ComPilots Aviation News Portal - Weather in the Deep South / Rose Marie Kern

I flew low altitude from Atlanta to Orlando once, and as I looked out of the window I saw lakes, ponds and streams sparkling like a chest of diamonds flung haphazardly on an emerald carpet before a fireplace. All that water! For a girl from the desert it was almost a religious experience.

And yet, this past year there were reports of widespread drought causing a rash of forest fires. How? How can that lush landscape dry up so much when the ocean hugs its edges, and at the same time, how can there be floods and storms from central Texas all the way to Kansas?

The answer lies in a little recognized weather system known as either a Bermuda or Azores High Pressure. This enormous system is parked over the Atlantic Ocean just north of the Equator and its diameter varies from mere hundreds of miles across to thousands. Believe it or not, a Bermuda High can determine where and how a hurricane will make landfall.

Picture a wide brimmed popcorn bowl turned upside down on a map of the Atlantic, with the center just east of the Bermuda Islands. The winds flow clockwise, or anti-cyclonic around the edges of the bowl. Under the bowl the air is very dry and stable.

If the bowl is west of its usual place over Florida, the dry high pressure sits on top of the Sunshine State and refuses to allow all that lovely ocean moisture to revitalize the land, instead the moisture is flung westward into the Gulf States where flooding ensues from Texas up to Kansas.

Just as the Bermuda High can sweep hurricanes from the Atlantic into the United States, it also drags warm tropical air northward. Because the edge of the high sits farther west than normal (closer to the U.S. East Coast), hot humid air is circulating farther inland, leaving the Midwest hot and sticky.

If the bowl of high pressure is over or east of Bermuda then Florida, Georgia and the Carolinas get their normal amount of rain. Now let’s look at how it affects hurricanes.

June through November is considered “Hurricane Season”. During this time a band of abnormally warm water spans the Atlantic Ocean from Africa to Central America. This warm region—Hurricane Alley—is the birthplace of hurricanes in the Atlantic, and unusually warm waters means that storms have plenty of fuel to develop.

A normal Bermuda High often leads to hurricanes moving up the east coast and back out to sea. If the Bermuda High expands to the south and west, it has the power to steer hurricanes into the Gulf of Mexico rather than up the east coast or curving out to sea. This almost always ensures landfall.

The intensity of a land falling hurricane is expressed in terms of categories that relate to wind speeds and potential damage. The Saffir-Simpson Hurricane Scale expresses hurricane strength from a Category 1, which has winds only about 74 to 95 knots and does relatively minor damage, to Category 5 with windspeeds of over 155 knots and can cause massive widespread damage to buildings and shorelines. The strongest winds usually occur in the right side of the eyewall of the hurricane.

You can keep track of what is happening in the Atlantic by accessing the following website from NOAA: http://aviationweather.gov/obs/sat/intl. This will allow you to choose from several satellite pictures. Watch the equatorial band where the hurricanes form, then as they move northwest you can “see” the edges of the Bermuda High as it steers them.

The deep south has no real mountain ranges until you get north to Tennessee, so weather flowing along the usual patterns from west to east encounters nothing to slow it down. Even without any major fronts, troughs, or low pressure zones, moisture from the gulf and from the Atlantic may hang over the land for weeks – which plants love but people and airplanes hate.

Remember that Density Altitude problems are not just caused by high field elevations. Heat and moisture both play their parts in making it difficult to break ground effect.

Rose Marie Kern has worked in ATC for over 25 years. If you’d like to ask her a question, contact her at author@rosemariekern.com.