by Bill Webb (Crackerboy)
“Climate is what you expect. Weather is what you get.”
Here in Florida the rainy season officially begins on May 20, and runs through the end of October. During this period we get from 85-90% of our total annual rainfall of about 60 inches. We’re entering the season of the southeast trade winds, the balmy breezes that circulate around the Bermuda high, passing over the warm waters of the sub-tropical Atlantic, then across the warm shallows of the Bahamas. This southeasterly flow, combined with the surface low pressure over land caused by strong thermals, brings huge quantities of warm, moist air across the peninsula from the east. At the same time a weaker flow from the Gulf of Mexico brings similar conditions to the West Coast.
As the land heats throughout the day, more and more of the humid air is carried upward as the air near the ground becomes heated to higher temperatures than the air immediately above, causing it to rise. The rising air cools as it expands in the lower pressures of higher altitudes. This causes the moisture in the air to condense into tiny droplets and form clouds.
When the moisture condenses it releases great quantities of additional heat (latent heat of condensation), overcoming the cooling effect of the expansion, and carrying the air higher. At the same time, as the column of air rises, more warm, wet air flows in at the bottom. If there is enough moisture and enough heat the process continues, bringing in still greater quantities of water vapor. The droplets in the clouds combine into larger drops, and as they are carried upward, into larger drops yet.
When the weight of the water drops becomes great enough, the rising air can no longer support them and they begin to fall as rain. In the case of the most powerful updrafts, drops can be carried to altitudes where they freeze. If they remain above the freezing level, they accumulate layer after layer of ice, eventually falling to the ground as hail. (This is different from sleet, which occurs when rain freezes while passing through cold air near the ground.)
The cold raindrops carry cold air along with them as they fall, creating downdrafts. When these cold “waterfalls” hit the ground they spread out, causing the chill wind that blows out of thunderstorms. This wind also counteracts the inflow of warmer air, while at the same time cooling the ground, assisted by the rain. Eventually the cooling at ground level overcomes the tendency of the air to rise, and the storm—at least in that area—begins to dissipate.
The winds aloft in the upper atmosphere generally blow from west to east. As thunderstorms grow higher, they begin to be affected more by the winds aloft than by the winds at the surface. This is why thunderstorms tend to form in the west and move toward the east.
In Florida the inflowing air from the Atlantic and Gulf tends to form lines of moderate activity in the mornings known as “sea breeze fronts.” As these lines of clouds move inland from their respective coasts, they eventually collide toward the center of the peninsula and produce a line of towering thunderstorms. These storms, under the influence of the winds aloft, move toward the east or northeast in a line, with tops that can reach 50,000 feet (15,240 meters) or more. Their powerful downdrafts can produce surface winds of sixty to seventy miles per hour in localized areas, and tornadoes are not uncommon, especially in storms that approach from the southwest. (This is true in other parts of the country as well.)
This image of a lightning strike next to a tornado was taken near Lake Okeechobee in 1991. It has been “Photoshopped” and used in a variety of ways since. This is the original.
Florida has the most violent thunderstorms in the world. Were it not so flat, the flooding alone would claim many lives each year. As it is, pouring water on South Florida is like pouring it onto a tabletop: it doesn’t flow off very fast, but neither does it get very deep. Street flooding is common, but it is rare for anyone to lose a life due to flooding unless they drive off a flooded bridge or road into a canal.
Lightning is a different story. On average, about 10 lives per year are lost to lightning in Florida, with about four times that many injuries. In a manner not well understood, the hundreds of tons of moving air, combined with the freezing layer, create a massive static electrical charge. When the difference in charges, either between two sections of cloud or between the cloud and the ground become great enough, the static electricity discharges as lightning.
The National Weather Service has the following to say about lightning safety:
How to stay safe when lightning is around: use the 30-30 Rule!
- The best defense is to plan ahead and avoid exposure to lightning when a thunderstorm occurs. Know where safe shelter is located and leave enough time to reach safe shelter before your danger level is high. Don’t be an isolated tall object, and don’t be connected to anything that may be an isolated tall object.
- NSSL’s scientists and collaborators did a study to find out how close is too close. They found that 80% of the next lightning strikes in a storm are within 2 to 3 miles of each other in Florida, but as far as 6 miles from each other in Oklahoma. Use the ’flash-to-bang’ method to find the distance to lightning. Safe shelter must be reached by the time a flash is within 30 seconds flash-to-bang. In most cases, then, when you can hear thunder you are no longer safe.
- But there is often blue sky in some direction while lightning is occurring nearby, and it may not be raining, so pay much more attention to the lightning than the rain. A particularly difficult situation is the first flash from a storm–watch for a storm that is growing quickly, such as when a storm is becoming very dark at its base or is growing very tall.
- An equally dangerous situation is when a storm appears to be finished, and only light rain and/or occasional thunder are heard, but the cloud overhead continues to be fairly dark. The most common situation for a lightning death or injury in Florida was found NOT to be in the heaviest rain area with lots of flashes, but after or before the time when rain and lightning was the most intense. So, the weak storm without too many flashes, or early or late in the life of a larger storm, is most dangerous.
- The best shelter is a substantial building that has plumbing and wiring–in other words, one that is used or lived in by people for a major portion of the day. An unsafe building for lightning has only a roof and some supports, but no wiring or pipes extending into the ground. A vehicle with a metal roof provides good shelter, and is much better than being in the open or in an ungrounded building, but is not as good as being in a building that is grounded by wires and pipes.
Combined with the low, dark clouds, the wind, and torrential rain, lightning can turn a Florida thunderstorm into a most impressive and memorable experience. But there’s still nothing like a big thunder-bumper outside, a tin roof, and a nice soft bed. Man, that’s sleepin’ weather!
Y’ll be careful now, y’hear?