How Hurricanes and Typhoons Form

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Podcast Transcript

Every year parts of the planet are hit by devastating typhoons and hurricanes. They can cause billions of dollars of damage and can take hundreds if not thousands of lives. 

But why do these storms exist? What causes their distinctive spiral shape with an eye in the middle? And why do they only appear in certain parts of the world at certain times of the year? 

And while we’re at it, what is the difference between a hurricane and a typhoon anyway? 

Learn more about hurricanes and typhoons and how they can become so deadly on this episode of Everything Everywhere Daily.

Let’s start with the easiest question to answer, what is the difference between a hurricane and a typhoon? 

The answer is….that they are the same thing. They are just different words used in different parts of the world to describe the same phenomenon. 

Both hurricanes and typhoons are classified as tropical cyclones, which are wind speeds beyond a set value, more on that in a bit. 

A hurricane is a tropical cyclone that forms in the Atlantic Ocean. Almost every hurricane forms north of the equator, usually around the Caribbean or Gulf of Mexico

There has been only one recorded hurricane-force tropical cyclone in the southern hemisphere. Hurricane Catarina, not to be confused with Hurricane Katrina, was a category 2 hurricane that hit the southern coast of Brazil in March of 2004.

A typhoon is a tropical cyclone that forms in the Pacific Ocean north of the equator. These will usually form near East Asia and frequently will hit the Philippines, Vietnam, China, and Japan

Tropical cyclones which are south of the equator in the Pacific Ocean, or that form in the Indian Ocean, are just known as cyclones. 

So, hurricanes, typhoons, and cyclones are all the same meteorological phenomena. 

Europe and Antarctica are really the only continents that are immune from tropical cyclones, as they are not in the tropics. Occasionally a hurricane might make its way far north enough to reach Britain, but by that time it would just be tropical depression or just a storm. 

In addition to the names, one of the confusing things about these storms is that there are different systems to categorize them all over the world. It seems like something that would really benefit from a global standards, but one hasn’t developed. 

There are five different intensity scales based on wind speed that are used in different parts of the world. 

The best known for listeners of this podcast would be the Saffir–Simpson scale which is used for storms in the Atlantic as well as the central and eastern Pacific. This is the system that ranks hurricanes on a scale from 1 to 5, with tropical storms and tropical depressions below that. 

There is another system used in East Asia, one in India, a French one used in the southwest Indian Ocean, and one used in Australia and Fiji. 

In the Northern Hemisphere, the season for tropical cyclones is from June to November, although rare storms have appeared in May and December.

In the Southern Hemisphere’s season is usually from November to April. 

Due to geography, there are more tropical cyclones in the Northern Hemisphere than in the southern hemisphere.  The location of land masses and ocean currents leads to more storms forming north of the equator. 

Tropical cyclones almost never cross the equator. I’m just hedging my bets by saying “almost never” because one has never been observed to actually do that. 

In fact, almost no tropical cyclones form within 5 degrees north or south of the equator.  The lack of cyclones near the equator is due to the Coriolis effect, again, more on that in a bit. 

So, the big question is why these storms form and make them so powerful. 

It all starts with warm water. 

This is why tropical cyclones are tropical. The tropics is the area between the tropic of Cancer in the north and the Tropic of Capricorn in the south, where the sun hits the Earth at its most direct angle. Each tropic line is 23°26? above or below the equator, which is exactly the same as the tilt of the Earth. 

For a tropical cyclone to form, ocean water must have a temperature of about 80 degrees Fahrenheit or 26.5 degrees Celsius.  For a cyclone to sustain itself, this temperature usually has to exist for the top 50 meters of water. 

When the water temperature gets this warm, it evaporates on the surface and rises into the air.  This warm, wet water will rise until it hits the troposphere, where it will cool and precipitate out as rainfall. 

The rising hot, warm air causes a low-pressure region near the surface of the water. The higher pressure air around the low-pressure region flows into it, where it will then rise up. 

This low pressure is really the engine behind a cyclone. A low-pressure region with a surrounding high-pressure region will continue to drive warm wet air up and then send it back down in the form of rain. 

One other condition is that there can’t be too much wind in the upper atmosphere. If there is too much wind shear, the entire system can blow out like a candle before it gets started.

However, there is more to it than that. These storms spin and move. What causes that? 

The other thing that causes tropical cyclones is the Coriolis effect. 

For all practical purposes, we can’t experience the rotation of the Earth by standing on it.  However, the Earth does spin faster at the Equator, just like how a record will move faster at the edge than it does at the center.

The rotation of the Earth does affect the air in the atmosphere. Because the air at lower latitudes is moving faster than the air at higher latitudes, the wet air will start to rotate as it moves to the low-pressure region, conserving the angular momentum of the air.

In the northern hemisphere, all tropical cyclones rotate counterclockwise. In the southern hemisphere, they all rotate clockwise. 

The Coriolis effect also explains several other things. 

The reason why no tropical cyclones form within 5% on either side of the equator is that the Coriolis effect is minimal here. Water can still evaporate, but it just becomes a raincloud.

Likewise, it is almost impossible for a tropical cyclone to cross the equator for the same reason. If it did cross the equator, it would have to reverse its spin, and the storm would just collapse. 

If you look at a map of all the tropical cyclones in the world, you will see a band right along the equator with nothing and then storms above and below the band.

Due to the Coriolis effect and the rotation of the Earth, tropical storms will always travel east to west, and they will usually wander to the north or south, depending on the hemisphere they are in. 

As the storm develops, a low-pressure region in the center becomes very pronounced. This is known as the eye of the storm. The winds in this region are very calm. This region actually starts to pull cold air down from a higher pressure region above.

Immediately outside of the eye is the eyewall. This is the region of the storm with the highest wind speeds. 

While the high winds from a tropical cyclone can be extremely dangerous, the greatest damage can actually come from the storm surge. A storm surge is a type of flood where the high winds blow water into the shore. The highest storm surge ever recorded was in Australia in 1899 when a storm surge of 44 feet or 13.4 meters was recorded.

The magnitude of the storm surge depends on the storm’s wind speed, the shore’s orientation to the storm, and the tides. A storm surge that happens during a high tide is known as a storm tide. 

This low-pressure, high-pressure cycle can keep driving the storm so long as there is warm wet water to keep fueling it.

A tropical cyclone will eventually die. How it dies will depend on what it runs into. 

If a storm runs into land, it will be deprived of warm water, dying out and eventually just becoming a rain storm. 

If a storm is in the middle of the ocean and veers north or south, it will eventually hit cold water or drier air, which will also kill the storm. 

There have been several tropical cyclones that have been incredibly destructive. 

One of the strongest storms in history was the 2015 Hurricane Patricia. It had the highest wind speeds ever recorded in a tropical cyclone at 215 miles per hour or 345 kilometers per hour. It also had the second lowest pressure ever recorded at 872 millibars.

The lowest pressure ever recorded was in Typhoon Tip in 1979, with 870 millibars. This was also the lowest atmospheric pressure ever recorded on Earth. 

The deadliest storm in history was the 1970 Bolha Cyclone which hit Bangladesh and India. Over half a million people were killed on November 12, 1970. 

One of the recognizable features of hurricanes in the North Atlantic is the names given to storms. 

In the early 20th century, meteorologists had a complex naming system for every storm based on the latitude and longitude of where it originated. During the second world war, army meteorologists in the Pacific began to give storms women’s names to make them easier to remember.

The system worked so well that in 1953 the National Hurricane Center began using the same system for Atlantic hurricanes. 

The storms are named in alphabetical order, so the season’s first storm begins with A, the second with B, etc.

Eventually, they had a hard time coming up with women’s names, so in 1978 they began using men’s names for half of the storms. A storm name tends to be reused about every six years, as there is a six-year rotation of names. 

Starting in 1979, if a storm is particularly devastating, which is decided by the World Meteorological Organization Hurricane Committee, then its name is retired. 

As of the time of recording, 95 names have been retired. The season with the most retired storm names was 2005, with five: Dennis, Katrina, Rita, Stan, and Wilma.

Other naming systems are also used by other countries, including Japan, India, and the Philippines. 

Hurricanes and typhoons are the most powerful things on Earth. Factoring in the wind, rain, and lighting, an average hurricane has enough energy as 200 times the entire energy consumption on the planet. They expend more energy than a volcanic eruption or all but the most powerful earthquakes.

Hurricanes and typhoons are a fact of life in many parts of the world. They are awesome and worthy of respect.