Folklore for Forecasting?
Before the advent of the science of meteorology, folklore provided forecasting guidelines for generations of farmers and mariners. Since literacy levels were low, these predictive criteria were put in the form of easily remembered doggerel. Like most folklore, they have a kernel -- or more -- of truth to explain and justify them. Consider:
"Red sky at night, a sailor's delight;
Red sky in the morning, a sailor's sure warning."
Red skies occur when the sun shines through clear air right at the horizon, but illuminates the bottoms of clouds in areas of disturbed weather. Since most major weather systems (local squalls and tropical cyclones excepted) move from west to east, when the sky is red at sunrise it is because the eastern horizon is clear while a weather system approaches from the west. The opposite is true when the sun sets behind a clear horizon and illuminates the undersides of the clouds of a departing front.
"Mackerel sky and mares' tails,
Make lofty ships carry low sails."
Mackerel sky is created by altocumulus clouds, which have white or gray patches and are usually round in shape; mares' tails are cirrus clouds, ice clouds above the freezing point that look like delicate white feathers or streamers. These two cloud types are the first to appear at the approach of a cold front or low pressure system.
"When rain comes before the wind dories, gear and vessel mind;
When wind comes before the rain, soon you'll set your sails again."
Typically, substantial rain that begins before the wind picks up is another precursor of an approaching front, and days of bad weather can follow. If it begins to blow, then starts to rain, this is often the symptom of the approach of a local rainstorm or squall, which will likely be over in minutes or hours.
Well, here in South Florida we have our own folk saying (in the modern era of blank verse, don't expect a rhyme):
"When the first cold front of the fall passes through latitude 25, hurricane season has begun to end."
The 25th parallel is the approximate latitude of Miami. Cold fronts march from northwest to southeast throughout the year, but during the South Florida rainy season they seldom even reach mid-Florida. The main reason is that our summer visitor, the ubiquitous Bermuda High I've spoken of several times, blocks the fronts from moving as far south as Miami, so the presence of a cold front usually indicates the degradation of the blocking ridge, and that process normally continues through the winter. That doesn't end the hurricane season for the entire Atlantic/Caribbean basin, but it does reduce the risk of a Florida landfall -- and canceled or delayed cruise departures -- for three reasons:
The weakening of the ridge, or its contraction further east, allows cyclones moving west from the Cape Verde spawning grounds to make an earlier turn toward the pole, keeping them either out to sea or on a track that makes landfall north of Florida.
Behind a typical cold front is dry air. That is why the winter season has shorter periods of rainfall than summer in Florida; the periodic fronts inject dryer, more stable air into the region. This dry air entrains into any nearby cyclone, muzzling the hurricane generation engine.
Though there isn't much temperature change with the earliest fronts, eventually they do bring significant temperature drops, which, in turn, begin to cool the local waters.
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