Pea Soup or Peanut Butter?
Fog has enthralled mariners around the globe for centuries, both for its danger and its beauty. Both visual and audial fog signals have been used since antiquity. Among these are iconic lighthouses, sea buoys, horns, whistles and even cannons. Dense fog can reduce visibility to near zero. In urban and industrial regions, smoky fog (smog) can be not only blinding, but lethal, especially to the elderly, young or infirm. Thick, yellow-green smog was common in and around London, England from the late 19th century through the middle of the 20th century thanks to widespread coal fire smoke mixing with Thames Valley fog. This “London Fog” or “Pea Soup Fog” has become much less common during the last 50 years or so, thanks to better clean air standards. Perhaps some of you may remember the dialogue between Prospector Yukon Cornelius and Hermey the Elf from the 1964 Rankin/Bass animated television Christmas feature Rudolph the Red-Nosed Reindeer [Yukon: “This fog’s as thick as peanut butter! Hermey: “You mean pea soup.” Yukon: “You eat what you like, and I’ll eat what I like!”].
One simple (and accurate) definition of fog is “a cloud at ground level”. The more formal definition from the American Meteorological Society is: “water droplets suspended in the atmosphere in the vicinity of the earth’s surface that affect visibility”. The term “dense fog” is reserved for those fogs which reduce visibility to one nautical mile or less (at sea). Marine navigation on most vessels is affected in one way or another when the visibility gets this low. For this reason, your Florida Keys National Weather Service will issue a “Dense Fog Advisory” for a particular marine zone when fog is expected to reduce horizontal surface visibility to one nautical mile or less.
Fog is quite rare in the Florida Keys. Heavy fog (fog that reduces visibility to ¼ mile or less) is even rarer. On average, only 2–3 such days occur each year. In some years, no such fog is observed. In fact, the Florida Keys are one of the least foggy places in the continental United States. The foggiest locations include the Pacific Northwest, New England, and portions of the Ohio Valley and central Appalachians. One of the foggiest places on the planet is over the Grand Banks off Newfoundland. Here, warm, moist air from over the Gulf Stream frequently meets the cool, moist air from over the Labrador Current. The close proximity of these two ocean currents is also one of the primary reasons that fisheries there are so prolific.
Fog across the coastal waters surrounding the Florida Keys most likely occurs during the winter months of December, January, and February. It is during these months that local sea surface temperatures (and the temperature of the air immediately atop the water surface) dip to their lowest values of the year. When a sudden return of moist, tropical air returns after a period of cold weather, the stage is set for sea fog. Sea fog typically develops over Florida Bay or the extreme southeastern Gulf of Mexico, as well as the shallower nearshore waters adjoining the islands. It is much less common over the warmer, fast-flowing waters of the Gulf Stream/Florida Current. Reports from National Park Service personnel in the Dry Tortugas indicate that dense winter sea fog has occasionally enshrouded the area for two or three days in a row! The islands comprising the Dry Tortugas lie along the southern portion of the “West Florida Shelf”, the continental shelf sloping under the Gulf of Mexico westward of the Florida Peninsula. The shallower “shelf waters” tend to both cool and warm faster than the deeper Loop Current or Straits of Florida to the west and south, respectively. In the winter, successive cold air outbreaks over the southeastern Gulf of Mexico may lower the temperature of the sea surface (and the air immediately above it) into the 60s. When a warm front subsequently moves up from the south, the arriving tropical air mass will enrich the cool air with moisture, thereby increasing relative humidity to 100%, causing condensation to trump evaporation, which results in fog. Occasionally, the fog bank will quite literally “lift” when drier air begins to mix into the layer of air near the surface, thereby leading to a tipping of the evaporation-condensation balance in favor of evaporation. The resulting cloud bases may only be a few hundred feet off the ground. This type of cloud is called “stratus”, derived from Latin, and meaning “spread out” or “covered, as if with a blanket”). Stratus clouds are diffuse, generally grey and shapeless, and very low (based less than 1500 feet above ground). They usually form in a relatively continuous layer, and, not surprisingly, look like a fog bank based above the surface.
Should a rare dense fog visit Florida Keys waterways this season, the Florida Keys National Weather Service and U.S. Coast Guard would like to remind boaters to reduce speed, turn on running lights, and be careful. Please refer to the latest marine weather warnings, advisories, and forecasts on the web at www.weather.gov/keywest or on marine VHF channels 2, 4, or 5. Follow the Florida Keys National Weather Service on Facebook at www.facebook.com/us.nationalweatherservice.keywest.gov and on Twitter at www.twitter.com/nwskeywest), or call us at 305-295-1316. Finally, remember to be weather-ready, and stay safe!
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