May is a Transition Month

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May is a transition month, weather-wise, in the Florida Keys, bridging a warm and often breezy subtropical spring with a long, sultry, and typically wet summer. The sun has slowly but surely been climbing higher in the sky since the winter solstice in December. Weather patterns are changing all across the northern hemisphere, with snow cover retreating northward. Oceans, lakes, and rivers are steadily warming, storing solar energy as they do. Land areas are warming even faster, with afternoon air temperatures in the Florida Keys typically approaching the upper 80s by Memorial Day weekend. The breeze slackens, on average, during May, as compared with the windier winter and early spring months. The reduced wind speeds and hotter weather lead to a proliferation of local wind circulations created by “differential heating” between sea and land. These local winds take the form of sea breezes during the day and land breezes at night. May is the month in the Keys when sea surface temperatures typically breach the 80°F mark for the first time since the preceding November, making the ocean “swimmable” for many once again. We also witness the first weak tropical waves emerging off the coast of West Africa during May. These tropical waves will attain increasing size, intensity, and westward extent during the months ahead. Hurricane season, after all, begins on June 1. The average monthly wind speed drops significantly during May, down to about 11 knots, the lowest in over seven months. And, May cold fronts are almost unheard of in the Florida Keys. In fact, air temperatures below 75°F are increasingly rare by mid-month. Perhaps the biggest weather change occurring during May is the transition from “dry season” to “wet season.” The first week of the month often features cloudless skies with plenty of evaporation. Brushfires typically are a concern over the Florida peninsula as the hot sun beats down on ground which has been drying for several months. However, as the Gulf of Mexico, Atlantic Ocean, and Straits of Florida gradually warm, they become very effective at humidifying the air above. By the middle of the month, give or take a week or two, shower and thunderstorm coverage is on the increase, developing especially near sea and land breezes. The sea and land breeze circulations are strongest over the Florida peninsula where the sea-land temperature differences are usually greatest. Sea-breeze “fronts” develop, and move inland along the east and west coasts of the peninsula. Sea-breeze fronts loft the air out ahead of them, and when this air is warm enough, moist enough, and unstable enough, cumulus clouds develop and grow, often rapidly, into giant cumulonimbus clouds (i.e. thunderstorm clouds). Some of you, no doubt, are familiar with these majestic mountains of water vapor which can be seen with the naked eye, boiling and rolling over the Everglades north of the Keys on most summer days, and on many a May day, especially later in the month. Occasionally, these storms turn south, bringing a bout of foul weather to a Florida Keys island community or two. Infrequently, these storms may reach severe levels. A so-called severe thunderstorm is defined by the National Weather Service as a thunderstorm with wind gusts of at least 50 knots (or 58 miles per hour) or hail of at least one inch in diameter. The most relevant aspect of the “May transition” for the Florida Keys mariner is the emergence of a somewhat less predictable wind and rain regime, arising from the coincidence of a high-energy surface air mass with a predominance of local wind circulations. Therefore, it is especially important to keep an eye to the sky and stay abreast of the latest weather and forecast updates. The Florida Keys rainy season features many hours and days of sun and fun, punctuated occasionally, perhaps violently, by a few minutes of lightning, wind, and rain. By the way, National Safe Boating Week is May 19– 25. For more information, visit http://www.nws.noaa.gov/safeboating. Also, June 1 is just around the corner, and next month, we will cover Hurricane Season 2012. Until then, remember to be weather-ready, and stay safe!