Weather Wise: Hurricanes

Hey everybody! Have you ever been curious how a small thunderstorm in the open ocean can intensify and become a swirling force of nature capable of wreaking havoc on an entire coast line? In this, the latest posting in a series of weather education blogs we like to call Weather Wise, we’re gonna take a closer look at one of the most powerful forces of nature, Hurricanes.

Hurricane research, tracking and forecasting has come a long way in recent years. New technology has allowed us to spot these storms looming in open waters long before they come ashore. In 1900, Galveston, Texas was blindsided by a category 4 hurricane with winds estimated at 145 mph. The storm surge on that particular storm was responsible for the deaths of nearly 8,000 Texans. The Galveston Hurricane is still known as the deadliest natural disaster to ever strike the United states. Fortunately, our technology has improved enough over the last century so that hurricanes like this one will not go unnoticed. Satellites allow us to keep eyes on these monsters from space, 24 hours a day, 7 days a week. Computers help to process the large amounts of weather data and produce models to tell the meteorologists where the hurricane will track. These technologies and others like them, in partnership with several scientists have helped to increase the warning times and pinpoint the long stretches of coast lines that will be affected, days before the hurricane strikes. This has helped to cut back on needless deaths in recent years.

File:Galveston Hurricane (1900) SWA.JPG

-Surface Map of the Galveston Hurricane just before it made landfall.

So what is a hurricane exactly? When the oceans near the Equator get hotter than a $2 pistol, and other atmospheric conditions become favorable, thunderstorms form with ease in what you can call a breeding ground for hurricanes. Compare it to a college house party — when the music gets turned on, (and I’m not talking about Bob Segar’s Greatest Hits,) before too long you have a living room that gets transformed into a dance floor. Those party animals start bumping into friends and inviting others nearby and before long, you have a raging kegger on your hands. The same goes for the tropics. Once water temperatures near 80°, the slightest breeze can easily send some of that low level moisture mixing into the atmosphere. Once that moist air parcel rises and condenses, a cloud forms. It may sound silly, but the first step in hurricane formation is a cloud.

-Artist's depiction of such a cloud.

The general area of thunderstorm formation is a meteorological mouthful. The Intertropical Convergence Zone, or ITCZ for short, is an area primed for thunderstorms. In this same area just north of the equator (June-Nov), the southward and northward flowing tradewinds come together to form a crease in the atmosphere. At the same time, a jet of air flows across the Atlantic Ocean from north western Africa. Sometimes, this causes a kink to form in the crease, a beep in the boop, or we could just make things easy and call this a Wave. These atmospheric waves help to get our simple clouds and small thunderstorms to intensify and form tropical disturbances, the next big step in hurricane formation.

Fast forward from your lonely cloud, through the simple thunderstorm phase and all the way to a small cluster of thunderstorms known as the disturbance. This small shapeless blob may begin to form a closed circulation, or simply put, the thunderstorms start to tango and spin around one another. The cause of this spin is the Earth’s rotation on it’s axis, similar to why the water in your toilet bowl spins counterclockwise in the northern hemisphere, (or clockwise in the Southern Hemisphere.) Soon enough, the pressure in the center of these thunderstorms begins to drop, forming a center of Low Pressure.  The storms spinning around the low begin to organize further and winds become sustained from 29-39 miles per hour, and what began as a small cloud has now turned into a tropical depression.

The National Hurricane Center starts to take note of the further organizing system and they might begin to start dispatching Hurricane Hunters to investigate. The probes they release into the storm are not containers filled with Prozac to treat the depression, * Que Rim Shot*, rather they are chock full of instruments to gather data on the impending threat. Sure enough, they note a closed circulation and that the overall shape is now more circular and that they are clearly rotating around an increasingly lower pressure center. The winds now pick up to sustained speeds greater than 39 miles per hour, and thus a Tropical Storm is born. At this time, the National Hurricane Center gives this developing monster a name.

-Note that the names alternate back and forth between male and female. Also, there is a separate list of names for storms that form in the Pacific Ocean.

Tropical Storms alone can drop torrential downpours and cause massive amounts of flooding inland if they make landfall, but they still pose a smaller threat than a hurricane in most cases. A hurricane is just a glorified tropical storm. Pressures in the center continue to drop and satellite images continue to show a developing “eye.”  So what separates a tropical storm from a full fledged hurricane? If this spinning top of thunderstorms contains sustained winds of 74 miles per hour or greater, the tropical storm gets upgraded to hurricane status.

Structure of airflow around the eye of a hurricane

-Cross Section of a Hurricane.

Hurricanes contain a wide variety of hazards that pose a danger to us as humans. Whether it’s flooding from the rains above, or flooding from the storm surge, winds toppling the trees or the embedded tornadoes tossing them, hurricanes really can pack a punch. Some people call tornadoes, “Fingers of God.” If that’s the case, a hurricane should be known as “God’s Fist.”

The most dangerous and deadly hazard presented by hurricanes that make landfall is the storm surge. Simply put, a storm surge is a large dome of water that rises under a hurricane due to the low pressure that moves over the shoreline causing extensive damage to beaches and structures. Like I stated at the top of this post, this is what wreaked havoc in Galveston over 100 years ago. Obviously the damage reaches it’s maximum potential if the hurricane also makes landfall at the same time as a high tide. There are 3 factors to determine the intensity of the surge. Wind speeds, water depth and the intensity of the low pressure center all determine the height of the surge. For a larger storm surge, you would look for high wind speeds, an intense center of low pressure and shallow waters. Category 5 winds combined with the other factors can produce a towering storm surge 25 feet high. The most intense surge of water occurs near the low pressure system and in the quadrant of the hurricane where the winds are blowing towards the shore. The surge is powerful and comes ashore like a small scale tsunami, destroying everything in it’s path. This is why forecasting lead times and coast line evacuations are extremely important.

Storm Surge

-Storm Surge - Courtesy Lutgens & Tarbuck, The Atmosphere, 7th ed.

The winds alone in an average sized  hurricane, 74 mph or greater, can spread out over 100 miles and the tropical strength winds, 39-73 mph can extend several hundred miles from the center of the storm. The hurricanes are categorized by their sustained wind speeds. The Saffir-Simpson Scale rates these storms from 1-5 with a Category 1 storm having winds from 74-95 mph and a Category 5 hurricane having sustained winds 155 mph and higher. The winds are measured by the anemometer, which can resemble a model airplane without wings, or 3 spinning ice cream scoops. The wind damage itself is exponential as it increases in speed. 120 mile per hour winds are not going to do twice the damage as 60 mph winds, rather they will do almost 100 times the damage.

-A cup anemometer, also known to small town folk as the "Whirly-Gig." Yes, while chasing we have gotten several people ask us what the whirly-gig does.

Embedded within the spiral rain bands are another, usually small scale threats… tornadoes. As the hurricane spins on it’s axis, there are usually isolated cases of some “spin-ups” or small tornadoes. Most of the time these do not go reported due to the limited visibility and the fact that the damage is hard to disseminate from damage the hurricane itself caused.

The final punch that the hurricane packs is torrential coastal and inland flooding. If a roaring storm surge, hidden tornadoes and winds so strong you can’t walk weren’t bad enough, add to the mix rainfall rates of more than a few inches an hour and you have a lot of cleaning up to do when it’s all said and done.  The slower the hurricane is moving, the more potential for flooding you have. It’s very difficult to accurately predict the amount of rainfall locations will receive that are impacted by a hurricane, but generally an average storm will drop anywhere from 6-12 inches of rain or more over a 12 hour period depending on speed.

So from disturbance to depression, from tropical storm to category 5 hurricane, the easiest way to avoid any and all of the above risks is simple. Move inland. The storms rely on the warm ocean surface to fuel their growth and sustainability. You will often notice that when a hurricane moves over a large land mass or onto the United States mainland, the storm rapidly deintensifies. So if you are wanting to avoid such risks, pass on buying the ocean front property.

Always Remember, some are Weather Wise and some are otherwise… don’t be the latter. Again, if you have any questions, comments, complaints or suggestions, please email them to Brad@SWATChasers.com. If your questions are good enough they may be featured in the next Weather Wise segment!

-Brad Maushart

www.SWATChasers.com and www.SWATTours.com

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