It’s easy to assume that the further into the tropics you go, the more at risk you are of encountering one of nature’s most ferocious creations—a hurricane.
However, due to a quirk of nature, it turns out that the equator itself is one of the safest places in the world in terms of hurricane risk, and that no hurricane has ever crossed it.
This bizarre fact went viral on Twitter after science communicator Hank Green retweeted a map posted by user @Bonecondor showing the routes and severity of hurricanes around the world, amassing 1.4 million views and over 10,000 likes.
The reason for this hurricane-free equatorial zone is due to the Coriolis effect, a force that acts on our atmosphere due to the fact that the Earth rotates faster at the equator than at the poles, according to National Geographic. This means that air currents and storms bend to the right in the northern hemisphere and to the left in the southern hemisphere, the degree to which depends on the air speed and size of the storm system.
“The Coriolis effect influences why tropical cyclones do not cross the equator. But, the Coriolis effect impacts storms in a few ways,” Chris Slocum, a physical scientist at National Oceanic and Atmospheric Administration (NOAA), told Newsweek.
“Thinking about the ingredients that a tropical cyclone needs to form, you need warm ocean waters as an energy source and low vertical wind shear (a measure of the change in wind speed and direction with height in the atmosphere) plus some background spin. Coriolis is weak near the equator and increases near the poles. So, while the oceans are balmy and the shear is low at the equator, there isn’t enough background spin to organize thunderstorms into tropical cyclones. Coriolis provides some of this background spin. In the North Atlantic, African easterly waves have a fair amount of spin allowing the waves to potentially develop into tropical cyclones.”
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The Coriolis effect is essentially zero at the equator, Mathew Barlow, a professor of environmental earth and atmospheric sciences at the University of Massachusetts-Lowell, told Newsweek.
“The closest recorded formation was about 100 miles away from the equator, to give a sense of distances. Because they’re forming away from the equator, they’re generally being affected by winds that steer them away from the equator,” he said.
The Coriolis effect also causes storms to move away from the equator due to “beta drift.”
“This causes storms to move away from the equator (to the north and west in the Northern Hemisphere),” Slocum said. “And, the tropics lack large-scale patterns like a cold front to counteract this motion and push a tropical cyclone across the equator.”
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It was initially believed that storms could not form within 5 degrees latitude of the equator, but more recent research has found that to be not quite true.
“With the satellite era and complete global coverage by geostationary satellites, we realized that tropical cyclones can occur close to the equator. These near-equator storms tend to be small and disorganized, mostly because these storms do not have access to this background spin provided by the Coriolis effect. However, even these storms do not cross,” Slocum said.
It is physically possible that a storm could cross the equator if it were, figuratively and literally, the perfect storm.
“It’s probably possible for a hurricane to cross the equator if there were the right (unlikely but possible) set of circumstances: a hurricane of at least moderate strength very close to the equator, and a larger-scale wind field pushing it toward the equator,” Barlow said.
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The viral map also shows the difference in strength of the storms in the Southern Hemisphere compared to those in the Northern Hemisphere. This is due to the conditions needed for a storm to arise and the lack of availability of those conditions in the Southern Hemisphere.
“This goes back to the idea of ingredients that are favorable for tropical cyclones,” Slocum said. “If we focus on warm oceans and vertical wind shear, the oceans in the Southern Hemisphere are warm over a large region and warm enough to support tropical cyclone formation and intensification. But, with respect to shear, the Southern Hemisphere is more hostile and unfavorable for tropical cyclones. When the Southern Hemisphere oceans reach their highest temperatures for the year (roughly February/March), the lowest average shear in the Southern Hemisphere is nearly double what the average values are in the Northern Hemisphere when the highest temperatures are observed (roughly August/September). This makes it harder for storms to form and harder for storms to intensify.”
It is also related to the locations where storms tend to form in the Southern Hemisphere, being closer to land, giving the storm less distance to travel over the ocean and gather strength.
“Tropical cyclones developed over the Western North Atlantic and Western North Pacific are normally stronger in intensity because of the high Sea Surface Temperature of the Western Hemisphere Warm Pool and the Tropical Warm Pool of the Western North Pacific Ocean, and once formed have longer distances to travel over vast ocean surface to garner energy before making landfall,” Norman Kin-Wai Cheung, a senior lecturer in hazards, environment and geography at the Education University of Hong Kong, told Newsweek. “However, the tropical cyclones formed in the Gulf of Carpentaria, the Coral Sea and off the northwest coast of Australia have relatively shorter distances to strength their power.”
Storms across the world, in both the Northern and Southern Hemispheres, are expected to get stronger and more destructive with the effects of climate change.
“Tropical cyclones are indeed affected in different ways by climate change: the storms are becoming more intense, and there are higher levels of precipitation associated with them. It is expected that this will continue in the future. The storm surge associated with tropical cyclones are also expected to increase,” Suzana Camargo, a research professor of ocean and climate physics at Columbia University, told Newsweek. “These are robust projections globally.”
This is because of increased ocean and air temperatures, as well as sea level rising due to melting sea ice.
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“Climate change is clearly increasing the upper limit on hurricane strength and rain rate due to both the increasing temperature of the ocean, which provides the energy for the storms, and the increasing temperature of the atmosphere, which allows for more intense rain,” Barlow said. “Additionally, the warming increases average sea levels and so makes storm surge worse. Other aspects are less clear, including possible increases or decreases in the overall number of hurricanes in a year, although we do expect the proportion of hurricanes that are major storms to increase.”
These increasingly vicious hurricanes are expected to result in more damage to property, infrastructure, and human life.
“We are seeing increased human exposure to tropical cyclone related hazards and impacts, Slocum said. “Storm surge and flooding cause the most deaths during landfall. With climate change, sea level rise increases the impact of storm surge, and slower-moving storms that are trending to be wetter increase flood risk in that more intense rain falls for longer periods. These impacts are highly regional and unequally distributed because of coastal structure and local topography.”
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