Wind is the movement of air from areas of high pressure to areas of low pressure. While air is made up of gases, in many ways it behaves like a liquid. It flows over and around obstructions, seeking the path of least resistance. Wind will blow more strongly out of valleys and will be almost nonexistent on the leeward side of a high hill.
The wind is rarely perfectly steady. Depending on the surfaces it passes over, the stability or instability of the air, weather systems, and even the effects of other boats, the wind is constantly changing in both strength and direction.
The wind itself is invisible, but its effects are not. When you’re sailing, it’s important to be aware of the strength and direction of the wind in order to harness its energy efficiently and sail safely.
There are many ways to tell the direction of the wind. Wind blowing across the water causes friction on the surface, forming small ripples perpendicular to the direction of the wind. (Larger waves are caused by the longer-term effects of the wind and current.) Learning to determine the wind’s direction by looking at the water’s surface takes much practice, but it’s the most accurate method. Other helpful indications are flags, smoke, and other sailboats.
There are a couple of simple tools that can help you find the direction of the apparent wind. Telltales are lengths of yarn or strips of nylon tied to the shrouds and backstay. A masthead fly, with a wind arrow, goes at the top of the mast and points into the wind.
You can also use your sails to find wind direction. When you ease your sails, they will luff and line up with the wind. Gradually turn your boat toward the wind; you’ll be straight head-to-wind when the sails are luffing on the boat’s centerline.
One telling indicator of wind strength is when whitecaps (white tufts on the waves) just begin to form. This occurs around 12 to 14 knots, a point at which many small boat’s begin to get less stable. Inexperienced sailors shouldn’t be out alone when there are whitecaps.