Dissecting the Wind
Why does the wind report from Sand Key differ so much from the Key West airport observation, even though only nine miles separate the two locations? The same question could be asked of the Sombrero Key and Marathon airport observations, with only four miles distance between them. Why is the wind instrument on my vessel always reading higher than the marine forecast? And, what is a “sustained” wind versus a “gust” or a “squall”. Hopefully, the answers to these questions and others will be answered in this article, as we explore and dissect the wind.
Wind is a timely subject for us in the Florida Keys. November is, after all, the windiest month on average in the Florida Keys. Last month, we reviewed the Beaufort Scale, which describes in simple, poetic clarity the great variation which exists among a range of wind speeds starting at calm and ending at hurricane. When the hum of a persistent breeze fades, an eerie stillness settles across both land and sea. At the other end of the spectrum, when a strong gale grows into a hurricane, the scene transforms dramatically into one of relentless agitation and violence. A 75-mph sustained wind results in a persistent sonic blast that cannot be ignored. Through most of the year in the Florida Keys, wind speeds range between about 5 and 25 knots. Dead calm conditions are relatively infrequent, as are gales. True hurricane conditions are rare.
Wind is one of the more fickle weather elements in the Keys, and its “character” occasionally can be difficult to pin down, especially as breezes freshen from “gentle” to “strong”. Wind speed is measured by a device called an “anemometer”. The actual wind speed at any location depends on several factors, including altitude, exposure, and gustiness. The wind right above the surface typically is lower than even 20 or 30 feet above the surface, and much lower than 100 feet elevation. Also, wind over an open expanse of water tends to be higher and steadier than the wind along the coast or inland. Trees and houses disturb the wind, resulting in a more turbulent flow of air over land. This leads to a less steady breeze, with more peaks and lulls. Also, during the daytime, heating from the sun causes air over land to warm faster than air over water, leading to sea breezes near the coast and thermals (turbulent vertical wind) toward island interiors. Subsequently, the wind character can be quite gusty over and near land masses during the daytime. At night, the land cools faster than the water, and this can lead to a “decoupling” of the air mass near the ground from the air just a few hundred feet high, resulting in a situation where near calm conditions “inland” are coincident in time with a moderate breeze alongshore and a fresh breeze well offshore. Particularly gusty conditions are associated with very turbulent air flow which usually happens when the air temperature cools rapidly with increasing height, such as which occurs over the Gulf Stream after a cold front.
Another aspect of the assessment and comparison of wind speeds from different instruments, locations, and exposures is the issue of “averaging period”. The sustained wind reported by most National Weather Service anemometers is really an average wind over a two-minute period. Wind is always changing with peaks and lulls occurring all the time. The two-minute average is useful for assessing the impact of the wind on things like aircraft or power lines. However, within that two-minute average you will find gusts which exceed the average typically by 20–40%. The percentage increase that a gust has over the sustained wind is called the “gust factor”, and it becomes more important as the average wind speed increases. Think of it this way: a gust factor of 30 percent with a 10-knot wind will yield a 13-knot gust, whereas the same gust factor applied to a 25-knot wind yields a wind gust of 33 knots! Gusts are important, as any mariner is well aware.
The wind speeds included in National Weather Service marine forecasts are “sustained” winds valid at 10 meters (or 33 feet) above mean sea level. Many wind observations are taken at about this altitude, which is one of the reasons this height is used. Another reason is that a diverse collection of captains, guides, ship masters, and recreational boaters utilize these forecasts. Some operate vessels less than 15 feet in length right on the surface, while cruise ships, cargo ships, and tankers can rise over 100 feet above the sea. Specific wind gusts are included in the forecast usually when the sustained winds reach about 25 knots. The descriptor “and gusty” is used on occasion to indicate wind gusts of 5 to 10 knots over the sustained wind when the sustained winds are forecast between 15 and 25 knots.
To view a handy, hourly weather graph of forecast wind speeds at popular marine locations around the Florida Keys, bookmark the following site on your web browser or smartphone:
http://www.srh.noaa.gov/key/?n=marineweathergraph
Hurricane season ends November 30th. Remember to be weather-ready, and stay safe!
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