Explainer: Winds and where they come from | Science News for Students

Explainer: Winds and where they come from

Here’s an explanation for how and why the wind blows
Jan 22, 2018 — 6:30 am EST
windy palm trees
Temperature and pressure are critical factors in the creation of a windy day.
behindlens/iStockphoto

Hear that flag snapping sharply against the flagpole? See those kites flying high overhead? Feel that cooling breeze coming in off of the water?

Wind is all around us. It arrives in many shapes and forms. Wind may be an elegant mood-setter or a furious early warning of a dangerous storm. Although few people give much thought to wind — unless it’s threatening — those rivers of moving air drive the weather in ways that rule our environment.

There are many different types of wind. Each forms in different ways. But essential to all are changes in air pressure.

weather map
Zones of high (H) and low (L) pressure are labeled on this weather map.
NOAA/Wikimedia Commons

TV weather forecasters regularly point on maps to areas of high and low pressure. And that makes sense because changes in air pressure are what lead to wind — the flow of air. In fact, wind is Mother Nature’s way of equalizing differences in air pressure.

Air pressure is the force that air exerts toward whatever contains it. The pressure of air in a balloon is higher than that of the air outside. That’s why most of the air will leave a balloon whenever it gets a hole. When it comes to the atmosphere, air pressure describes the weight of air over a given site. It is determined by that parcel of air’s temperature, volume and density.

Expanding air produces regions of “high pressure.” These push nearby air away. Contracting air creates zones of “low pressure.” They pull nearby air inward. That’s why the wind blows: It moves from regions of high pressure to those where pressure is lower. The zone between the high- and low-pressure areas is known as a pressure gradient, or a zone over which the pressure varies from high to low.

Thermal wind balance

Thermal wind is the first of four main types of atmospheric flow. The most complex type of wind, it drives weather systems across the globe. It’s born from differences in the temperatures between the equator and the poles.

Picture a column of air from the ground to the top of the troposphere (TRO-puhs-sfeer) — that layer of atmosphere in which we live. As the sun beats down on it, this air heats up and expands. That makes the top of the column rise. This is common near the equator. If a column of air cools, such as at the poles, it contracts and shrinks. That same stack of air — still weighing the same amount — will now be shorter and denser.

This means that imaginary surfaces of constant density slope down toward the poles. That slope isn’t constant. These lines rise up and down like bumps and wrinkles in a blanket, depending on local conditions. But the general downward slope allows masses of air to slide toward the poles.

Thermal wind is what is created as those masses flow down this slope, carrying heat away from the equator. Meteorologists refer to this natural movement of solar energy out of the equator as “poleward heat transport.” Without it, most folks living outside the tropics would be buried beneath a sheet of ice. The equator would also be hot as a furnace.

As sun-warmed air rises near the equator and begins to move toward the poles, it also starts to drift eastward. This is due to Earth’s spin. It swirls the air from west to east around the planet.

a diagram showing how the Earth's spin causes air to flow in the Northern and Southern hemispheres
The Earth’s spin causes air to flow a bit to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
NOAA

That poleward-moving air also speeds up — dramatically. This is because Earth is an oblique (Oh-BLEEK) spheroid. If you took horizontal slices of the planet, those slices would be widest at the equator and narrowest at the poles. As Earth’s radius “shrinks” as one approaches the poles, the air has to speed up. This is because the air gets funneled into a smaller and smaller path. As it does so, its flow rate increases. (This process is due to what’s known as the conservation of angular momentum.) In the Northern Hemisphere, this makes the air flow to the right with increasing speed. This swirling action is known as the Coriolis force.

Earth’s rotation and the change in the planet’s radius mean that moving air will always want to turn a bit to the right in the Northern Hemisphere (and the opposite direction in the Southern Hemisphere). This affects everything. A football tossed from one end of a stadium to another will naturally deflect 1.26 centimeters (a half inch) to the right! It’s also why winds in the upper atmosphere are relatively weak near the equator. Closer to the mid-latitudes, they howl. They’ve curved so much to the right that they often are speeding eastward at an impressive clip.

The jet stream

This is how the jet stream forms. This current of air snakes around the planet at speeds greater than 322 kilometers (200 miles) per hour. It’s found winding its way directly overhead of the strongest temperature contrasts at the surface.

This temperature gradient creates a steep density “hill” in the atmosphere where the air quickly sloshes down. The more rapidly it moves, the more the northern jet stream curves east. It’s just like riding a bicycle down a hill: The steeper the slope, the faster you go.

But as the air moves poleward, it never actually gets to the poles. Instead, it curves to the right rapidly because of Earth’s rotation and that Coriolis force. As a result, the jet stream meanders as it circles the Earth in each hemisphere. In the North, it moves air west to east in a circle around the mid-latitudes (and the opposite in the Southern Hemisphere), changing its path from season to season.

Poleward of the jet stream, the atmosphere is turbulent. Dozens of “eddies” of high and low pressure rotate around the globe, dragging wacky weather with them. On the equator side, the flow is described as “laminar.” That means it’s relaxed, and not chaotic.

Along this temperature boundary, a fierce atmospheric battleground develops. Colliding air masses of different temperatures spin up cyclones and other severe weather. Indeed, that’s why meteorologists refer to the jet stream’s position as a “storm track.”

The position of the jet stream influences the type of weather a region encounters. Consider the Northern Hemisphere, for instance. From December through February, the sun doesn’t reach the North Pole. This allows an extensive dome of super-cold air to bank up nearby. Atmospheric scientists refer to this flowing pool of cold air and low pressure as the polar vortex. It swells in size during winter. And when this flow of cold air surges southward, it pushes the jet stream into southern Canada and the northern United States. That can bring seemingly endless snowstorms to the upper Midwest and Northeast during the dead of winter.

Geostrophic winds

In summer, the poles warm. This weakens the temperature gradient between these zones and the equator. The jet stream responds by retreating some 1,600 kilometers (a thousand miles) northward. Now, the weather in the lower 48 U.S. states calms down. Sure, scattered thunderstorms erupt from time to time. But there are no huge storm systems spanning 1,600 kilometers or more to influence day-to-day events. Instead, the weather becomes geostrophic (GEE-oh-STRO-fik) — meaning relatively tranquil.

lightning
Summer can bring thunderstorms that light up the night sky. In cooler months, this risk of huge storm systems tends to diminish.
Jurkos/iStockphoto

Ordinarily, air would flow from high pressure to low pressure. It would move across a pressure gradient. So the driving force would be known as the pressure gradient force. But the Coriolis force is still at play. So as parcels of air try to move down the gradient, they’re tugged to the right in the Northern Hemisphere (and the opposite direction in the southern one). These two forces cancel out. Like a perfectly-matched game of tug-of-war, the air isn’t yanked in either direction. It just meanders slowly around large pressure systems.

As a result, the air ends up circling around high- or low-pressure systems without moving toward or away from them. Closer to the surface, the flow is slightly ageostrophic (meaning the winds are no longer in complete balance), due to the effects of friction with things at or near the surface.

Other large-scale wind-balancing effects

Sometimes, however, a low-pressure system spins so fast that a third force develops. It’s the same outward shove you feel on a merry-go-round or a vehicle rounding a corner. This is centrifugal force.

Rings of air in constant balance between these two forces spin around a storm’s center indefinitely. Their rather constant distance from the center is due to what’s known as cyclostrophic (Sy-klo-STROW-fik) balance. This represents a harmony — complementary actions — of the pressure-gradient and centrifugal forces.

On rare occasions, the Coriolis, centrifugal and pressure-gradient forces can all counteract one another. This perfect trifecta marks what scientists call gradient wind balance. It’s not worth a lot of fanfare. It does, however, dictate which way air parcels will move along the outer edges of a cyclone, any spinning column of air.

Clearly, there are a lot of moving parts that control the way the wind blows.

Local winds

The last category of winds are the ones you experience every day. And they’re different depending on where you are. Head down to the beach, for instance. On sunny days in the afternoon, air over land warms and rises. Cooler air sitting above the ocean rushes in to coastal regions, filling the void caused by the air rising over land.

This generates a line of puffy little cumulus (KEWM-u-lus) clouds that die out after the sun sets. Along peninsulas like Florida, colliding sea breezes can result in convergent winds. These colliding air masses force pockets of moist air high up into the atmosphere, forming thunderstorms. That’s why folks in the Southeast always carry umbrellas, even on sunny mornings. The “self-destruct” sunshine routinely generates scattered afternoon boomers.

Florida storm
Afternoon thunderstorms such as this one are common in Florida.
Marc Averette/Wikimedia Commons (CC BY 3.0)

The same process that sparks these storms reverses overnight. Since the ground cools faster than the water, the direction of the flow of air reverses. Instead of a sea breeze, a “land breeze” develops. Now, storms move out from the land, to the ocean. That’s the reason many people along the Gulf Coast can enjoy gorgeous offshore displays of evening lightning.

Wind also can vary locally along stationary fronts. These are the very sharp boundaries between regions of warm and cold air. Sometimes, stationary fronts can become hung up in valleys. When they do, the warm and cold air masses — winds — can slosh back and forth. Like water and oil in a bowl, they don’t mix. Instead, they just push each other back and forth like angry ocean waves. This can trigger dramatic temperature swings within short periods of time.

One particularly noteworthy example came from the Black Hills of South Dakota on January 22, 1943. A stationary front had established itself along the foothills in the western part of the state. According to the local National Weather Service office in Rapid City, the temperature skyrocketed from -20° Celsius (-4° Fahrenheit) at 7:32 a.m. to 7.2 °C (45 °F) just two minutes later. That afternoon, as the front retreated, over a span of just 27 minutes the temperature plummeted 32.2 degrees C (58 degrees F).

Similar wild swings in the mercury were noted across that region throughout the afternoon. Motorists reportedly had trouble driving because their windshields would fog over — or even crack — when crossing between warm and cold pockets. (Imagine trying to dress for the weather that day.)

Regardless of where you are or what season it is, the wind holds a lot of information. Its direction, temperature and speed all offer valuable clues about the state of the atmosphere. Next time you’re outside, take a second to pay attention to Mother Nature. There’s a lot she has to tell you if you note what’s blowin’ in the wind.

The jet stream (red) meanders over a 30-day period in this NASA visualization of atmospheric winds in the Northern Hemisphere.
EarthDirect/NASA

Power Words

(for more about Power Words, click here)

air masses     Large volumes of air, sometimes covering many hundreds or thousands of square kilometers (square miles), that typically have a consistent temperature or water-vapor content. Air masses are often classified by their source, such as continental, arctic or tropical. Air masses and other weather systems are steered across Earth’s surface by jet streams and by differences in atmospheric pressure.

air pressure     The force exerted by the weight of air molecules.

angular momentum     A moving object is said to have momentum. When that object is rotating, the term becomes angular momentum. And that movement will not alter its speed unless acted on by another force.

atmosphere     The envelope of gases surrounding Earth or another planet.

axis     The line about which something rotates. On a wheel, the axis would go straight through the center and stick out on either side. (in mathematics) An axis is a line to the side or bottom of a graph; it is labeled to explain the graph’s meaning and the units of measurement.

centrifugal force     A force that seems to pull a rotating body — or something on a rotating object (such as a rider of an amusement park ride) — away from the center of rotation.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

complementary     To match or fit with something else to complete it.

conservation of angular momentum     The angular momentum of a system — the energy used to power rotation — will not change unless acted on by some outside force. This holds even as movements within the system may change (for instance, as a rotating figure skater extends her arms and legs toward the body or out from it).

constant     Continuous or uninterrupted.

convergent     (v. converge) An adjective for things that are approaching each other; coming together.

Coriolis force (or Coriolis effect)    An apparent force that’s due to Earth's rotation on its axis. This movement shifts moving objects (such as airborn footballs or winds) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

cumulus     A type of especially puffy cloud that sometimes look like floating mounds of cotton. Its base may be flat and fairly close to the ground (just 1,000 meters, or 3,300 feet up). The cloud’s top crown tends to look like rounded towers. the head of a cauliflower, it is called cumulus congestus or towering cumulus. When these clouds develop into giant moisture-filled towers, they evolve into what’s known as cumulonimbus clouds, the type from which thunderstorms emerge.

current     A fluid — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of electricity moving through some point over a particular period of time.

cyclone     A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.

cyclostrophic       An adjective that describes winds whose motions are held in balance by the matched, counteracting effects of the local pressure-gradient force and the rotational (centripetal) force.

density     The measure of how condensed some object is, found by dividing its mass by its volume.

eddy     A circular motion that develops in some liquid or gas and that moves in a direction opposite to the main current. This may create a whirlpool.

equator     An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

friction     The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage a surface of some material as it rubs against another.

fronts     (in Earth sciences) The boundaries that separate air masses — parcels of air having very different densities. Fronts extend both horizontally (up and down) and vertically (along a plane).

geostrophic     (in meteorology) A term for winds whose direction and speed are determined by a balance between the pressure-gradient force (due to air pressure) and the Coriolis force created by Earth’s rotation.

gradient     From the word “grade,” it describes the incline, slope or degree of increase in some measure (such as temperature, pressure or even color) that develops as one moves in time, position or along some scale.

horizontal     A line or plane that runs left to right, much as the horizon appears to do when gazing into the distance.

hurricane     A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.

jet stream     A fast-flowing, high-altitude air current. On Earth, the major jet streams flow from west to east in the mid-latitude regions of the Northern and Southern Hemispheres.

laminar     A flow that takes a smooth, linear path. It’s the opposite of a turbulent path.

latitude     The distance from the equator measured in degrees (up to 90).

literally     A term that means precisely what it says. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

meander     A bend in a stream or to move slowly and with no straight path in mind.

mercury     Sometimes called quicksilver, mercury is an element with the atomic number 80. At room temperature, this silvery metal is a liquid. Mercury is also very toxic. Because it was used in many old-style thermometers, “mercury” can also be used as a slang term for temperature.

meteorologist     Someone who studies weather and climate events.

mid-latitudes     That part of the globe that lies midway between Earth’s tropical and polar regions. Most people live in these temperate regions and most of the world’s food is produced here.

momentum     A measure of the motion of something, made by multiplying its mass and velocity. Changing the speed or direction of an object will also alter its momentum.

oblique     An adjective that describes a direction meaning, off at an angle. It is neither parallel nor perpendicular to something, but at a slant from it.

peninsula     A parcel of land that is that is attached to the mainland but surrounded by water on three sides.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

polar vortex     A semi-permanent weather system involving a large air mass in Earth’s upper atmosphere. It consists of an area of low atmospheric pressure. In the Northern Hemisphere, this tends to center near Canada's Baffin Island and over northeast Siberia. Winter strengthens the vortex, because that’s when the temperature difference between the poles and mid-latitudes is greatest.

poles     (in Earth science) The cold regions of the planet that exist farthest from the equator.

poleward     A term for direction, meaning toward the poles (such as the North Pole or South Pole).

pressure     Force applied uniformly over a surface, measured as force per unit of area.

pressure gradient force     A force that initiates the movement of air masses from areas of higher pressure to areas of lower pressure.

radius     A straight line from the center to the circumference of a circle or sphere.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

slope     (in mathematics) The degree to which some line rises or falls from a strictly horizontal direction. A line that appears to rise as it moves to the right has a positive slope. One that appears to fall as runs to the right has a negative slope. Vertical lines have neither. Their slope is described as undefined.

spheroid     A term for something that is roundish, but not perfectly spherical.

stationary front     (in meteorology) The site of the boundary between two slowly moving (or even static) air masses, each of which has very different temperatures and which do not mix.

subtle     Some feature that may be important, but can be hard to see or describe. For instance, the first cellular changes that signal the start of a cancer may be visible but subtle — small and hard to distinguish from nearby healthy tissues.

thermal     Of or relating to heat. (in meteorology) A relatively small-scale, rising air current produced when Earth’s surface is heated. Thermals are a common source of low level turbulence for aircraft.

tropics     The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.

troposphere     The lowest level of Earth's atmosphere. It runs from the planet's surface to a height of  6 to 18 kilometers (4 to 11 miles), depending on the latitude. It's the region where the air is thickest and where most weather occurs. Air currents moving through this region often flow not only horizontally, but often vertically (up and down).

turbulent     (n. turbulence)  An adjective for the unpredictable fluctuation of a fluid (including air) in which its velocity varies irregularly instead of maintaining a steady or calm flow.

void     An empty space or cavity.

vortex     (plural: vortices) A swirling whirlpool of some liquid or gas. Tornadoes are vortices, and so are the tornado-like swirls inside a glass of tea that’s been stirred with a spoon. Smoke rings are donut-shaped vortices.

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

Citation

Report: National Weather Service. The Black Hills Remarkable Temperature Change of January 22, 1943. Weather Forecast Office, Rapid City, S.D.