The Warm, Wet Winds of August

by 14

August is the hottest month in the Florida Keys, both at sea and ashore, according to historical weather data. The sea surface in August averages a bath-like 87 degrees, while island air temperatures typically hover near 90 degrees for much of the day. The setting sun offers little relief, as temperatures above 80 degrees are common even in the dark of night. Tropical downpours may provide the only respite from the heat. Indeed, August is smack in the middle of the Florida Keys rainy season, with five to six inches of rain falling during the month in an average year. Heat and moisture energize atmospheric circulations in August, leading to four warm, wet winds: the gentle easterly breeze, the summer squall, the waterspout, and the tropical cyclone. The average wind speed along the Florida Reef tract in August is just under nine knots, and the wind blows from the east or southeast about 60 percent of the time. This type of wind has a steady character, blows across a large area, and is generated by a large-scale weather system. In the Florida Keys, this weather system typically is the high-pressure cell oriented east-to-west from near Bermuda to the Carolinas. This breeze typically freshens from north to south, as both latitude and air pressure decrease toward the Equator. Summer squalls may interrupt the gentle easterly breeze, on occasion. A typical summer squall will feature rapidly shifting and gusty winds. Surges of wind from a gentle breeze to near gale are common. Scattered summer squalls are typical of the Florida Keys rainy season, and develop as heated, moist air converges and subsequently rises into an atmosphere which cools rapidly with increasing elevation. First, puffy white cumulus clouds will appear. Soon, they will grow into towering masses of water in all of its phases (vapor, liquid, and ice). Eventually, the liquid portion will dominate, and fall as rain, bringing cooler air from aloft down to the surface. When the mass of rain and coolness hits the surface, air initially will rush out as a “gust front” in all directions. Occasionally, the tranquil-looking cumulus cloud can grow into a giant taller than Mount Everest when sufficiently injected by a feed of hot, moist air from beneath. Such a cloud is referred to as “cumulonimbus”, following the Latin-based classification so common in science. The enormous cumulonimbus can hold thousands of tons of ice and liquid water aloft thanks to a vertical wind called an “updraft”. The updraft can span eight miles in the vertical. The upper portion resides in a very inhospitable place with subzero temperatures, ice, and violent turbulence. Meanwhile, the base of the updraft is anchored in the hot, moist air at the earth’s surface. Infrequently, cumulonimbus squalls may become violent, delivering frequent lightning strikes, blinding torrents of rain, shifting winds with gusts exceeding 40 knots, and rough seas. A waterspout is a tornado over water – an offshore tornado. Research has shown that Florida Keys waterspouts exhibit a wide range of wind speeds, from below 40 knots to in excess of 150 knots. Most Florida Keys waterspouts develop beneath towering lines of cumulus or cumulonimbus clouds. Dark, flat-bottomed cumuliform clouds tend to spawn waterspouts, just at the hint of rain. Waterspout birth, maturation, and decay is tied closely to the updraft life cycle of a given rain cloud, and the updraft growth must be fast enough, strong enough, and in the right location for waterspouts to form. In all cases, waterspout should be considered dangerous, and avoided. August is the month in which long-time residents begin to recall names like Andrew (1992) and Katrina (2005). It is hurricane season, after all. Tropical storms and hurricanes born over the warm, tropical waters of the Atlantic Ocean and Caribbean Sea typically will move westward or northwestward, occasionally posing a threat to the Florida Keys. The winds in a tropical storm start at gale force (34 knots). The top end is 63 knots (a “hurricane” starts with a sustained wind of 64 knots). A 40-knot gale certainly will generate rough seas and hazardous marine conditions. At 60 knots, the interface between the upper portion of the sea and the lower portion of the atmosphere begins to blur as howling winds rip into towering waves, resulting in an accelerating horizontal stream of wind, foam, and water. Fortunately, 60-knot storms are rare in the Florida Keys. However, one should be wise to the four warm, wet winds of August in the Florida Keys. As always, be weather-ready, and stay safe!