Hitting the Books: How hurricanes work

Hurricane season is currently in full swing across the Gulf Coast and Eastern Seaboard. Following a disconcertingly quiet start in June, meteorologists still expect a busier-than-usual stretch before the windy weather (hopefully) winds down at the end of November. Meteorologists like Matthew Cappucci who, in his new book, Looking Up: The True Adventures of a Storm-Chasing Weather Nerd, recounts his career as a storm chaser — from childhood obsession to adulthood obsession as a means of gainful employment. In the excerpt below, Cappucci explains the inner workings of tropical storms.

Excerpted from Looking Up: The True Adventures of a Storm-Chasing Weather Nerd by Matthew Cappucci. Published by Pegasus Books. Copyright © 2022 by Matthew Cappucci. All rights reserved.

Hurricanes are heat engines. They derive their fury from warm ocean waters in the tropics, where sea surface temperatures routinely hover in the mid- to upper-eighties between July and October. Hurricanes and tropical storms fall under the umbrella of tropical cyclones. They can be catastrophic, but they have a purpose—some scholars estimate they’re responsible for as much as 10 percent of the Earth’s annual equator-to-pole heat transport.

Hurricanes are different from mid-latitude systems. So-called extratropical, or nontropical, storms depend upon variations in air temperature and density to form, and feed off of changing winds. Hurricanes require a calm environment with gentle upper-level winds and a nearly uniform temperature field. Ironic as it may sound, the planet’s worst windstorms are born out of an abundance of tranquility.

The first ingredient is a tropical wave, or clump of thunderstorms. Early in hurricane season, tropical waves can spin up on the tail end of cold fronts surging off the East Coast. During the heart of hurricane season in August and September, they commonly materialize off the coast of Africa in the Atlantic’s Main Development Region. By October and November, sneaky homegrown threats can surreptitiously gel in the Gulf of Mexico or Caribbean.

Every individual thunderstorm cell within a tropical wave has an updraft and a downdraft. The downward rush of cool air collapsing out of one cell can suffocate a neighboring cell, spelling its demise. In order for thunderstorms to coexist in close proximity, they must organize. The most efficient way of doing so is through orienting themselves around a common center, with individual cells’ updrafts and downdrafts working in tandem.

When a center forms, a broken band of thunderstorms begins to materialize around it. Warm, moist air rises within those storms, most rapidly as one approaches the broader system’s low-level center. That causes atmospheric pressure to drop, since air is being evacuated and mass removed. From there, the system begins to breathe.

Air moves from high pressure to low pressure. That vacuums air inward toward the center. Because of the Coriolis force, a product of the Earth’s spin, parcels of air take a curved path into the fledgling cyclone’s center. That’s what causes the system to rotate.

Hurricanes spin counterclockwise in the Northern Hemisphere, and clockwise south of the equator. Though the hottest ocean waters in the world are found on the equator, a hurricane could never form there. That’s because the Coriolis force is zero on the equator; there’d be nothing to get a storm to twist.

As pockets of air from outside the nascent tropical cyclone spiral into the vortex, they expand as barometric pressure decreases. That releases heat into the atmosphere, causing clouds and rain. Ordinarily that would result in a drop in temperature of an air parcel, but because it’s in contact with toasty ocean waters, it maintains a constant temperature; it’s heated at the same rate that it’s losing temperature to its surroundings. As long as a storm is over the open water and sea surface temperatures are sufficiently mild, it can continue to extract oceanic heat content.

Rainfall rates within tropical cyclones can exceed four inches per hour thanks to high precipitation efficiency. Because the entire atmospheric column is saturated, there’s little evaporation to eat away at a raindrop on the way down. As a result, inland freshwater flooding is the number one source of fatalities from tropical cyclones.

The strongest winds are found toward the middle of a tropical storm or hurricane in the eyewall. The greatest pressure gradient, or change of air pressure with distance, is located there. The sharper the gradient, the stronger the winds. That’s because air is rushing down the gradient. Think about skiing — you’ll ski faster if there’s a steeper slope.

When maximum sustained winds surpass 39 mph, the system is designated a tropical storm. Only once winds cross 74 mph is it designated a hurricane. Major hurricanes have winds of 111 mph or greater and correspond to Category 3 strength. A Category 5 contains extreme winds topping 157 mph.

Since the winds are derived from air rushing in to fill a void, or deficit of air, the fiercest hurricanes are usually those with the lowest air pressures. The most punishing hurricanes and typhoons may have a minimum central barometric pressure about 90 percent of ambient air pressure outside the storm. That means 10 percent of the atmosphere’s mass is missing.

Picture stirring your cup of coffee with a teaspoon. You know that dip in the middle of the whirlpool? The deeper the dip, or fluid deficit, the faster the fluid must be spinning. Hurricanes are the same. But what prevents that dip from filling in? Hurricane eyewalls are in cyclostrophic balance.

That means a perfect stasis of forces makes it virtually impossible to “fill in” a storm in steady state. Because of their narrow radius of curvature, parcels of air swirling around the eye experience an incredible outward-directed centrifugal force that exactly equals the inward tug of the pressure gradient force. That leaves them to trace continuous circles.

If you’ve ever experienced a change in altitude, such as flying on an airplane, or even traveling to the top of a skyscraper, you probably noticed your ears popping. That’s because they were adjusting to the drop in air pressure with height. Now imagine all the air below that height vanished. That’s the equivalent air pressure in the eye a major hurricane. The disparity in air pressure is why a hurricane is, in the words of Buddy the Elf, “sucky. Very sucky.”

Sometimes hurricanes undergo eyewall replacement cycles, which entail an eyewall shriveling and crumbling into the eye while a new eyewall forms around it and contracts, taking the place of its predecessor. This usually results in a dual wind maximum near the storm’s center as well as a brief plateau in intensification.

In addition to the scouring winds found inside the eyewall, tornadoes, tornado-scale vortices, mini swirls, and other poorly understood small-scale wind phenomena can whip around the eye and result in strips of extreme damage. A mini swirl may be only a couple yards wide, but a 70 mph whirlwind moving in a background wind of 100 mph can result in a narrow path of 170 mph demolition. Their existence was first hypothesized following the passage of Category 5 Hurricane Andrew through south Florida in 1992, and modern-day efforts to study hurricane eyewalls using mobile Doppler radar units have shed light on their existence. Within a hurricane’s eye, air sinks and warms, drying out and creating a dearth of cloud cover. It’s not uncommon to see clearing skies or even sunshine. The air is hot and still, an oasis of peace enveloped in a hoop of hell.

There’s such a discontinuity between the raucous winds of the eyewall and deathly stillness of the eye that the atmosphere struggles to transition. The eyes of hurricanes are often filled with mesovortices, or smaller eddies a few miles across, that help flux and dissipate angular momentum into the eye. Sometimes four or five mesovortices can cram into the eye, contorting the eyewall into a clover-like shape. That makes for a period of extraordinary whiplash on the inner edge of the eyewall as alternating clefts of calamitous wind and calm punctuate the eye’s arrival.