or extratropical cyclones. Like hurricanes, these mid-latitude storm systems often have a counter-clockwise circulation and even a defined 'eye.' They're just as powerful as hurricanes, too. Extratropical cyclones were responsible for the great blizzard of 1888, the sinking of the Edmund Fitzgerald, and the 1991 Perfect Storm.
A narrow and fast-moving wind current called the polar jet stream, which circulates at the margins of the polar vortex, affects the path of storm fronts and extratropical cyclones. One important storm track for extratropical cyclones produces the Chinook winds, which originate and carry strong storms known as Alberta clippers that track across the Canadian interior and the upper Midwest. If a northeasterly wind such as the jet stream should catch a storm system and direct it up the eastern seaboard, we often call that storm a nor'easter.
Atmospheric Rivers
In 2022, the eruption of an underwater volcano in the South Pacific threw enough water vapor into the atmosphere to alter its composition and influence weather patterns around the globe. Prevailing wind currents carried that moisture for thousands of miles, creating continent-spanning channels of clouds that could drop a month's worth of rain or more in just a few hours. These storms, called atmospheric rivers, are almost too big to be called storms at all. When an atmospheric river takes a path over land, the monsoon-class rainstorms it can spawn are among the heaviest rainfall events known.
Winds on Other Planets
What is the wind like on other planets? Mars and other rocky planets with atmospheres have winds much like those of Earth. Photos from the spacecraft we've sent to orbit and explore Mars show that even with Mars' rarefied atmosphere, its winds can carry enough particulate to erode rocks, raise sand dunes—and bury a Mars lander's solar panels in ultra-fine dust.
On a gas giant, the situation is different. Based on what we know about Jupiter and its kin, gas giants don't have surfaces like Earth has a surface, with a defined transition between solid crust and atmosphere. Bands of wind encircle gas giants, moving at hundreds, even thousands, of miles per hour. NASA's Juno spacecraft captured this image of Jupiter's wind belts in September of 2023:
Gas giants are nearly perfect spheres, so their wind belts can start to have strange effects on the planets' outer layers, something like a Spirograph. In our own solar system, Jupiter and Saturn both have striking geometric storms at their poles.
Saturn's polar hexagon and the titanic cyclones around the poles of Jupiter are generated by the same forces that circulate Earth's atmosphere and oceans.
The Solar Wind
Powered by internal fusion engines, stars like our Sun convert their own mass to energy, which they radiate away into space as the solar wind. The solar wind is created by the outward expansion of plasma (a collection of charged particles) from the Sun's corona (the outermost layer of its atmosphere). This plasma is continually heated by the Sun's internal fusion engine, so much that its energy is enough to push it outward against the Sun's gravity. It then travels along the Sun's magnetic field lines that extend radially outward. The same radial lines determine the direction of a comet's ion tail.
Normally, Earth's magnetic field shields us from most space weather and the solar wind. However, the Sun's surface activity waxes and wanes through an 11-year solar cycle (which will reach its next peak in 2025). Sometimes turbulence in the Sun's outer layers creates "coronal holes," magnetic disturbances so large they could swallow the Earth whole. When those magnetic disturbances finally equalize, they often release a coronal mass ejection. Moving at about a million miles per hour, these gouts of plasma can reach as far as Mars, carried on the outward stream of solar wind.
Sometimes, because of how a sunspot is pointing when it lets go, a coronal mass ejection threatens humans on Earth. In 1859, a giant solar explosion known as the Carrington Event shut down electrical systems on the entire sunward side of the planet. The Carrington Event is the most intense geomagnetic storm in recorded history. During the event, the aurora borealis was visible as far south as central Mexico. It was so bright that people reported being able to read a newspaper by its light, even in the middle of the night.
“There was a ghastly splendor over the horizon of the North, from which fantastic spires of light shot up, and a rosy glow extended, like a vapor tinged with fire, to the zenith,” wrote the Cincinnati Daily Commercial at the time. Telegraphs threw sparks and operated by themselves, allowing operators to carry on conversations for several hours without their telegraphs connected to any power or battery.
Ultimately, stellar and terrestrial winds are possible because gases and liquids obey the laws of moving fluids. Complex interactions between surface geometry, temperature, moisture, and the atmosphere's composition all influence the wind patterns on Earth's surface at any given time.
Then again, Calvin's dad might be right.
