Night had fallen several hours earlier. The full moon was bright enough to cast shadows on the deck. But as we reached along through the tropics, an ominous line of black clouds slowly obliterated the stars behind us. A squall line was overrunning us from behind.
Over the years I’ve picked my way through scores of squall lines and have negotiated thousands of menacing-looking clouds. Every time I do so, though, it is still with a sense of drama and uncertainty. While squall lines have many common characteristics, each is unique. Some approach from astern. Others we plunge into headlong. Some are benign, while others seem almost malicious, with radical wind shifts and gusts capable of tearing a sail to shreds in a matter of seconds.
Whether they qualify as storm cells, thunderstorms, towering cumulus clouds or full-on squall lines, squalls occur throughout the globe. How can we distinguish the truly malicious from the merely inconvenient? How can we best deal with them? There are more than a few clues to look for, and knowing how to read them will not only help you avoid trouble, but even allow you to use squalls for your own purposes.
Squall lines are merely storm cells arranged in a row. With weather satellite imagery you can see them from thousands of miles away. Their high, cold clouds signal the possible presence of towering cumulus clouds, those huge, billowing formations that can reach 50,000 feet. Commercial aircraft will detour around the biggest of them, but sailboats often do not have that option.
Knowing that they’re coming can help you prepare well in advance. As the clouds grow in size, they indicate an unstable air column: the taller and more ragged-looking the cloud, the greater the convective instability, or rapidly rising and falling air, within it. This instability creates surface wind as air moves toward the bottom of the cloud and rises, bringing moisture up into a colder air mass where it condenses in the form of clouds or rain. As the rising air cools, it also becomes denser. This denser, heavier air then sinks back toward the earth’s surface. If the changes in temperature are radical enough, a downburst, or very rapid sinking of cold air from the cloud can result.
At 0200, as the wind picked up during the passage I was describing earlier, I could feel the air temperature plunge 15 degrees. The warm night was suddenly cool as cold air dropped down from high above. The squall was definitely approaching.
Sometimes squall lines will precede cold fronts by 100 or 150 miles, but the cold air that is often associated with squalls is usually from aloft rather than from the cold air mass behind it. Radar can see the rain that may be associated with a squall or cloud that has gusting winds under it, but neither satellite imagery nor radar will tell you the complete story. You can have wind without rain or rain without wind. My own general rule is: what you see is what you get.
If you are faced with a huge towering cumulus cloud with a flat anvil-like top you can be sure there is convective instability in the cloud, with ventilation at the top—which means significant cooling of the rising air mass and circulation, or spinning of the air like a mini low-pressure system. The cold air will probably be dropping, creating a divergent wind at the base of the cloud as the air sinks, slams into the surface of the water and then spreads out. Usually, the strongest winds are those along the cloud’s leading edge. Often the lighter winds associated with the cloud are behind it as it departs the scene. Check to see if the cloud is billowing actively and has a “ragged” bubbling appearance. If so, it’s probably best to steer clear by going behind it if possible. If evasive action is not possible, make sure you err on the side of safety and reduce sail early on. Downpouring rain is another indication of a possibly unstable air mass.
That said, I’ve certainly been in pouring rain when there was no wind. To determine whether or not there’s wind under the cloud, check the slant of the rain. As you do so, make sure you interpret what you see correctly. Is that slant the result wind or sunlight streaming through the cloud? Where’s the sun located? Is the cloud itself moving and progressively dumping rain onto a windless sea? If you see rain slanting in several different directions, it’s probable there is circulation taking place, and the wind direction will vary under the cloud depending on location. Under that cloud, you’d probably have a complete calm right under the middle, again like a mini-low-pressure system—think the eye of a hurricane. Those situations are also best avoided by going behind the cloud if possible. It’s important to be aware that winds of 35 knots or more are commonplace under a towering cumulus cloud, especially along the leading ledge.
Since you don’t have Doppler radar on your boat, you’ll have to use your eyes and deductive reasoning. Check the surface of the water under the cloud. Is it darker than the surrounding water? If so, is it because it’s in a shadow, or is it being disturbed by a strong wind? Are there whitecaps close to the cloud indicating more wind?
If necessary, take bearings on the cloud. If the bearing is changing, you’re either going in front of or behind it. If there’s a break in the clouds that you’d like to try to work your way toward, increase or decrease your boatspeed, or change your heading, until you maintain a stable bearing with that break.
When you know what you’re dealing with, you can plan your tactics accordingly. Conditions under some clouds and squall lines are more vicious than under others. If the signs indicate that there is a great deal of wind or radically changing wind directions, you’ll want to proceed with extreme caution. Again, these signs include a flat, anvil top, billowing cloud, rain, and white caps or dark sea state near the cloud. Remember that going under the middle of the cloud may mean that there’s no wind in the center or there may be a 180-degree wind shift if there is a rising column of air there due to convection.
Of course, I’ve managed to sail right under the middle of towering cumulus clouds—not by choice but rather by unfortunate chance—and the scenario has proven to be less than ideal. Winds quickly built from one side of the boat, perhaps on a starboard tack. Then, under the epicenter of the cloud, the winds dropped off to nothing with sails slatting and boat speed dropping to near zero. As the cloud passed overhead, the wind instantly gusted and shifted 180 degrees, heeling us over on port tack.
When approaching a large cumulus cloud, you are advised to not only reef down, but to make sure your runners are clear, preventers are ready to be eased and re-deployed, and everything is secure belowdecks. It’s also a good idea to clip into your harness. Like reefing, this is a job that is more easily done in advance of the storm rather than during it. Windshifts happen quickly. Advanced preparation puts you in control rather than at the mercy of the conditions.
It may be OK to go in front of the cloud, but you should take bearings on it to stay out of the center. The problem with going in front of a cloud is that if you slow down or it speeds up or gets larger, you could be trapped. Additionally, the strongest winds are usually along a cloud’s leading edge. Going behind the cloud is usually the best and safest approach. You have more options to get away from it whenever you want, and you can exercise a certain degree of control over how much extra wind or shift you get by sailing closer or farther from the cloud.
Similarly, when confronted by a squall line, you may be able to choose the point at which you cross the line or allow it to overrun your position. Do you want to shoot for a gap in the larger clouds? Or, if that is too much out of your way, do you want to go under a particular part of the cloud? Try to go behind to weather the worst of it.
Occasionally, there are situations in which you’d like to stay with the cloud or run down the face of the squall line. If you’ve been sailing in light conditions, perhaps the wind under the cloud is a godsend. During TransPac Races, some successful teams have used radar to spot rain clouds and ride along with them through the night. They then try to distance themselves from the cloud shortly before dawn when the cloud starts to evaporate and seems to suck the wind out of the neighborhood. In the doldrums, I’ve successfully played “connect the dots,” sailing from cloud to cloud, using the only wind in the area to work my way out of the light spots.
Crossing a squall line can be nerve-wracking or exciting. The experience can bring trouble, or it can bring increased awareness and a sense of satisfaction. Try to understand what you’re dealing with and plan your tactics accordingly.
Top photo by Onne van der Wal