|We were sailing quite happily on the Bay when the August skies began to darken in the west. Clouds at the leading edge of the storm seemed to boil, and as the black wall drew closer we could see the whitecaps beneath. Our pleasant little daysail was about to turn ugly, and it was an experience we won't soon forget. While the crew began reducing sail, I double-checked our position and the locations of other boats around us. The Chesapeake has its share of shallow spots, and in late summer there's no lack of traffic. No doubt we'd soon lose visibility. Without panic we were ready when the first cool blast of wind arrived. Gusty winds peaked at about 35 knots, and then a driving rain hammered the deck. Thirty minutes later the wind had died and the squall had passed. We raised sail and resumed our course across the Bay. In open waters in a sturdy cruising boat, a squall is often an opportunity to top off the water tanks. But for smaller boats - especially in congested coastal waters - these storms can pose a serious threat. Anchors drag, sailboats damage their rigs and, worse, boaters fall overboard and drown. In our region, the squall lines that precede a cold front can be particularly hazardous. Knowing how to read the clouds can help mariners predict the severity of the weather ahead and prepare accordingly. |
A squall forms when warm, moist air over the land or sea rises, cools and condenses to form clouds. If the air at the base of the cloud is very warm, pushing the cloud to great heights, the currents of frigid air aloft rapidly cool the rising air, which then sinks, creating strong winds at the base of the cloud. Downbursts of air slam down from above, generating speeds over 40 mph and hitting the earth's surface with little warning. Torrential rain and reduced visibility are a given. Hail and tornadoes or water spouts are distinct possibilities. The severity of a storm is directly related to its updraft. The colder the top of the air column and the hotter the bottom air, the more volatile the air mass. When a cold front approaches from over the Gulf Stream, for example, its updraft strength is enhanced by the warm water, causing the storm system to develop quickly. (The same thing happens when cold jet-stream winds blow across the tops of cumulus clouds that form over land on a hot muggy summer day. That's where thunderstorms and tornadoes come from.) Boaters need to be particularly concerned about line squalls, which spring up along the leading edge of a cold front. It's possible to outmaneuver a single storm cell, but multiple cells are hard to avoid. Squall lines can also precede a cold front by as many as 100 miles and several hours. Most commonly, you'll find squalls in the warm sector of a low-pressure system, as shown in the illustration at right. Monitoring a weather channel so you'll know when these low pressures systems are approaching is a wise move for any boater.
Squalls are rowdy power-houses on the move, but by observing the cloud mass as it approaches you can gather information about what is coming. Towering cumulus clouds, especially those with flat anvil-shaped tops, can bring gusts from 35 to 40 knots. If the cloud mass seems ragged and turbulent - as if it's boiling - there's a lot of convection going on, which means even heftier wind speeds at the bottom. Exceptionally large cumulus clouds can generate "downbursts" that clock 50- to 60-knot winds. As the storm approaches, look at the surface of the water around it. Sometimes a darker color is just caused by the cloud's shadow, but often enough it indicates rough water and, by inference, coming wind. If you can see whitecaps you know you're in for a blow. The wind under a cloud is often strongest along its leading edge. Directly under the cloud, winds will often be both shifty and gusty.
If you're in a sailboat on open water as a squall approaches, you'll probably have little choice about which side of the cloud to choose. Rather than waste precious time trying to outrun the inevitable, ready your boat for what is to come. Start your engine and shorten sail. Make sure the decks are clear for a sudden tack and be ready to center the boom to keep it from swinging wildly across the beam of the boat. You might want to simply douse all sail if your engine has enough power to keep you headed into the wind and waves without the help of canvas. If your engine is the least bit unreliable, keep a bit of sail up so you can maintain steerage should it conk out. If you're on a powerboat, you might be able to reach shelter before a storm hits, but there's a good chance you'll end up powering through a driving rain with reduced visibility. Know your position, speed and heading, and above all, make sure you're aware of any other boats that are on the move nearby. Learning how squalls form and how to avoid them not only makes your boating more enjoyable, it offers a great introduction to the complex - and often fascinating - weather patterns that affect the Bay. Whether you want to stay dry or fill your water tanks, a little squall wisdom can go a long way.
Bill Biewenga is a professional captain and weather router with more than 300,000 nautical miles beneath his keel. He is the author of Weather for Sailors from North U Press. His website is www.wxadvantage.com