Wx Watch: Bermuda Blocking

Some highs aren't so nice

September 1, 2005

Let me tell you about one of the biggest large-scale summer weather systems that can affect the entire eastern half of the United States. It can cause huge thunderstorm complexes, make density altitudes soar to dangerous levels, and drop visibilities to instrument meteorological conditions values in haze. Worst of all, this type of system creates all these hazards day after day, and can last for days or weeks at a time.

Care to guess what this threatening system is? A mesoscale convective complex? Nope. Squall lines? No again. Fast-moving cold fronts? Sorry.

We're talking about the Bermuda high, perhaps the largest of the high-pressure systems affecting the continental United States. While most pilots automatically equate high pressure with good flying weather, this is not always the case. And one of these exceptions is the Bermuda high. This is a high that can extend from the mid-Atlantic Ocean to the Mississippi River. It's the circulation around this high, and its stagnant nature, that can cause problems for pilots.

It's called the Bermuda high because the center of its high pressure — the area where barometric pressures and altimeter settings are the highest — is situated more or less in the vicinity of Bermuda. The ocean's moisture is at the heart of the Bermuda high's mischief. The clockwise circulation around the Bermuda high sends all that warm, damp ocean air around the "back side" (western portions) of the high, and deep into the entire eastern United States. There, afternoon heating causes this moist air mass to de-stabilize.

This sends vast amounts of unstable air skyward, which in turn causes condensation, its associated release of latent heat, more lifting, and — ultimately — massive cumulus buildups. The result can be widespread air-mass thunderstorms over large portions of the affected areas. These typically form in the early afternoon hours, reach their peak intensities by late afternoon and early evening, and die down after sunset.

The Bermuda high is a semipermanent feature of summer weather, meaning that it can persist for long periods of time. A recent look at some of the surface analysis charts for early June 2005 showed that a Bermuda high dominated eastern U.S. weather for a full week — from June 5 through June 12. On each of these days, thunderstorms affected huge sections of the eastern states, from Wisconsin to Florida, and from Maine to Louisiana. As the storm-soaked ground cooled overnight, low-level moisture condensed into ground fog by daybreak in many locations, causing many airports to report low-IFR conditions (ceilings below 500 feet, visibilities below one statute mile).

At the same time, daytime high temperatures reached the 90-degree Fahrenheit mark in the mid-Atlantic and central states, and the high 80s elsewhere in the affected areas. With clear skies above any lower layers of cloud or fog, rising temperatures serve to heat up the surface and evaporate, or "burn off" any low-level scuzziness left from the previous night's rainfall. But it's those same rising temperatures that set off the day's convective cycle — and create high density altitudes.

Not even the arrival of Tropical Storm Arlene (at the Florida Panhandle on June 11) caused this Bermuda high to break its stride. It was so big, and so motionless, that it persisted for another few days, only to subside and reassert itself again from June 24 to June 27.

When it's in sway, a Bermuda high can prevent the usual west-to-east progression of surface fronts. This puts it in a category meteorologists call blocking highs. With a high this strong, day after day of sweltering temperatures persist under inversion layers that trap a soup of haze and pollutants beneath it. Meanwhile, any frontal relief stays at bay in Canada, northern New England, or the Midwest.

You can locate Bermuda highs on any surface analysis chart. You'll see the high's center marked by an H symbol somewhere off the East Coast, and see its isobars radiating out across the eastern half of the country. For further confirmation, check constant pressure charts — such as the 500-millibar chart, which corresponds to 18,000 feet msl — and you'll see that the Bermuda high's dome can extend into the flight levels.

On days when Bermuda highs take over, be sure to make an extra careful check of all convective forecast products. These include convective sigmets, convective outlooks, and the collaborative convective forecast product, or CCFP, all viewable online ( http://adds.aviationweather.noaa.gov/convection). Of course, you'll also want to check the latest Nexrad radar imagery updates prior to launching, as well as obtain a complete weather briefing on all other weather and regulatory aspects relevant to your flight.

And remember, high pressure can be ugly!

E-mail the author at tom.horne@aopa.org.

Links to additional information about high-pressure systems on AOPA Online ( www.aopa.org/pilot/links.shtml).

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.