The 6 conditions needed for a hurricane to form

How do hurricanes form?

Hurricane AndrewHurricanes--also known as tropical cyclones, and in the Pacific Ocean as typhoons--form over warm tropical and subtropical waters in the Atlantic and Pacific Oceans. The scientific term for how hurricanes form and develop is tropical cyclogenesis.

Although scientists do not yet completely understand the full process of how hurricanes form, it is known that there are six conditions involved in forming a hurricane. It is important to add that some hurricanes do form without all six conditions present, but most hurricanes are formed when all six conditions occur together in the atmosphere.

How Do Hurricanes Form: Water Temperature

One of the primary factors involved in the formation of a hurricane is water temperature, or more specifically, the surface temperature of the ocean where a hurricane is formed.

Under normal conditions, hurricanes require an ocean surface temperature of just above 79 degrees Fahrenheit or more (contrast this with the average surface temperature of the world's oceans, which is about 61 degrees F), from the surface to about 50 meters deep. This warm water evaporates and begins to condense, or form condensation, at higher levels in the sky, releasing heat and giving the hurricane its source of energy. This is why a hurricane tends to weaken considerably when it travels over land for a significant period of time, as it gets cut off from its source of energy, warm ocean water.

As a result of hurricanes' need for warm water, hurricane season tends to coincide roughly with summer, with the peak coming late in the summer, when ocean waters near the equator tend to be at their warmest.

How Do Hurricanes Form: Unstable Atmosphere

Related to the previous factor, How Do Hurricanes Form: Water Temperature, is another factor, an unstable atmosphere above the site where the hurricane is forming.

The warm, moist air rising from the warm surface of the ocean as part of the evaporation process eventually cools and condenses. This action of moist air rising, cooling and condensing makes the atmosphere unstable, leading at first to thunderstorms and eventually, if other conditions are favorable for it, to the formation of a tropical storm and finally a hurricane.

How Do Hurricanes Form: Area of Disturbance

Hurricane Katrina 1During hurricane season, it is fairly common to see weather reports on the news where a meteorologist talks about an area of disturbance in the tropics. This area of disturbance--sometimes also called a tropical wave--usually refers to an area where the weather is bad, with lots of clouds, low barometric, or air pressure, and plenty of rain, even thunderstorms.

Meteorologists and hurricane trackers keep a close eye on such areas during hurricane season because these areas of disturbance can be the "seeds," or the beginning, of a hurricane, provided some of the other factors that form a hurricane are present as well.

How Do Hurricanes Form: High Humidity

Another factor in the formation of a hurricane is high humidity in the air. More specifically, in the lower to middle levels of the atmosphere that scientists call the troposphere. The troposphere is the part of the atmosphere closest to the surface of the planet earth, where most of our weather, including hurricanes and tropical storms, is formed.

As you can imagine, the first factor listed above, How Do Hurricanes Form: Water Temperature, plays an important role in creating high humidity in the troposphere. As ocean water is warmed up in the tropics and it evaporates, it saturates the air above the ocean, making it more and more humid and moist. The higher the humidity and moisture in the air, the more favorable the conditions are for a hurricane to form.

How Do Hurricanes Form: The Coriolis Effect

You may have heard about the Coriolis effect before. The Coriolis effect is the effect that the Earth's rotation has on objects that are moving on the Earth's surface. It works differently in the northern hemisphere and southern hemisphere, causing moving objects in the northern hemisphere to rotate in a counterclockwise direction, while moving objects in the southern hemisphere rotate in a clockwise direction.

The Coriolis effect is best known as the effect that supposedly makes water in a sink or toilet spin one way or another as it empties out, but scientists have proven this is not necessarily true. However, it is true that the Coriolis effect is responsible for making hurricanes in the northern hemisphere rotate counterclockwise.

The Coriolis effect also helps hurricanes form in another important way. As noted in the section above How Do Hurricanes Form: Area of Disturbance, an area of disturbance with low air pressure is needed in order for a hurricane to form. This low-pressure area has a vacuum-like quality to it, as it "attracts" air from outside its center, creating winds blowing in towards the center of the low pressure area.

Meanwhile, the Coriolis effect has a tendency to bounce the winds back out. If the winds attracted by the low pressure come into balance with the deflection caused by the Coriolis effect, you get what is called a cyclonic flow. The cyclonic flow, when combined with the evaporation of warm ocean water and condensation described in the section How Do Hurricanes Form: Water Temperature above, can feed off of itself and grow to form a hurricane and potentially turn it into an intense hurricane of Category 3 or higher.

It should also be noted that because the Coriolis effect gets much weaker as you approach the Earth's equator, it is very rare for a hurricane to form anywhere near the equator. Also, the Coriolis effect is one of the factors involved in steering a hurricane.

How Do Hurricanes Form: Wind Shear

Wind shear is a short-term variation in wind speed and direction which can happen anywhere in the troposphere, from near the surface of the Earth to the upper edge of the troposphere, which can be as high up as 12 miles near the equator.

Wind shear can be caused by a variety of factors and if you've ever flown on an airplane, it is often the reason for the turbulence you might experience in-flight.

Weak wind shear, with winds of about 20 miles per hour or less, is actually necessary for a hurricane to form. But wind shear that is significantly higher can destroy an existing hurricane or prevent a developing hurricane from forming in the first place.

The El Niño effect can play a role in this as well. In years during which an El Niño phenomenon is in effect, there tends to be higher wind shear present over the North Atlantic Ocean. As a result, fewer hurricanes than normal tend to form in the Atlantic Ocean when El Niño is around.

How Do Hurricanes Form: Conclusion

Hurricanes are a complex natural phenomenon requiring a number of conditions to be just right for their formation. As dangerous and menacing as they may be, they are a necessary mechanism by which the Earth maintains comparatively stable temperatures across the planet, by taking heat away from the tropics and carrying it to temperate locations.

Think of a hurricane as a large heat engine, formed and powered by mighty natural forces, destructive but essential to our environment.

Hurricane Katrina's Landfall in Louisiana

Hurricane Katrina, the most expensive natural disaster in United States history and one of the nation's deadliest hurricanes ever, made landfall near New Orleans, Louisiana in August 2005.
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