I live in the Caribbean so I have plenty of experience of hurricanes but have never understood how clouds begin to rotate and turn into hurricanes and why storms in northern latitudes not rotate into hurricanes but can have very strong winds and rain. It has been explained to me before, but not like I’m 5, so I never quite got it. Thanks for any answers.
In: Planetary Science
You are asking about something called cyclogenesis, or the development of a cyclone, in the tropics.
Six things must happen for a tropical cyclone, the precursor to a tropical storm and hurricane, to form:
1. Sea surface temperature of at least 26.5 C, which is pretty warm.
2. Atmospheric instability, or essentially warm air at the surface and cold air above
3. High humidity near the surface
4. Enough Coriolis force, so it needs to be a few degrees above/below the equator
5. A pre-existing low pressure disturbance
6. Not too much wind shear, which would tear it apart before developing.
Let’s break it down.
1. Warm sea surface temperature. The top layer of the ocean, ideally the top 50 meters, needs to be at least about 26.5C/80F so that the water can evaporate fast enough to feed the cyclone. All storms need moisture in the air, not for rain, but because as it rises, the water will condense (turn from gas to liquid), which releases latent heat, which causes it to rise faster, which releases more latent heat, and so on. It’s a positive feedback loop that all proper storms need. The average global sea surface temperature is 16C/60F so the conditions for cyclogenesis are pretty warm, but that’s happening more and more thanks to a warming climate. Essentially, the fuel that feeds a hurricane is a warm ocean.
2. Atmospheric instability. A stable atmosphere has cool air at the bottom and warm air at top, because warm air rises due to it being less dense. This is rare, however, because the sun warms the Earth’s surface, which in turn warms the atmosphere over the Earth’s surface, so you get warm air at the bottom and cool air at the top. This is necessary because when the warm surface air starts rising, it needs to be warmer than the surrounding higher air so that it rises even faster, which is how the initial “thunderstorm” forms for a cyclone.
3. High humidity. The lower levels of the atmosphere need to be humid because the moisture acts like gasoline on the fire of a cyclone. The upper atmosphere needs to be dry because when this lower humid atmosphere rises into the dry atmosphere, the water will condense out, which releases its latent heat, which makes it even more warmer than the surrounding cool dry atmosphere, so that it rises even faster. If the cyclone sucks up dry air at the surface, it’ll lose this latent heat gasoline fuel, and snuff itself out.
4. Coriolis force. Coriolis force makes air rotate when it moves *away* from the equator because the Earth is rotating slower the further away from the equator you go. So you have air over the equator that’s barely moving over the ground with just a little wind. When that air moves north, now the ground is spinning around slower than the ground near the equator, which means that now the air, which was moving slow with the equator ground, will now be moving fast. The initial low pressure disturbance (see next step) needs to be at least ~500km/300miles/4.5 degrees north of the equator to give the warm humid surface air its initial rotation, or cyclonic movement.
5. Pre-existing low pressure disturbance. There needs to be an area of low pressure at the surface to kick start that whole process. This usually comes from a tropical wave, not of water, but of air. Think of it like a “kink” in the trade winds. Most usually form from a unique feature of Africa called the African Easterly Jet, which is like a mini jet stream 10,000 feet over the Sahara. The edges of this jet stream form these disturbances (a disturbance is really just an area with a really sharp change in some atmospheric property in a very small area) that then head west over the Atlantic. Most hurricanes can trace their origins all the way back to the Sahara.
6. No wind shear. If there’s too much wind shear, essentially really fast wind aloft compared to the surface, then it will tear apart any rising storms before it can fully develop.
So let’s see how a typical hurricane would form:
1. It starts with a disturbance over Africa. Let’s say the African Easterly Jet, as it moves north for the summer, suddenly speeds up on a really hot day. This creates a really strong wind gradient that creates a disturbance that breaks off from the jet and now starts blowing west over the Atlantic.
2. As it goes west, it hovers a few degrees above the equator, so as the air gets sucked into this atmospheric disturbance, the air starts rotating. At the middle of this rotation, the air rises. With winds lower than 38 miles per hour, it is now called a tropical depression
3. As the tropical depression moves west, the sea surface temperature goes up because the Atlantic Ocean circulates clockwise, meaning the water at the west end of the ocean at the equator has been traveling westbound over the equator since Africa, giving it the maximum amount of sunlight, warming it. This warm water evaporates faster than cooler water, making the air getting sucked into this tropical depression more and more humid, which triggers makes the air rise even faster, which means the air gets sucked in even faster. Now with winds between 39-73 mph, it’s called a tropical storm.
4. As the tropical storm keeps moving west, it encounters really cool and dry air really high up. This amplifies the rising humid air, making it rise faster, strengthening the tropical storm.
5. As this air rises, there isn’t much wind shear to tear it apart, so it becomes really vertical. Now the winds get even faster because the water over the Caribbean and Gulf of Mexico is so much warmer, making it a Hurricane.
I’m a slut for atmospheric dynamics, with a college degree in Atmospheric and Oceanic Sciences, although I went into a different field (military flying). In another life, I would’ve become a hurricane scientist, focused exactly on how they form.
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