Into the EYE OF THE STORM: Learn why hurricanes are one of the most powerful and destructive tempests on Earth.
Damage costs are still being assessed for 2018, but hurricanes Florence and Michael are expected to join some of these fierce storms in the top 10. And hurricanes are causing more casualties now than ever--Atlantic storms directly and indirectly killed more than 3,000 people in 2017 alone.
When a Storm Is Brewing
A hurricane is an extreme type of tropical cyclone. A cyclone is a low-pressure area that spins counterclockwise (clockwise in the Southern Hemisphere), sucking air into its center. "Tropical" means the storm has developed in the very warm and humid air located above tropical or subtropical waters.
When meteorologists describe a storm as a "hurricane," it implies a certain intensity; hurricanes have minimum sustained surface winds of 74 mph. "Sustained winds" refers to the average wind speed over one minute. Because it takes time to build up to these speeds, a storm passes through several classifications before it's deemed a hurricane. In its earliest stages, these systems are called "tropical disturbances"--areas of convection (showers and thunderstorms) with no surface circulation. When circulation develops, but sustained surface winds are less than 39 mph, the system is referred to as a "tropical depression." Once winds increase to at least 39 mph, the system is classified as a tropical storm and given a name. (See "What's in a Name?" on Page 70.)
When winds reach a minimum speed of 74 mph, meteorologists officially deem the system a hurricane, and start discussing categories. A hurricane's "category" refers to the intensity of the storm as ranked on the Saffir-Simpson Scale (see chart on Page 72). This ranking is based on sustained wind speed, and gives the public an idea of the type of destruction that's possible if the storm hits land.
In 2017, there were two Category 5 storms: Irma and Maria. At its peak, Irma had sustained winds of 185 mph and gusts over 200 mph. Both storms made landfall at full strength with devastating results--Irma at Barbuda, and Maria at Dominica. In 2018, both Florence and Michael peaked as Category 4 storms. While Florence weakened to a Category 1 before making landfall in North Carolina, Michael came ashore at Mexico Beach, Florida, at full strength, and was the third most intense storm ever to make landfall in the United States.
The name used to identify a storm varies by location. "Hurricanes" are storms specifically in the Atlantic Ocean, Caribbean Sea, the Gulf of Mexico, and the central and eastern North Pacific to the International Dateline. West of the Dateline, these storms are "typhoons." In the South Pacific and Indian oceans, they're "tropical cyclones," or simply, "cyclones."
The official Atlantic hurricane season runs from June 1 through November 30, with a peak of activity between mid-August and mid-September. In an average year, the Atlantic Basin sees 12 named storms, six hurricanes, and three major (Category 3 or stronger) hurricanes. In 2017, there were 17 named storms; out of these storms, 10 became hurricanes, with six progressing into major hurricanes. Similarly, 2018 saw 15 named storms, with eight advancing to hurricanes, two of which were major hurricanes.
Inside the Cyclone
On a large-scale, there are several ingredients involved in the formation of a hurricane. Water needs to be at a temperature of 80 degrees Fahrenheit or warmer to provide the moisture that "feeds" the convection and drives the storm. Interestingly, the formation of a storm structure depends on light ambient winds aloft; if these winds are too strong, they can weaken or even destroy the storm's vertical structure, as well as inhibit convection. Finally, you need something to start the spinning motion; the Coriolis Effect provides this. A phenomenon resulting from Earth's rotation, the Coriolis Effect causes air sucked in by tropical cyclones to spin out, creating the rotating patterns that famously indicate a hurricane. This force is why hurricanes rarely form on or near the equator, where the Coriolis Effect is minimal.
Even if overall conditions are favorable for development, a catalyst is necessary--something to get things going. In summer, tropical waves of low pressure move off the west coast of Africa and head westward across the Atlantic. When these initiate convection, it yields a mass of showers and thunderstorms. In time, a cyclonic circulation can develop aloft, and if it works its way down to the surface, an incipient tropical cyclone forms.
The structure of a hurricane is very complex. Rather than a solid mass of rain, a hurricane is comprised of bands of showers and thunderstorms, rotating counterclockwise around a center and extending outward as far as several hundred miles. Closer to the center of the storm, the bands become stronger and more numerous. A partial or complete ring of convection surrounds the center of the storm. This is the eyewall, where you'll find the strongest winds and heaviest rain. Once you penetrate this layer of heavy rain, you enter a relatively calm center known as the "eye" of the storm. Here, the air sinks rather than rises. In the strongest storms, the eye is clear, with blue skies overhead. The eye varies in size, from less than 10 miles across to more than 40 miles wide.
Tropical cyclones bring with them a variety of threats. The most obvious danger comes from the strong winds indicated by the Saffir-Simpson Scale. Winds of 50 mph or greater can take down trees and result in power outages. Inland areas, where strong winds are less frequent, will see more damage with lesser wind speeds.
Although strong winds can produce major damage, the storm surge typically brings the majority of destruction along the immediate coastline. This push of water inland is associated with tropical cyclones, and caused by strong onshore winds. It's not just the flooding itself that causes damage; the pounding of massive waves in addition to the elevated sea level can destroy even well-built structures. The actual height of the storm surge depends on a number of factors, including the size and strength of the storm, and the height of the tide during the initial hit. In 2005, Hurricane Katrina produced a storm surge of nearly 27.8 feet at Pass Christian on the Mississippi Gulf Coast. The surge pushed inland at least 6 miles. A buoy off the coast measured a wave height of 55 feet.
Freshwater flooding due to excessive rainfall has become one of the greatest threats from tropical systems in recent years. Interestingly, rainfall amounts don't correlate with the strength of the storm. Dying hurricanes or even storms of lesser strength can produce prodigious amounts of rain. The threat can extend inland many miles from the coast and last for days. According to the NHC's Tropical Cyclone Report on Hurricane Harvey, it was "the most significant tropical cyclone rainfall event in U.S. history." The highest storm total rainfall report from Harvey was 60.58 inches near Nederland, Texas. Second in rainfall events to Harvey was Florence in 2018, with Elizabethtown, North Carolina, recording 35.93 inches.
In some cases, a hurricane can produce tornadoes when it reaches land. Although typically not as strong as conventional tornadoes, these can still cause quite a bit of damage. They most often occur in the right-front quadrant of the storm. Hurricane Harvey produced 52 tornadoes in 2017.
Tracking the Storm
Since 1960, weather satellites have been our early warning system for tropical cyclones, constantly monitoring even the most remote ocean areas. Meteorologists compare active storms with images of former tropical systems from these satellites to make reasonably accurate estimates of intensity. Once a system gets close enough to land, the famed National Oceanic and Atmospheric Administration (NOAA) Hurricane Hunters fly directly into the storm. Although satellite imagery provides much information, researchers need to get into the system to determine its exact strength and movement. It sounds dangerous, but with experienced pilots, these missions aren't overly hazardous.
After it's established that a system has the potential of becoming a tropical cyclone, the NHC starts sending out bulletins online every six hours describing the system, giving a forecast on intensity changes and future movement. If the storm is endangering land, the frequency of the bulletins increases to every three hours. The public advisory gives a plain-language account of the storm, including current strength, location, and forecasts for future strength. This information is also provided in map form to make it more visually comprehensible for the public.
If a tropical system is close enough to threaten coastal areas, the NHC issues either a watch or warning, depending on the storm's potential impacts. A watch means that tropical storm force winds may affect a given area within 48 hours. A warning means that tropical storm force winds are confidently expected within the given area in 36 hours or less.
Track forecasts are usually quite accurate. In 2018, NHC forecasters predicted a direct hit on Puerto Rico by Maria multiple days in advance, and landfall forecasts for Michael were accurate within 10 miles. Intensity forecasting tends to be trickier, since researchers don't fully understand the internal workings of a hurricane. Meteorologists use the term "rapid intensification" to refer to a storm system that's gained several categories of strength in a single day.
A research group at Colorado State University is working on long-range hurricane forecasts. Typically, in the spring, the team releases a report of tropical activity for the upcoming hurricane season. The El Nino/La Nina cycle is a major factor. The NHC also updates its forecast several times during the summer, with marginal accuracy.
Although it's impossible to relate individual events, such as hurricanes, to long-term trends, such as climate change, we can make some basic assumptions. As ocean temperatures increase, there'll be more energy available for tropical cyclones. Rising sea levels also increase the likelihood of storm surge flooding. While too many other factors are involved to make this significant for all storms or for a full hurricane season, researching these connections helps meteorologists predict the growing intensity of hurricanes.
Every year, hurricanes sweep down the coasts of the country, causing havoc and destruction at each turn. After countless catastrophic hurricanes, and likely with more to come, these storms have proven to be some of the most fearsome systems on the planet.
By Dr. Ed Brotak
For 30-plus years, Ed Brotak taught thousands of college students about weather, and helped many of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife and daughters.
What's in a Name?
The practice of naming storms started in 1953 to help the public better recognize individual storms. Today, an international committee of the World Meteorological Organization comes up with the lists of names used around the world. (There are different sets of names for the Atlantic Basin, the eastern and central Pacific, the western Pacific, and so on.) If a storm is extremely destructive, the name is permanently retired.
Saffir-Simpson Scale CATEGORY WIND SPEED DAMAGE (mph) 1 74 to 95 mph Very dangerous winds, producing some damage. 2 96 to 110 mph Extremely dangerous winds, causing extensive damage. 3 111 to 129 mph Devastating damage will occur. 4 130 to 156 mph Catastrophic damage is expected. 5 157 mph or more Catastrophic damage. Courtesy of National Hurricane Center
For the latest official information on tropical systems, visit the National Hurricane Center's website at www.NHC.noaa.gov.
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|Date:||May 1, 2019|
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