Prevailing Winds: The Trades

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The striking constancy of the easterly wind is a characteristic of Florida Keys marine weather which is quickly apparent to those boaters and guides hailing from areas north of the Florida Peninsula. It turns out that persistent easterly breezes are common throughout vast regions of the subtropics and tropics around the globe. The term “trade wind” was coined centuries ago when European sailing ship captains noticed a persistent following wind once they reached subtropical latitudes during their “trade” voyages to the New World. In parts of Hawaii, the easterly breeze is so persistent that weather forecasters often do not bother to include a wind direction in their forecasts (e.g., “moderate trades becoming fresh tonight”). The native Divi Divi trees in Aruba bend westward owing to both the persistence and strength of the trade winds at that southern Caribbean location. Trade winds are so persistent across the Caribbean basin that most of the larger island ports were built on their leeward or western sides. In fact, both Old Town Key West and Key West Harbor were built and remain on the western (and usually, leeward) side of the island. An analysis of over 20 years of hourly wind observations at the Sombrero Key Lighthouse on the Florida Reef tract south of Marathon indicates that winds blow with an easterly component (from 020° to 160° on the compass) over 70% of the time. Winds often blow fresh out of the northeast during the winter months, while they are generally lighter and from the east or southeast during the summer months. The easterly winds often are not limited to just the surface layer of the atmosphere. During the summer, they commonly extend upward to 20,000 feet or higher above the ground. This has consequences for local summertime weather since many tropical waves, tropical storms, and hurricanes move westward in the trade wind belt. Occasionally, dust from the huge Saharan Desert is transported all the way to the Florida Keys, over 3000 miles away. When this happens, the celestial dome above the Florida Keys takes on an eerie, grayish hue, lying in dramatic contrast to the more common, crisp, blue skies. The persistence of the trade winds can be attributed to the huge, semi-permanent high pressure cells that dominate surface weather maps over the subtropical Atlantic Ocean. The “Bermuda High” is a term used for the high pressure cell centered most often within a few hundred miles of the subtropical island nation of Bermuda in the North Atlantic. In the Northern Hemisphere, winds spiral clockwise and outward around a high pressure cell. High pressure cells affecting the Florida Keys are usually located to our north, northeast, or northwest, depending on the time of year. As a result, winds usually blow out of the northeast, east, or southeast, depending on the exact location and shape of a high pressure cell on a given day. Winds can also be modified by local effects such as sea breezes and the gusty winds in and near showers and thunderstorms. The winds blowing through the islands of the Florida Keys produce countless combinations of “windward” and “leeward” exposures. On a windy day, these situations offer a larger number of navigational options to experienced guides and captains in the Keys, relative to their counterparts operating in coastal waters adjacent to “straighter” coastlines. The Florida Keys island chain extends in a southwesterly arc from the southern tip of the Florida Peninsula. Of course, scores and scores of smaller islands and islets populate the adjacent coastal waters of Florida Bay, Hawk Channel, and the extreme southeastern Gulf of Mexico. Many of these small islands are surrounded by very shallow water. It turns out that wave growth is limited significantly in shallow water, especially on the leeward sides of islands. As a result, the seas can differ greatly across short distances on a windy day. Take, for example, an east-northeast wind which has been blowing at 20 to 25 knots for several days. You will easily find seas exceeding 8 feet in the Gulf Stream right off the Florida Reefs in such a wind regime. Also, Hawk Channel seas may reach 4 to 6 feet in some areas given the east-northeast fetch. However, the waters would be much calmer just on the other side of the Overseas Highway because most of Florida Bay is less than six feet deep, as are large expanses of the Gulf-side flats from Marathon to Halfmoon Shoal. Again, wave growth is limited by depth. In fact, if the water depth is less than about 15 feet, your National Weather Service Coastal Waters Forecasts will describe the sea by using “chop” phrasing rather than actual wave heights (e.g., “a light to moderate chop”, “choppy”, or “rough”). Safety on the water depends on not only winds and waves, but also the area in which you operate, the characteristics of your vessel, the experience and diligence of the captain or guide, other marine traffic, and the use of life jackets. For any trip on the water, remember to be weather-ready, and stay safe! wc-NEWprevailwinds-(1)


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