Jul
19

Weather Wisdom: The Circle of Life of a Hurricane

Weather Wisdom: The Circle of Life of a Hurricane

July 19
By

“Great storms announce themselves with a single breeze …” is a line from the 1985 the movie Ladyhawke. Although the Bishop of Aquila was talking about a potential civil uprising, the quote can be used to describe a hurricane.

A hurricane does not form with the snap of a finger. Many times it begins as a single or group of thunderstorms in a process that can take only a few days to more than a week. Likewise, the life cycle of tropical cyclones (from tropical depression to tropical storm to hurricane to death) can last only a day or two or for as long as a month.

Hurricane development and its lifespan depend on factors such as atmospheric conditions and geography. For example, if the upper air patterns are unfavorable or water temperatures are too cool, hurricane development will be stunted, if it happens at all. As for geography, interaction with land, especially landfall, almost certainly means the eventual death of a storm will come sooner rather than later.

Let’s take a look at the evolution of a hurricane, keeping in mind that the start of the process doesn’t mean that a storm will turn into a hurricane and the process could stop at any time.

Tropical Disturbance or Wave

A tropical disturbance is an organized area of thunderstorm activity that could develop into a depression or storm. Disturbances can form on old frontal boundaries or from a thunderstorm or group of storms.

While tropical waves are basically disturbances, the term is used primarily to describe areas of disturbed weather that move off the African west coast. Tropical waves, which ironically have origins tied to the Sahara Desert, move west through the Atlantic Ocean. Some of these waves keep moving west through the Caribbean across Central America to turn into hurricanes in the Eastern Pacific Ocean.

 

tracking hurricane Rita

Life Cycle of a Storm. Image Courtesy of NOAA and CIMSS

Tropical Depression

This is when a storm really starts cooking. An area of disturbed weather becomes a depression when it develops a closed low pressure (an area of circular winds). Tropical depressions’ wind speeds are generally at least 25 mph. While a depression doesn’t have much wind speed, it can dump a lot of rain on land areas. Here is a word of caution: There have been instances during storm development that a storm went from a disturbance to a tropical storm, skipping classification as a depression.

Tropical Storm

This occurs when a tropical system’s circulation becomes better organized and wind speeds reach 39 mph. Once the tropical storm threshold is reached, a storm receives a name.  The wind speed range for a tropical storm is 39 to 73 mph. As a tropical storm becomes stronger, its potential to cause wind damage increases, and like a depression, it can dump tremendous amounts for rain.

Hurricane

A hurricane is the most severe classification of tropical cyclone. A hurricane’s destructive arsenal includes high winds, heavy rains, tornadoes and the storm surge. Hurricane status is reached when a storm’s winds reach 74 mph. A Hurricane’s eye (the point of lowest pressure that is characterized by calm winds) will become more defined as the storm gets stronger.

A scale based on wind speed, the Saffir-Simpson Hurricane Wind Scale, is used to estimate potential property damage. The Saffir-Simpson scale ranks hurricanes 1 (74-95 mph) through 5 (157 mph or greater.) Category 3-5 storms are considered to be major hurricanes, and it is wise for coastal residents to head to the hills as quick as possible. However, category 1 and 2 storms should not be taken lightly.

Worth pointing out is that while hurricanes are ranked based on wind speed, the storm surge is the deadliest threat. According to the National Hurricane Center, storm surges have killed more people in the United States than all other hurricane threats combined since 1900. Wind speed plays a role in the height of the surge, but the geography of the coastline plays a role, too. For example, Category 2 Hurricane Ike in 2008 hit the Texas coast with a 20-foot surge. If Ike’s eye had crossed the coastline at Palm Beach, Fla., only an 8-foot surge would have been produced.

 

tracking Bertha

Developing Storm Image courtesy of NOAA

 The End of the Life Cycle

Assuming a storm grew up and became a hurricane, it will eventually cease to be hurricane. Its death can occur due to landfall or moving too far north over the water, depriving it of its main source of energy, warm water. Unfavorable atmospheric conditions can also sap a hurricane of its strength sending it to its demise.

In most cases, the life cycle is reversed and a hurricane fades into a gentle breeze,  leaving its mark in the history books.

If you’d like more about hurricanes, be sure to check out our Ebook “Five Lessons about Disaster Recovery from Hurricane Sandy.”