Explainer: The furious eye(wall) of a hurricane or typhoon | Science News for Students

Explainer: The furious eye(wall) of a hurricane or typhoon

From ‘mini-swirls’ to giant rolls of wind, the eyewall is where the real fury resides
Oct 12, 2018 — 12:38 pm EST
a radar map showing hurricane Harvey's eyewall

Doughnut of intense storm activity swirls about Harvey’s eyewall (green ring at center with smattering of red and yellow). This radar imagery captured hours before the hurricane’s landfall near Rockport, Texas on August 26, 2017. 

National Weather Service, GR2 Analyst, M. Cappucci

People often use the phrase “eye of the storm.” It’s a term that defines part of a hurricane. It’s that small zone of calm in the midst of chaos, ferocious rains and battering destruction. The wall of winds that swirl around this quiet respite are the polar opposite of this eye. Indeed, they lash out with the cyclone’s greatest fury.

That’s saying a lot, because even the outer regions of hurricanes combine Mother Nature’s wildest weather. Their winds can blow ferociously. When their direction is right, these can sweep destructive storm surges inland across coastlines. Their clouds can dump a meter (upwards of 3 feet) of rain — or more — on inland communities. Their unstable winds can even spawn tornadoes by the dozens.

Unstable air — turbulence and rising motion — is key to building and strengthening hurricanes.

The atmosphere naturally cools the farther away you rise from the planet’s surface. That’s why ice crystals may grow outside of windows of a cloud-level airplane — even when it’s a hot summer day at ground level. When the air near the ground is extra warm, it will rise up to pierce through some of the cooler air above. This can create a localized plume of rising air known as an updraft. That’s one surefire sign that the air is unstable.

Warm sea surface temperatures and fairly unstable air are major ingredients in the recipe for a hurricane. Those conditions can serve to fuel quickly rising storm clouds.

Scientists refer to hurricanes are barotropic (Bear-oh-TROH-pik). Such storms form from vertical instabilities. That means there is no real forcing mechanism to move the air sideways. Instead, the air plumes only blossom upwards thanks to extra-chilly air aloft.

To grow, a hurricane must suck in more air. This air spirals in a counterclockwise fashion toward the center. And as it nears the middle, the air accelerates faster and faster. It speeds up just as an ice skater does when she pulls in her arms and legs.

By the time a pocket of air approaches the center, it’s now howling at destructive speeds. This air loses heat to the storm. That energy flows to the cloud-free “eye” of the storm, then exits up and out the top. Inside the eye, the winds disappear. A bit of the air curls back down towards the ground and erodes any moisture, eating away at clouds. Sometimes blue skies appear directly overhead.

Circling just outside the eye are the winds that make up the eyewall. They’re the scariest, nastiest, gnarliest part of the storm. They form an unbroken line of extremely powerful downpours. In strong hurricanes, these winds can roar to 225 kilometers (140 miles) per hour.

an illustration of the structure of a hurricane
Here’s an artist’s depiction of the structure of a hurricane or typhoon. Warm air (pink ribbon) gets pulled into the bottom of the storm. It spirals up and out of the eye (center) where it cools (turns blue).
Kelvingsong/Wikimedia (CC BY 3.0)

Twirling masses of air

Despite how strong these storms are, one thing is often missing: lightning.

With a storm so intense, one would expect its clouds to trigger plenty of lightning. Most don’t. And it all has to do with the motion of the air pockets — known as parcels — spiraling into the eyewall.

Ordinary thunderstorms develop vertically, meaning upright from the ground. It’s a bit like a bubble of air rising from the bottom of a pan of boiling water. In hurricanes, however, there is so much rotational energy that the air doesn’t climb up directly. Instead, it takes a swirly, roundabout path.

a radar image showing Hurricance Harvey's tall storm clouds outside the eye of the storm
Radar data showing a  horizontal slice through Hurricane Harvey, last year. It shows intense, tall storm clouds on either side of a calm, tranquil eye. The diagram combines 16 horizontal scans and stitches them together as one vertical slice. This revealed the structure of the storm.
National Weather Service, GR2 Analyst, M. Cappucci

Parcels of air swirl slantwise into the storm, inward from all directions. All the while, they rise.

So while they reach the height of typical thunderstorms — 10 to 12 kilometers (6.2 to 7.5 miles) — the rising motion isn’t quite as strong, given that they’re circling like a merry-go-round. In order to spark lightning, there need to be lots of straight-up-and-down rising motion.

That’s why eyewalls only spit out sporadic bolts when a storm is intensifying — when more air is moving in the upwards direction rather than around and around. Scientists can actually gauge whether a storm is strengthening by probing how electrified its clouds are. (They do that by scanning those clouds with Doppler weather radar.)

But eyewalls don’t just produce winds with epic speed. Their winds also blow in many different directions.

Whirling fury may neighbor quiet zones

A typical hurricane eyewall tends to be about 16 kilometers (10 miles) thick. And as that eyewall moves across a site, the storm’s winds can explode within a matter of seconds.

When such strong winds hit land, they slow a bit. That’s due to friction. In the air well above us, there’s little to slow down rushing pockets of air. But near the ground, air masses can encounter all sorts of things. Trees, houses, cars and everything else serve as obstacles to the wind. Air passing over this lowest kilometer (0.6 mile) or so to the ground “feels” the effects of surface drag. That part of the atmosphere is known as the Ekman layer.

Due to the change in wind speed with height, there also can be friction between different layers of moving air. Scientists refer to this as wind shear. It’s a turning of the winds or a change in their speed with height.

Imagine you hold a pencil between your two hands. What would happen if you moved your hands in opposite directions? The pencil would rotate. The same thing happens to air masses within a storm.

We can’t necessarily see it. But people can certainly feel the results.

a radar scan of Hurricane Andrew
This radar scan of Hurricane Andrew in 1992 shows the super furious Cat-5 storm making landfall near Homestead, Fla. The location of the National Hurricane Center – NHC – is plotted. This was the last data received before the National Weather Service’s radar was destroyed by the storm. The catastrophically strong eyewall is visible as an unbroken band of dark red.
National Weather Service

During Hurricane Andrew in 1992, for instance, areas of extreme damage emerged in swaths next to strips of land that escaped relatively unharmed. Each alternating “stripe” was a few hundred meters (perhaps 1,000 feet) across. They could be a kilometer or two long. Engineers coined the term roll vortex to describe what they thought was happening.

A vortex is a spinning or rotating mass of air. Much like the pencil spun in your hands, researchers hypothesized that long tube-like horizontal vortices of air could develop in the Ekman layer of a hurricane. These invisible vortices could stretch a few kilometers, and span some 300 meters (1,000 feet) across.

Later research would show much larger and more oblong roll vortices forming in less intense hurricanes. The parallel rolls would line up a few kilometers apart. That’s according to Ian Morrison and Steven Businger, researchers at the University of Hawaii at Manoa in Honolulu. Near the ground, these tubes could enhance wind speeds — a lot. And sometimes, they would hover over the same site for hours on end. That explains why some neighborhoods can see wicked winds, while a nearby community could miss the action entirely.

Why don’t these vortices move along with the storm? Well, think about a stone in a river. Downstream of that rock or obstacle, a series of miniature rolls or ripples forms. Even though the river’s current is moving swiftly, interruptions in the flow can cause vortices to form on a largely unchanging spot above it. The same process is responsible for the formation of roll vortices in hurricanes. When houses, mobile homes or any structures “interrupt” the normal flow of wind, stationary vortices may emerge.

Twirling into true twisters

But that’s not the only oddity within the eyewall. Inside those internal storms that make up the eyewall, scientists have seen evidence of tornado-like vortices causing a ruckus.

It’s long been known that tropical storms that come ashore can generate tornadoes. Swarms of them can develop in the outer rain bands once a cyclone makes landfall. It’s all thanks to that wind shear within the storm. That shear effect tends to be strongest in the forward right quadrant (one-fourth) of the storm. The vorticity — or “spin energy” — in that region can cause individual thunderstorm cells to rotate. The result? A tornado emerges within a hurricane. And like Harvey in 2017, some tropical cyclones have become prolific tornado-makers.

But eyewall twisters are different. Tornadoes shouldn’t be able to form in this part of the hurricane. Renowned tornado expert Tetsuya “Ted” Fujita was called to weigh in on the unusual damage seen in the wake of 1992’s Hurricane Andrew. And Fujita discovered something novel — mystery whirlwinds.

Fujita called them mini-swirls.

Mini-swirls may look and act like a tornado, but they form differently. Even more novel: They’re not connected to the storm clouds above.

Sometimes, small eddies may form near the ground when the wind blows around an object. Hikers may observe little vortices of dust, grass or leaves meandering across a field on a windy day. Inside the hurricane though, these swirling eddies can grow. And grow. And grow.

Because an eyewall’s winds just above the ground are so strong, they exert an upward “pull” on the air near the ground. That can stretch the tiny vortex upward a few hundred meters (yards). Suddenly it’s not so tiny.

Angular momentum is a phrase that defines the energy in a moving object that rotates. Because angular momentum (energy) is conserved, wind speeds rise dramatically as the vortex is yanked up. (Remember that figure skater that twirls ever faster as she brings her arms and legs close in to her body.) That can lead to winds of up to 129 kilometers (80 miles) per hour.

That alone may not sound so high. But imagine getting hit by one of these rotating through an eyewall where the ambient winds were already moving at 193 kilometers (120 miles) per hour. That combination could produce narrow paths of destruction a few meters wide where winds would have briefly reached 322 kilometers (200 miles) per hour!

Because of how quickly mini-swirls move, they may only impact an area for a few tenths of a second. But that’s enough to cause extreme damage. These mini-cyclones within the cyclone were one big reason why Hurricane Andrew featured damage unlike typical hurricanes.

Evidence of mini-swirls also appeared in the devastation left across the Florida peninsula in 2017 by Hurricane Irma. One was caught live on television. Mike Bettes was roadcasting from Naples, Fla., when he found himself face to face with a mini swirl. At the time, this meteorologist for The Weather Channel was standing inside the eyewall of Irma.

“You were just in the eyewall of a hurricane,” noted an anchor from the TV station’s studio. Then suddenly a whirling mass of condensing water caused Bettes to lose his footing. Whipping across the street at incredible speed, the vortex slammed just meters (yards) away from Bettes. It eventually bent a palm tree and caused more offscreen damage. Bettes escaped unscathed.

Power Words

(for more about Power Words, click here)

accelerate     To experience a change in velocity (speed).

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.

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.

barotropic     (in meteorology) An adjective that refers to connecting air regions having the same air pressure (barometric pressure).

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

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. (in computing) A network of computers (hardware), known as servers, which are connected to the internet. They can be used to store data and computer programs (software) that can be accessed by one or many people at once, and from anywhere in the world.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

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

develop     To emerge or come into being, either naturally or through human intervention, such as by manufacturing.

drag     A slowing force exerted by air or other fluid surrounding a moving object.

Ekman layer     A region of the atmosphere near to Earth's surface that encounters horizontal friction as it passes over the surface (the sea surface, for instance, or maybe a house or trees).

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

eye      (in atmospheric sciences) The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud.

eyewall     Also known as a wall cloud, it’s an organized band or ring of cumulonimbus clouds that surround the eye, or light-wind center, of a tropical cyclone.

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.

gauge     A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.

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.

lightning     A flash of light triggered by the discharge of electricity that occurs between clouds or between a cloud and something on Earth’s surface. The electrical current can cause a flash heating of the air, which can create a sharp crack of thunder.

meteorologist     Someone who studies weather and climate events.

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.

novel     Something that is clever or unusual and new, as in never seen before.

parallel     An adjective that describes two things that are side by side and have the same distance between their parts. In the word “all,” the final two letters are parallel lines. Or two things, events or processes that have much in common if compared side by side.

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

radar     A system for calculating the position, distance or other important characteristic of a distant object. It works by sending out periodic radio waves that bounce off of the object and then measuring how long it takes that bounced signal to return. Radar can detect moving objects, like airplanes. It also can be used to map the shape of land — even land covered by ice.

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

spawn     To release or fertilize something (usually eggs) in the environment.

sporadic     An adjective that describes events that occur infrequently and at unpredictable intervals.

storm surge     A storm-generated rise in water above normal tidal level. In most cases, the largest cause of storm surge is strong onshore winds in a hurricane or tropical storm.

survey     (v.) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.

swarm     A large number of animals that have amassed and now move together. People sometimes use the term to refer to huge numbers of other things, even tornadoes.

tornado     A violently rotating column of air extending from the ground to a thunderstorm above.

tropical cyclone     A strong, rotating storm. These usually form over tropical areas around the equator where the water is warm. Tropical cyclones have strong winds of more than 119 kilometers (74 miles) per hour and usually have heavy rain. Large ones in the Atlantic are known as hurricanes. Those in the Pacific are termed typhoons.

turbulence     The chaotic, swirling flow of air. Airplanes that run into turbulence high above ground can give passengers a bumpy ride.

vertical     A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.

vortices     (singular: vortex) Swirling whirlpools 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.

wake     An area of disturbed air or water left behind an object (such as a boat or animal) moving through it.

water vapor     Water in its gaseous state, capable of being suspended in the air.

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: Wilmington experiencing its highest winds since 1960… National Weather Service Wilmington, NC. September 14, 2018. 

Blog: Greg Nordstrom. Hurricanes on the Mind! Eye of the Storm. LDTC Storm Chaser. June 8, 2010.

Journal: I. Morrison et al. An observational case for the prevalence of roll vortices in the hurricane boundary layer. Bulletin of the American Meteorological Society.  Vol. 62, August 2005, p. 2662. doi: 10.1175/JAS3508.1.

Report: J.L. Franklin, M.L. Black and K. Valde. Eyewall wind profiles in hurricanes determined by GPS dropwindsondes. National Hurricane Center Report. April 2000.

Journal: R.M. Wakimoto and P.G. Black. Damage survey of Hurricane Andrew and its relationship to the eyewall. Bulletin of the American Meteorological Society. Vol. 75.  January 1994, p. 189.