Behind the Forecast: How hurricanes form

Behind the Forecast: How hurricanes form
Science Behind the Forecast airs each week on 89.3 WFPL on Fridays at 7:45 a.m.

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

Science Behind the Forecast: How hurricanes form

LOUISVILLE, KY (WAVE) - We’re smack dab in the middle of hurricane season and the Atlantic and Pacific Oceans are very active. The Atlantic hurricane season typically peaks from mid-August to late October. Since hurricanes are the main thing dominating weather news at the moment, let’s break down what ingredients that are needed for a hurricane to form.

The tropics were very active to start September. In this image, Florence had just made landfall in North Carolina.
The tropics were very active to start September. In this image, Florence had just made landfall in North Carolina. (National Hurricane Center)

First we need a tropical wave. These are areas of energy with general rotation in the atmosphere, basically areas of low pressure, that form because of temperature contrasts between the hot air over the Saharan Desert and the cooler, humid forested areas further south. These waves travel westward across the Atlantic Ocean with the help of the Bermuda High. These waves sometimes contain thunderstorms. As they begin to develop, these waves most often look like an upside down V on satellite images if you’re ever trying to spot one.

Next, you need warm water and there’s plenty of that across the Atlantic Ocean. The water must be at least 26.5 degrees Celsius, or just shy of 80 degrees Fahrenheit, over a depth of 50 meters to power the storm.

Sea surface temperatures were quite warm this week, indicating that Atlantic storms had plenty of energy to feed off of.
Sea surface temperatures were quite warm this week, indicating that Atlantic storms had plenty of energy to feed off of. (NASA)

Third, thunderstorms are of course a key part of a hurricane’s formation.

Lastly, low wind shear is needed. A big difference in wind speed or direction as you ascend into the atmosphere near a storm can easily weaken it. Wind shear inhibits the development of a storm or rips it apart once it’s formed.

So here’s how all these pieces work together. As a tropical wave moves westward, the warm ocean water is pulled into the storm forming an area of low pressure at the surface. This low pressure causes more warm, moist air to rise up into the storm which cools forming clouds and thunderstorms. As the water condenses, heat is released which continues to power the storm.

PREVIOUS SEGMENTS:

Once wind speeds reach 39 mph, the storm is classified as a tropical storm. This is when a tropical storm it gets a name from a list provided by the World Meteorological Organization. At 74 mph, the storm is officially a hurricane. Hurricanes are ranked by the Saffir-Simpson Hurricane Wind Scale. The 1 to 5 rating based on a hurricane’s sustained wind speed. If a hurricane has winds of 111 mph it is then a major hurricane, a category 3. In the western North Pacific Ocean, the term “super typhoon” is used to describe tropical cyclones with sustained winds more than 150 mph. The following table from the National Weather Service breaks down the categories and the impacts seen from each level of storm.

The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based on a hurricane's sustained wind speed
The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based on a hurricane's sustained wind speed (National Weather Service)

With Florence, there was plenty of warm water to feed off of. Across the Atlantic where it traveled, water temperatures were consistently over 25 degrees Celsius. It also had very little wind shear to deal with which is also why we saw the rapid intensification earlier this week.

The Atlantic hurricane season doesn’t end until November 30 so it’s important to remain vigilant. However, it is important to note that tropical storms can and have formed outside of hurricane season.

Copyright 2018 WAVE. All rights reserved.