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Because conditions on the water vary from one minute to another, it's important for performance-oriented sailors to be vigilant regarding the shape of the wind and waves and learn to respond accordingly.
In the world of sports, there's a great deal of predictability when it comes to the playing fields used in most competitions. In tennis, you serve from a baseline that is exactly the same distance away from the net no matter where in the world you go to play. In hurdling, it's nearly the same, though a major controversy erupted at the 2000 Summer Olympic Games when the height of the horse was set incorrectly by mere inches in one particular heat. But in competitive sailing, on two different days, or even two different hours, the same racecourse can often provide completely different conditions. Variations in wind strength and direction will obviously affect how you set your boat up, as well as your attitude, but one factor that may not be so immediately apparent is the shape of that wind.

Wind has different causes, so it fills in different shapes-lines, circles, swirls, and every possible combination of those three. Fortunately for sailors, most puffs follow the same pattern on a given racecourse for a given weather pattern, making it possible to apply the lessons learned from one puff to the next. No matter what shape it is, the goal remains the same for optimum performance-stay in the pressure and out of the lulls. So let's look at how you go about that for the three basic wind shapes. For the purpose of this discussion, let's assume that you and your crew can see every significant puff on the water in plenty of time to respond to it, so the only challenge is how to maximize this advantage.

Helicopter Puffs These to me are the most fascinating wind shapes since you can clearly see their entire perimeter from just a few feet off the water. Helicopter puffs are usually fairly random in their location and frequency. As escapees from a different breeze aloft, helicopter puffs will spread out as they hit the surface just as if they were actually spun off a rotor. This instability means two things: 1) the puff won't travel very far down the course, and 2) the breeze angles will change drastically within the puff. You are most likely to find the best pressure, and often a lift, right along the edges. So it makes sense to tack before a helicopter puff arrives, timing your tack to place your boat directly to leeward of the middle of the puff.

True helicopter puffs have a vertical component that causes the wind to spin outwards in a circular fashion when it hits the surface of the water. Savvy sailors know to understand its behavior by studying the wind on the water.
The size and shape of these puffs will vary widely, and how large they are relative to your boat will affect how you play them. If the puffs are very short-lived or you lose a lot of speed every time you tack, it's best to wait until you can find one that's worth altering course. If you are sailing a small boat like a Laser and the duration of each puff is more than a minute, you can connect the dots by using each puff to get you to the next one.

Helicopter puffs are usually fairly random in their appearance. From the starting area looking up the course, these puffs will often appear just as more pressure, and it isn't until you see them up close that they separate into distinct forms that I call blobs. Keep in mind where they come from (aloft) and try to correlate any change in cloud cover with their frequency; that will help you predict their next occurrence.

Downwind, it will be very important to keep an eye over your shoulder and alter course fairly drastically to line up with the next big helicopter puff. The edges of these puffs have the best pressure, so it is important to intersect one that's coming from upwind, rather than sailing into the middle of one ahead of you. And even though you may be lifted along the edges, you don't want to jibe unless you are lining up for the next puff. The bigger the pressure they bring, the harder it will be to get out of the holes that lie in between.

"More times than I can remember I've watched the water transform from a glassy golden shimmer to solid dark ripples the instant the breeze fills across it."

Wind Lines Have you ever watched a sea breeze fill in? This to me is the most amazing example of how quickly the water surface changes character. More times than I can remember I've watched as the water transforms from a glassy golden shimmer to solid dark blue ripples the instant the breeze fills across it. Although sea breeze comes in as a "line," this line is usually not quite perpendicular to the direction of travel. Often there are fingers of breeze that march ahead, unstable compared to the solid fill behind, but they're usually still a big step up in velocity from the flat calm they are replacing.

A sea breeze is the most obvious example of a wind line, but most wind lines follow this basic pattern. Unless the wind that forms a wind line is the result of a significant weather front, it is caused by geography, so a little local knowledge can help you identify the direction and time that a wind line will fill in. The most subtle wind lines I've ever seen are in Biscayne Bay in Miami. There a series of lines will fill from one side (left or right depending on the breeze direction), and the velocity change will only be one or two knots; but those that find a way to stay in the better pressure will always have a significant lead around the weather mark.

Usually the breeze is very unstable along the leading edge of a wind line, so it is best to dig into it even if you appear to be sailing in a tempting header. Often those sailors that tack along the first edge will fall out of the breeze, while those that are a bit more patient will remain lifted, in pressure, once they finally tack.

When sailing downwind, it's important to watch aft for the next wind change whether it's a puff, swirl, or a wind line.

Downwind, wind lines can be very frustrating since they fill in for those in the back of the fleet first. It is important to pick a side and to sail the same angle as the boats behind so that you get the wind as soon as it fills down the course. It is also important to keep an eye astern to stay in the best pressure and the best lane.

Swirls I call these "squirrelly puffs" since they don't fill along any particular (or predictable) path. Often a puff that looks headed right for you will veer off and suddenly drop astern, just like a windblown seagull. And some will actually jump over the top of your sailplane and continue on downwind, leaving you braced for nothing. The only hard and fast rule about this wind shape is that it forecasts unstable weather.

When dealing with swirls you must be able to shift gears quickly, so they don't blow you over when they hit. Staying in the velocity is important, but you must keep the boat going fast through the sometimes radical and sudden changes in pressure and angle.

Downwind, have one crew keep an eye astern as much as possible to warn you of potential knockdowns or opportunities to make gains by accelerating in the swirl. And don't be too hard on anyone calling the breeze in these conditions until you have tried it yourself!

A wind fly like the one on deck above can help you determine the immediate wind direction, but you'll need to study the water up the course to really get an idea of what the wind is doing.

Steady breeze is almost always the easiest wind form to sail in, but it is also the one that offers the fewest opportunities for gains. The small shifts ordinarily experienced when the wind is steady will usually not lead to many place changes and races conducted in these conditions will be won by those sailors who have superior starts and boat speed. In these conditions, the biggest priorities (both upwind and downwind) are to sail in clear air and to stick to the basics-set the boat up well for the conditions, get off the line and around the marks cleanly, and keep the bow headed toward the mark.

Because wind shape usually stays the same for a given racecourse and weather pattern, you can learn from each puff and try to apply that lesson to the next one. It's always best if you involve the whole crew in analyzing the shape of puffs as well as their strength and arrival time; anyone on the rail has a better view and can study the travel of the wind over a much longer distance than the driver. Use the gain or loss on your competition to gauge your success, and enjoy the process! Reading the shape of the wind is one of the many great challenges of sailboat racing, and practice will only make you better.

Suggested Reading:

Seeing the Wind by Bob Merrick

Understanding Wind Shear by Dobbs Davis

Getting a Handle on Wind by Dan Dickison

SailNet Store Section: Wind Instruments
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