How Hurricanes Work: The Science Explained

What Are Hurricanes?

Hurricanes are tropical cyclonic storms with sustained wind speeds of at least 74 miles per hour. When determining whether the threshold has been reached to call a storm a hurricane, wind gusts are not considered.

Usually, there is a progression that starts with a wave, then a depression, then a tropical storm, and finally a hurricane.

A wave occurs when the wind flows in a wave-shaped pattern. Normally, air flows in a straight or slightly curved direction. When air begins to rise due to thunderstorm formation, air is attracted to the reduced pressure. While it is still flowing from the east in the tropics, it is deflected as it passes the area of disturbed weather. This is caused by the air being attracted to the area of pressure reduction. The airflow is deflected from just flowing directly to the disturbance by the Coriolis effect, the deflection caused by the Earth’s rotation. If the airflow passes the disturbance, it resumes its original path, giving the appearance of a wave.

If the air completes the circle as it encounters a disturbed weather feature, it becomes a depression.

When the wind speed reaches thirty-nine miles per hour, the system is called a tropical storm.

The Coriolis Force Depends on Latitude

The Coriolis force deflects moving air to the right in the northern hemisphere and to the left in the southern hemisphere. The reason is the air moves with the Earth as it makes one rotation every twenty-four hours. So, if it moves north away from the equator, the air literally outruns the Earth, and if it moves south, the Earth outruns the air.

The farther from the equator, the greater the Coriolis force. Also, air flowing closer to the north or south will have maximum deflection.

Simple Understanding of Basic Forces

Understanding a hurricane involves understanding a thunderstorm. Air rises. This can be caused by heating the air, making it lighter per volume. It can also be caused by two air masses coming together, whereby it is squeezed upward.

Along the equator is the Intertropical Convergence Zone, where air from the northern and southern hemispheres collide. This is, in essence, a frontal boundary. Also, equatorial heat makes the air lighter, so rising is enhanced. The result is tropical thunderstorms, large storms that occur daily.

The Intertropical Convergence Zone moves with the sun, encroaching into the northern hemisphere in the summer. When the encroachment is far enough north, the air from the northern hemisphere has a Coriolis force to change its direction to the right. The same is true for air crossing the equator, but notice it is moving north, so right is opposite air moving south. This can cause low pressure on the Intertropical Convergence Zone.

Not all hurricanes originate in the Intertropical Convergence Zone; other situations can cause an initial reduction in pressure and lead to hurricane development.

Once low pressure is established, air flows towards the area of lower pressure under the force of gravity. As air moves, the Coriolis force deflects the air. Ideally, this would cause airflow to become circular. Another force in play is friction, which slows the wind. This has no effect of gravity causing the air to be attracted towards the lower pressure but does reduce the Coriolis force, which is dependent on wind speed. The result of including friction is that the air does not flow in a circular path, but rather air spirals inward. When it reaches the convective eyewall, the air rises.

Need for Outflow

Rising air needs a place to go. If there is no place to move the air after it has risen, there would be a blockage preventing more air from rising. This means air must be able to flow out of the top of a hurricane.

Water as Fuel

As air rises it cools due to a drop in pressure and entering a cooler environment. Cooler air cannot hold as much water in vapor form. Condensation occurs. Condensation continues to occur as air cools further due to additional rising.

When water vapor liquifies it releases heat, which then warms the rising air making it more buoyant. This causes further rising, leaving a partial vacuum that invites more air on the surface to rise. As air continues to rise more liquid condenses, so the rising air becomes even more buoyant. Eventually, the liquid freezes, releasing additional heat.

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The simple explanation is that heat released from the water changing from the gaseous state of matter to the liquid state, and the liquid state to the solid state, is the fuel in a mighty heat engine, the hurricane.

Role of Shear

If there is shear it can cause the rising air in the thunderstorms to be blown away from the bases of the storms. This can hamper or even shut down the system.

African Dust

Dust from Sahara Dessert dust storms have significance. Far out over the Atlantic the dust reflects enough sunlight to lower the temperature of the water surface. This cools the air in contact with the surface of the water. The result is the air can hold less water vapor, so there is less fuel for the system.

What Happens in the Eye

The spiraling air does not flow into the eye, a region of calm. The eyewall with its monstrous thunderstorms forces the spiraling air upward. The strong updrafts entrain some air from the eye, which in turn causes air from aloft to descend in the eye. This air effectively keeps the eye dry, and the weather in the eye is calm.

Storm Surge

The intuitive idea was originally that the low pressure made the ocean act as a barometer, supporting a bulge of water in the center of a hurricane. Physics indicates reduced pressure can indeed support a slight bulge of water, but not enough to explain storm surge. Instead, the main cause of storm surge is wind sweeps waves inward toward the center of a hurricane, building a larger bulge of water. The barriers, such as land, cause the surge to grow at landfall.

Location of Highest Winds

Wind speed in a rotating system that also is moving as a unit has two parts to consider—the tangential velocity of the rotating storm and the motion of the storm. In the northern hemisphere, the right side of a storm has tangential velocity in the same direction as the motion of the storm. Relative to the ground, the wind is the sum of the tangential velocity and the forward velocity.

On the left side of the storm, the tangential velocity is in the opposite direction when compared with the forward motion of the storm. On the left side of the storm, the forward motion must be subtracted from the tangential velocity.

The strongest winds are on the right side of a hurricane.

Tornadic Sector

The forward right quadrant of a landfalling hurricane often spins up tornadoes. These are generally, but not always, weaker EF0 and EF1 tornadoes. One thing to think about is these tornadoes can form a great distance from the center of the storm.

Why the Path Curves to the Right

Hurricanes cannot enter high-pressure regions. They are forced away by air flowing out from high-pressure regions. They can move around the perimeter of the high pressure.

At about thirty degrees from the equator, some air that has risen at the Intertropical Convergence Zone and some air that has risen from a band of lower pressure about forty-five degrees from the equator meet aloft. This air is colder and denser after losing heat, so it subsides as high pressure. This happens over a string of high-pressure domes that have breaks between them. Hurricanes often move through the gaps between high-pressure domes.

Whether following the curve of a high-pressure dome or moving towards a low-pressure region that can attract a hurricane, the path a hurricane takes generally curves to the right. This deflection to the right is a result of the Coriolis force.

Fluctuations

Hurricanes fluctuate in intensity. As they expand and contract over time, the maximum wind speed drops and intensifies. The cause is related to an ice skater spinning. When the arms of a skater are extended, the rotation slows; when held in close to the body, the rotation rate increases. The cause is the conservation of angular momentum. Compacting hurricanes intensify, and spreading hurricanes generally weaken.

During fluctuations, eyewalls can collapse and new eyewalls form. This results in the hurricane’s eye changing size.

Effect of Higher Wind Speed

Comparing two hurricanes with different windspeeds, it might seem a storm with a one hundred fifty mile an hour windspeed will do about twice the damage as a hurricane with a seventy-five mile an hour windspeed. That is not so. The hurricane with twice the windspeed as the other has winds with four times the kinetic energy as the other. Damage has more to do with energy than with force.

Citations

NOAA often explains some of these things during weather bulletins when a hurricane approaches. Information has been retained from these comments.

NOAA has much published information that support this article at https://www.noaa.gov/education/resource-collections/weather-atmosphere/hurricanes.

The NOAA YouTube material has also been used.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.