Why does a hurricane have an eye?

Hurricanes are interesting storms. While only present in parts of our world, other areas can get hurricane force winds – wind force 12, or 12 Beaufort, if you will. But experiencing a true hurricane is only reserved for some parts of the world – fortunately, perhaps.

The eye of hurricane Maysak, from the ISS, in 2015. Photo: Terry Virts/NASA/ISS, licensed to be in the public domain.

An interesting question that aligns with this weather phenomenon is maybe the next one: why does a hurricane have an eye? We know that hurricanes can be distinguished from regular storms because of this eye formation. But why does it happen? How does it emerge?

Very interesting questions, which we’ll dive into in this article 🙂

The nascence of a hurricane

Many parts of our world face the impact of hurricanes. Although the name hurricane is very common in the Western world, other parts of the world call them typhones – or cyclones, if you will. It’s important to understand that in all cases, we mean the same thing: a very powerful storm that can cause significant damage.

Most hurricanes live near the Equator. Hence, for hurricanes, that’s the Carribbean and parts of the United States. Very often, they pass alongside the U.S. east coast, diminishing in power. Subsequently, they appear in the regular pattern of low pressure areas (depressions) that strike Europe. Over there, people no longer face hurricane force winds or hurricane structures – but rather usually, a lot of rain.

Besides the east coast, did you know that hurricanes also occur in the west coast of the United States as well as in Mexico?

Now, let’s refocus to Carribbean and east-US based hurricanes. It makes things easier – because the principle of eye formation, which we cover in this article, is similar regardless of area, we should keep the area constant.

They’re connected to thunderstorms

If we want to understand how an eye forms, we must first take a look at how hurricanes form in the first place.

Did you know that hurricane formation is intimately related to the concept of a thunderstorm?

The eye of hurricane Katrina, as seen from space. Photo by NASA, licensed to be in the public domain.

For hurricanes in the Carribbean and the eastern parts of the United States, that would be thunderstorms near Africa.

In parts of Africa, temperatures are very high and so is moisture. We primarily find those parts near the Equator: Sierra Leone, Ghana, Côte d’Ivoire, and so on.

Thanks to the heat and the grand amount of sunshine that is present in this part of Africa, air becomes very buoyant and rising air takes place at large scale.

From our article about thunderstorms, we know that when air is forced to rise, it will eventually condense – and potentially grow into a thunderstorm. Initially, in Africa, we thus see many individual thunderstorms occur under the influence of heat. As they become larger, they will cluster into multicellular complexes. This is when things become interesting.

Shift towards the ocean

In Africa, these thunderstorms cause a lot of rain – as well as some other thunderstorm related phenomenons, such as lightning.

However, under influence of the trade winds, the storms leave Africa and move towards the west, and eventually reach the Atlantic Ocean.

The storm updrafts are now fed by very moist air (from the sea) which is still warm, as we’re near the Equator. By consequence, the storm complexes can grow intensely, even so that they cluster together in one massive complex.

At that point in time, the United States National Hurricane Center considers the complex to be a tropical disturbance – a complex at sufficient macro scale to be interesting meteorologically, but nothing special… yet.

Storm growth

While tropical disturbances can already cause substantial wind gusts, things can get more intense. Since it is continuously being fed with moist and warm air, the system can continue to grow – into a tropical depression, and eventually into a tropical storm. Even then, it’s no hurricane yet!

A map with storms in the 2020 hurricane season. At the time, many storms were active, and two disturbances can be seen.
A tropical depression over the Gulf of Mexico (on the left). Photo: NOAA GOES East, licensed to be in the public domain.

When tropical depressions and tropical storms make landfall, they can already cause quite some material damage. However, that’s nothing when compared to a hurricane, the last stage (okay, fair, there are still 5 more hurricane stages to go) of storm formation. Wind gusts of 300 kilometres per hour aren’t rare in those cases.

Hurricane eye formation

Now that we know how thunderstorms can eventually group together and form a hurricane through the stages of storm, depression and disturbance, we can look at the primary question of this article: why does a hurricane have an eye?

This is especially true for the more intense hurricanes, which show clear eyes on satellite imagery.

Here, it’s time to be fair with you:

Scientists do not yet agree fully about the precise mechanism that causes the eye of the storm. That is, why storms have an eye.

In fact, there are hundreds of theories which attempt to explain eye formation. However, while the why of the eye is not yet clear, scientists do understand today how the eye forms.

Here it is.

Core of the hurricane

Any storm is a low pressure area. Thanks to Coriolis force, all air swirling around the low pressure area swirls around it cyclonically, that is, against the direction of a clock.

In those cases, there must be a core somewhere.

Low pressure areas have cores, as can be clearly seen from this Icelandic low pressure area. Photo by NASA/GSFC, MODIS Rapid Response Team, Jacques Descloitres, licensed to be in the public domain.

Around this core, convective air movements are strong, and air moves upward. Much air is rising. However, when air reaches the Equilibrium Level that we covered in the linked article, it can’t rise any further – but can go down neither (as more air is pushed upward).

In those cases, air has to spread horizontally.

While most air moves away from the storms, due to an unknown reason some air does move towards the center, and sinks.

And when air sinks, the exact opposite happens compared to rising air – clouds disappear and the skies clear.

In short, sinking air near the core of a hurricane ensures that an eye begins to form. Especially when this process is accelerated due to storm intensity, we no longer observe clouds near the core.

Et voila – that’s how hurricanes get eyes!

References

Coriolis force. (2002, January 16). Wikipedia, the free encyclopedia. Retrieved September 13, 2020, from https://en.wikipedia.org/wiki/Coriolis_force

How does a thunderstorm form? (2020, August 19). Mr. Weather. https://mister-weather.com/2020/08/19/how-does-a-thunderstorm-form/

(n.d.). National Hurricane Center. https://www.nhc.noaa.gov/

Trade winds. (2004, May 8). Wikipedia, the free encyclopedia. Retrieved September 13, 2020, from https://en.wikipedia.org/wiki/Trade_winds

Vigh, J. L. (2006). FORMATION OF THE HURRICANE EYE.

What are the differences between single cell, multicell and supercell storms? (2020, August 24). Mr. Weather. https://mister-weather.com/2020/08/24/what-are-the-differences-between-single-cell-multicell-and-supercell-storms/

What are the updraft and downdraft of a thunderstorm? (2020, September 7). Mr. Weather. https://mister-weather.com/2020/09/07/what-are-the-updraft-and-downdraft-of-a-thunderstorm/

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