Hurricanes are a lot less understood then you might think. While they occur everywhere around the globe, and though they happen with some regularity every year, scientists still aren’t totally sure what drives them—our observational satellite records of these storms only go back to around the 1970s.
Understanding hurricanes, how they work, and what makes them go is still a very active area of research. They seem to be caused by a complex interaction between the ocean and the atmosphere. In order to form, they require a delicate balance of moisture, temperature, pressure and weather systems across almost an entire ocean. “They really are one of the more complicated phenomena because they bring together a lot of different process at once and they’re all important,” says Allison Wing, a professor of Earth, Ocean and Atmospheric Science at the University of Florida.
Over the next few weeks we’ll be taking a look at how these massive systems impact our planet and our lives: we’ll look at why they are so difficult to predict; we’ll tackle how cities could become less vulnerable to the flooding that these storms bring as they carry water from the ocean onto land; and we’ll get into the nitty-gritty of why scientists are having such a hard time pinpointing the impact that climate change is having on hurricanes around the world.
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But first up, let’s tackle some Hurricane 101 facts to clear up what we do know for sure, and what we’re still mystified by, when it comes to these monster storms.
Is a Hurricane the Same Thing as a Typhoon? What About a Cyclone?
The first thing to know is that while hurricanes happen all over the world, they’re not always called a hurricane. The umbrella term for these storms, the ones that scientists use when referring to them, is a tropical cyclone, because they form near the equator, between about 10 to 30 degrees of the north and south latitudes. But because of regional differences in naming conventions they can also be called a typhoon or a cyclone. In the Atlantic ocean and eastern Pacific they are called hurricanes, which originates from the name Huracan, a Mayan (or Taino, depending on who you ask) god of wind, storms, and fire. In the western Pacific, they are called Typhoons, which originate from the Chinese tai fung (meaning big wind) and the Arabic word ṭūfān (meaning storm). In the Indian Ocean, they are called cyclones, which was a coined by a British man in India as he described a large storm in 1789 that killed more than 20,000 people. He named it after the Greek word kyklon, which means ‘to move in a circle’.
But these are all the same type of storm: a cyclone that forms over the tropical ocean around an area of low pressure and warm air. Wind likes to flow from areas of high to low pressure, so as the pressure falls at the center of a cyclone (known as the eye) the wind speed increases around it. Warm water gets picked up by the winds, which evaporates and transfers its heat energy into the wind, which feeds the storm.
“Hurricane tracks are controlled by large-scale pressure fields in the atmosphere. In the Atlantic, the Bermuda-Azores High pressure system is a dominant reason why hurricanes move as they do. In the northern hemisphere, things move in a clockwise direction around high-pressure systems—so you see hurricanes come across the Atlantic and then curve to the north and then back to the east—they are moving around this high-pressure system. If this system is to the east, they recurve out at sea and don’t make landfall. If this system is shifted to the west, they come into the Caribbean or can make landfall in the U.S.,” says Jyotika Virmani, an atmospheric scientist and senior director of Planet and Environment at XPRIZE.
In the U.S., in addition to late-summer and autumn hurricanes, we also experience cyclone storms blowing in from the tropics in the winter; however they have cold air at their core and their wind fields are not uniform—whereas hurricanes are shaped like a bullseye, winter cyclones are shaped more like a comma.
Is a Hurricane Just a Water Tornado?
Hurricanes may sound like a warm, wet tornado, but they are not the same. Tornadoes are tiny in comparison and they get their energy from within other storms, most often a thunderstorm. They can also form within hurricanes themselves. Tornadoes are also relatively short-lived (most only last about 10 minutes), while hurricanes can last for days.
Can Hurricanes Happen At Any Time Of the Year?
Hurricanes need a very specific set of circumstances to form. And it’s the special characteristics of summer that give them their season. “We’ve known for years that there is a recipe. Ingredients and steps for formation and intensification,” says Rosimar Ríos-Berríos, an atmospheric scientist at the National Center for Atmospheric Research. “In terms of ingredients, the most important are the warm ocean temperatures, water vapor above the surface, and light windshear.”
During summer, the water of the ocean is warmer than the air in the atmosphere above it—warm enough to provide fuel to a storm. The atmosphere itself is warm enough to hold evaporating water. Meanwhile the wind speed nearby has to be juuuust right. For example, many of the more intense storms that hit the U.S. start with weather systems blowing off of Africa (because they pick up a lot of energy from warm water as they have to cross the entire ocean). But sometimes dust from the Sahara gets picked up in that wind, dries the air, and prevents hurricanes from forming.
“Strong winds in the upper atmosphere or in a different direction then ones in the lower atmosphere rip [a hurricane] apart. Sometimes the wind shear is too strong,” says Wing.
Why Don’t All Tropical Storms Become Hurricanes?
Here’s where it gets complicated—not all hurricanes form the exact same way. And not all hurricanes need the exact recipe of warm water + warm air + wind to grow in strength. “There’s a lot of weather disturbances coming off Africa but not all of them become hurricanes,” says Ríos-Berríos. Additionally, scientists aren’t entirely sure why some hurricanes become very strong and others don’t. In fact, Ríos-Berríos does research that specifically focuses on trying to understand why a storm might be missing one of the necessary elements and still manage to become strong. “You may have some vertical windshear,” she says, “but if the hurricane is over very warm temperatures and a lot of water vapor the hurricane may intensify.”
Why Do We Get Around 90 Hurricanes Every Year?
And then there’s the fact that scientists have no idea why we tend to have a relatively uniform number of hurricanes every year. According to Wing, “Every year around the globe the number [of cyclones] is usually around 90. It varies a little bit. But why isn’t is 200 or 900 or 50? We don't really know.”
Are Hurricanes Bad for the Planet?
One thing that can be confidently said about hurricanes is that, because of their massive size, which can span hundreds of miles, they are essential systems to the overall health of the planet. According to XPRIZE’s Virmani, “They are important. We can’t just stop them. They’re part of our natural ecosystem. The sun heats up the earth, it heats up the tropics more than the extra tropics, and hurricanes are the most effective way of releasing the heat into the extra tropics. It’s like a steam engine letting off steam. If you didn’t have the release mechanism it would stay cold up North, build up heat in the tropics, and it would be an imbalance.”
