TYPES OF STORMS

There are two basic types of thunderstorms, based on what causes the air to rise. The most common are called air-mass thunderstorms, which occur during warm weather. As the earth heats, thermals develop and cumulus clouds form, some turning into thunderstorms. Often they develop over land and then drift out over the water. Because this takes some time to happen, air-mass thunderstorms usually occur in the late afternoon or early evening.

Gene BjerkeA classic thunderstorm cloud builds vertically. The other type of thunderstorm is associated with frontal systems. While any front can produce thunderstorms, cold fronts are usually to blame. As the wedge of cold air moves into a warmer region, it forces the warm air up and begins the thunderstorm formation process. This usually happens 100 to 150 miles ahead of the front, though it can be as much as 300 miles ahead if the front is moving fast. The result is the creation of a squall line: a line of side-by-side thunderstorms that may be 100 or more miles long, typically running north-south or northeast-southwest. These are more dangerous because they are difficult to dodge.

From a boater's point of view, thunderstorms present two problems. First is the blast of cold wind coming out from under the cloud. There is often little or no wind just before it arrives, then you are suddenly hit with a blast from the opposite direction that can blow anywhere from 40 to 80 knots. The first gust is the strongest and can capsize those who are unprepared. The wind that follows will still be strong enough to build a short, choppy sea, and then the rains begin.

The second problem is lightning. A single bolt of lightning can contain 100,000 amps of electricity at 30 million volts. Boats–especially sailboats–are usually the tallest object out there. Luckily, relatively few boats get hit, and proper lightning protection can limit damage.