Jim W. Lee, Meteorologist Intern
NWS Key West, Florida
Lines of cumulus clouds are a common sight in the Florida Keys, particularly during the Summer months. These cloud lines often produce showers or thunderstorms, and are a favored location for the development of waterspouts.
There are several mechanisms that lead to cloud line formation. Some cloud lines form parallel to (or directly along) the island chain - this is most common in the Summer. Other cloud lines trail off of the islands, over the surrounding waters. On Saturday, May 15, 2004, a trailing cloud line formed over Big Pine Key, Summerland Key, and Cudjoe Key, and extended west northwest over the nearshore Gulf waters. That cloud line is circled in the satellite image below.
Cloud lines like this one, that are not parallel to the island chain, usually form when the low-level winds blow across the Keys, and the islands are warm enough to cause updrafts (in the form of cumulus clouds) to develop. In the Spring, land areas in the Keys are usually warmer than the surrounding waters during the afternoon, because the Sun heats land more rapidly than it heats water. In addition, the atmosphere is often unstable in the low levels at this time of year. As a result, the air flowing over the water is unstable, and only needs a triggering mechanism to be forced upward, forming cumulus updrafts. Once these updrafts form, they will linger after that mechanism is gone (or after they have moved away from it), because they will still be located in an unstable airmass. In this type of flow, the heated islands serve as the triggering mechanism, and cumulus clouds form along and just downwind of the Keys. As each cloud moves downwind and away from the islands, a new one forms in its place. That cloud then moves downwind, and another forms, and so forth, until there is a line of cumulus trailing downwind from the islands.
The two images below show the May 15 cloud line from two different perspectives. The first image was taken from Lower Sugarloaf Key, looking toward the northwest, while the second image was taken about ten minutes later from Upper Sugarloaf Key, looking toward the west northwest.
On May 15, low-level winds were strong, blowing out of the east southeast (100 to 110 degrees), toward the west northwest. As a result, updrafts formed over the Keys and moved off to the west northwest, creating a cloud line in that direction. But why did a cloud line become so well-developed in only one location? If you look at a map of the Keys (as shown in the satellite image), you will see that while most of the island chain is oriented from east northeast to west southwest, the stretch from Big Pine Key to Cudjoe Key is actually oriented from east southeast to west northwest. Thus the wind on May 15 flowed over a much larger area of land in that stretch than anywhere else, because it traveled right along that group of islands. The air was able to heat up enough over that large area to rise and create cumulus updrafts, whereas at other locations, there was not enough land area to cause cloud lines to develop.
While the processes responsible for cloud lines of this type are often different than those that form cloud lines parallel to the Keys in the Summer, any type of cloud line can produce rain, lightning, or waterspouts under the right conditions.