My crewmate and I were killing time waiting for the race to start, when I saw other sailors staring and pointing. Curious, I turned to see the biggest, blackest, meanest-looking thunderstorm cloud I had ever confronted, and it was bearing down on us. I changed course toward the nearby shore and told my one-man crew, Pete, to get the anchor ready to set if the wind fell. Instead, it shot up to about 60 knots.
Pete dropped the anchor while I clawed down the mainsail and pushed it, complete with boom, under the deck. The anchor must not have found bottom yet because we remained broadside to the wind. I was hiked out as far as I could go on our little 17-foot boat, with Pete lying on the weather deck, but the lee rail was buried in water from the pressure of the wind on just the bare mast. Minutes later–it seemed much longer–the weather let up, the anchor caught and we turned into the wind, giving us a chance to clear up the mess and observe the scene: most of the fleet had capsized (the papers estimated about 100 boats).
There were a number of stories to be told about the Governor's Cup, held on Virginia's York River all those years ago, but they all had the same lesson: thunderstorms should be taken seriously. Knowing how thunderstorms work, and how to react to them on the water, is a vital part of safe cruising.
HOW THUNDERSTORMS WORK
Thunderstorms need two things to form: heat and humidity. That is why they most often occur during spring and summer in humid parts of the country. Though they can occur anywhere in the United States, they are much less common in Alaska, Hawaii and on the West Coast.
Cumulus clouds (which take their name from the word, accumulation) form when warm, moist air rises and moisture condenses to form the cloud. Occasionally, a cumulus cloud will continue to build vertically, becoming a cumulonimbus cloud. At about 25,000 feet it encounters cooler temperatures and water droplets coalesce into larger drops that begin to fall. The friction of the falling drops, and their cooling effect, creates downdrafts.
They are rare, but thunderstorms are capable of producing waterspouts, as pictured here, or tornados. At its mature stage, the cloud is a roiling mass of updrafts and downdrafts with temperature changes within it and precipitation in the form of rain, snow or hail being tossed up and down. The friction of all this activity creates great quantities of static electricity, which provides lightning and thunder. The downdrafts become dominant and flow out of the bottom of the cloud, spreading out in all directions as they encounter the surface. Heavy rain, and sometimes hail, pours from the bottom of the cloud. In extreme cases thunderstorms can produce tornadoes or waterspouts.
As the cloud continues to build vertically, the top–now at about 40,000 feet–spreads ice crystals downwind in the characteristic anvil top. At this point, it is already dying. The downdrafts stop further intake of moist air and soon blow themselves out. The dangerous, mature phase of the storm usually lasts about an hour. However, blasts of cold wind from the dying storm can start new updrafts, producing a succession of thunderstorms.
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.
HANDLING HEAVY WEATHER
Your first job as skipper is to be aware of the possibility of thunderstorms. Watch the weather maps and reports for a couple of days before you go out to see if there are any cold fronts approaching, which can create squall lines. On the water, keep an eye on any cumulus clouds. Often, they indicate fair weather, but if some start to tower upwards and grow dark, they are potential thunderstorms.
Thunderstorms can move at speeds of 20 knots, meaning a cloud on the horizon can quickly overtake your boat.
If you see a thunderstorm approaching, do not hesitate: head directly for the nearest safe harbor. If you can see lightning, count the seconds until you hear the associated thunder. Divide this number by five and it will tell you the number of statute miles you are from the storm. Do not be complacent if the storm is several miles away. Thunderstorms can move at 20 knots, and a storm on the horizon can be upon you quickly. And lightning can strike several miles beyond the storm cloud itself.
If you can make port and tie in, do so–but make sure you can arrive in advance of the storm. A crowded harbor is not the place to be when the wind starts blowing. The chances of successfully docking are slim, and the wind can drive your boat into other vessels and obstacles. Another tactic, if the water is shallow, is to anchor and ride it out where you are. Just be sure the storm hook is set firmly and be ready to take action if it drags.
If you can't make port, you will have to deal with the storm. Air-mass thunderstorms are several miles across, but you may be able to avoid them if you move fast and early. No such luck with squall lines, which can stretch for hundreds of miles. If you are in a sailboat, get into your foul weather gear early and be sure the VHF and other safety gear are ready and waiting. Furl the sails and start your engine, because a single 60- or 70-knot gust can knock you flat, even if your sails are reefed. One tactic, if you have sea room, is to run off with the wind. You will be in the storm longer but the apparent wind will be less.
Thunderstorms are often preceeded by condidtions that include little or no wind, the proverbial "calm before the storm."
In a power boat, you have a better chance of outrunning a storm, but the same rules apply. If you have to ride it out, prepare the boat immediately. Take down any tall canvas that can act like a sail, or at least open up the panels. If you have the option to steer from a pilothouse or enclosed area, do so. Unless you are anchoring, head for deep water, in order to avoid being blown into shallows. Piloting will be different for every boat and every storm, but knowing your boat can help a great deal. If she doesn't perform well regularly in a following sea, that won't improve with heavy winds and waves. You can prepare yourself by reading up on heavy weather handling and thinking through the scenarios that may work best on your boat.
Whatever your circumstances, power or sail, everyone not needed on deck should be sent below. If you have an open boat, people should get as low as they can. The crew should stay away from metal objects such as masts and rigging. You may want to lower antennas and disconnect electronic devices that are not essential. Because lightning can strike away from the storm cloud itself, you should wait at least 30 minutes after the storm passes before continuing your journey.
And pass it will. Thunderstorms can be violent and dangerous, but they are short-lived. Seldom does one last more than an hour, and often the air is cooler and more pleasant afterward. In fact, if it isn't, suspect that there may be more storms lurking about.