Why Hurricanes Almost Never Form Near Or Cross The Equator

This post was originally published on Forbes.com on June 28, 2018

It’s time for another meteorological “101”  session. This time I want to address a question that I often receive as an atmospheric sciences professor and meteorologist. Why don’t hurricanes, typhoons, or cyclones form near the equator?

150 years of tropical cyclone tracks through 2006. NASA EARTH OBSERVATORY

The graphic above clearly reveals that hurricanes (Atlantic basin, E. Pacific), typhoons (W. Pacific), and cyclones (Indian Ocean, Australia) rarely if ever form between 5 deg North and 5 deg South latitudes, respectively. The prerequisite conditions for hurricanes are: warm, deep ocean waters (greater than 80°F / 27°C), an atmosphere cooling rapidly with altitude, moist middle layers of the atmosphere, low wind shear, and a pre-existing near surface disturbance. Even if these conditions are in place, a tropical cyclone is not likely to form if it is not at least 300 or so miles from the equator.

This is because of the lack of the Coriolis force. What is that? It is an apparent force caused by the rotation of the Earth. It is named after French mathematician Gaspard Gustave de Coriolis who investigated energy in rotating systems. It acts on moving air and water at timescales longer than a day or so. This means that your bathwater spiraling out of the tub is not likely being affected by the Coriolis force. Earthlings (us) will notice acceleration of an air or water mass to the right of forward motion in the Northern Hemisphere and to the left in the Southern Hemisphere. Because parts of the planet are moving at different speeds (yep), this has a profound effect on the Coriolis force. The scijinks.gov website notes,

“It takes Earth 24 hours to rotate one time. If you are standing a foot to the right of the North or South Pole, that means it would take 24 hours to move in a circle that is about six feet in circumference. That’s about 0.00005 miles per hour. Hop on down to the equator, though, and things are different. It still takes Earth the same 24 hours to make a rotation, but this time we are traveling the entire circumference of the planet, which is about 25,000 miles long. That means you are traveling almost 1040 miles per hour just by standing there.”

No Earth Rotation (left) vs Earth Rotation (right) and resultant Coriolis force. NOAA/MARSHALL SHEPHERD

The Coriolis force is quite different at the equator than it is at the Poles. In fact, the magnitude is zero at the equator. I hope you didn’t have lunch yet because I am about to throw a little math at you but don’t worry, it isn’t too bad. Coriolis force is expressed as Coriolis Force=2VΩsin A. V is velocity, Ω is angular velocity, and A is latitude. If you think back to high school math class, the sine of 0 (the latitude at the equator) is 0 also. This is why there is no Coriolis force at the equator and why hurricanes rarely form near the equator. The Coriolis force is simply too weak to move the air around low pressure. Air prefers to flow from high to low pressure. According to NOAA’s NWS Jetstream online school, there have been a few exceptions,

“…..Typhoon Vamei which formed near Singapore on December 27, 2001. Since tropical cyclone observations started in 1886 in the North Atlantic and 1945 in the western North Pacific, the previous recorded lowest latitude for a tropical cyclone was 3.3°N for Typhoon Sarah in 1956. With its circulation center at 1.5°N Typhoon Vamei’s circulation was on both sides of the equator. U.S. Naval ships reported maximum sustained surface wind of 87 mph and gust wind of up to 120 mph.”

Professor Paul Roundy at University at Albany-Suny also pointed out the fascinating case of Cyclone Agni in 2004 as well.

2004 Cyclone Agni. PAUL ROUNDY/SUNY ALBANY

Hopefully it is clear why formation of tropical cyclones is rare near the equator (if not, here’s a more visual demonstration), but can a hurricane cross the equator if it has already formed beyond 5 degrees latitude? Theoretically, the answer is yes but there is a “but.” Professor Gary Barnes addressed this very question on the University of Hawaii Department of Meteorology website. He says,

“Yes, because a well developed storm has plenty of spin that would dominate the weak Coriolis force near there. If it crossed the Coriolis force would be working against the initial direction of the spin, but it would be dominated by what we call the relative vorticity of the storm. Have we seen this happen? Hurricanes can move south and get close to the equator but I cannot find an example of one crossing in the Atlantic or eastern Pacific. In the Indian Ocean some come closer to pulling off this trick. Why don’t they cross? The variation in Coriolis with latitude – called the Beta effect – actually will move a hurricane to the NW in the northern hemisphere even if there is no large scale wind pushing the storm along! So, Coriolis not only seems to be a necessary ingredient to make a storm, but it may also pull them away from the equator making the crossing event a tough one to pull off.”

Though theoretically possible, practically it is not likely.

Hurricane directions by hemisphere. SCIJINKS.GOV

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