Florida's Rainy Season

 

Thunderstorms, T-showers, thunderboomers and gulley washers are just a few names for Florida’s heavy rain showers. Every summer afternoon, chances are very good that we will experience a heavy thundershower, usually with strong winds and lightning. Why is this?

In south Florida the rainy season lasts from May to September. This corresponds to the time of the year when the relative humidity is highest and the sun is at its most intense heating potential. By midafternoon, temperatures of the land will soar to well over 100 degrees. Try walking on the beach or on pavement in bare feet! The air above the land absorbs this heat by convection (direct transfer) and rises. This causes the afternoon sea breeze as moist air comes in from the gulf to fill this void. This warm air rises into a cooler atmosphere. As it does, it cools and the ability of the air to hold all that moisture drops. Rain droplets and ice crystals form. When the weight of these droplets gets high enough, uplifting air currents are insufficient to keep them in suspension and heavy rain falls to the earth. Now the reverse process occurs. Air temperatures at the surface begin to cool from the relatively cold rain falling. Temperatures can drop 20 degrees or more during the initial phase of the thunderstorm.  Serious downdrafts occur. So severe in fact that many aircraft crashes have been blamed on downdrafts near and in thundershowers.  This severe turbulence is also what causes the lightning associated with most T-showers. Friction between clouds and air masses causes static electricity differentials of millions of volts between clouds and between clouds and earth. Lightening is the arcing of this charge. The thunderclap is the sound of heated air expanding and then collapsing into the void created by the arc.  These types of thunderstorms are usually quite local. Often in our community, it can pour on the tennis courts, cancelling all tennis. But when I get home the pavement is dry or barely wet, showing the fickle nature of these storms.

Here is a typical summer afternoon as captured by my roof top weather station. Notice the morning and midday winds are out of the southeast. In midafternoon, the wind shifts to the west, bringing in the moist sea breeze. At 9 pm the torrent of rain begins, dropping ½ inch of rain in about 15 minutes (a rain rate of 1.79 inches per hour!). Wind gusts are substantial.  By 10 pm the rain has ceased, and the wind direction returns to the prevailing easterly direction.

Another type of summer thundershower is caused by the convergence of two air masses. Familiar to all beach goers is the sea breeze which comes in off the ocean every summer day by midday. Land heats quicker than water. So as the land heats up on a summer day, the air rises, and the cooler ocean air comes in to fill this low-pressure area. Sea breezes are generated on both the Atlantic and the Gulf Coasts. Now, the Florida peninsula is only 150 miles wide. So, these moist sea breezes meet over the mainland and the air is forced to rise by convergence. The same atmospheric cooling as in thundershowers now occurs and severe thundershowers happen. These type of showers are much larger, covering hundreds of square miles or more.

Rainfall rates during these summer storms can be truly prodigious. 1 ½ inches per hour are not uncommon. 3 to 4 inches per hour for short durations can also occur. Visibility can be cut to less than 20 feet! This can happen very suddenly. If driving on the interstate at 70 mph, this can be quite dangerous. If you slow down or stop, you run the risk of being hit from behind. Even pulling onto the shoulder can be difficult. When I’m driving and see a storm ahead, I will move to the slow lane, slow to 20 mph, put on my emergency flashers and hope the person behind me has noticed. (Adaptive cruise control, if active, will prevent me from hitting a slow or stopped vehicle in front.)

But all in all, the power and awe of a Florida thunder shower is truly exciting. Kerri and Elliot would agree.