Understanding the Wind

Wind is what makes a sailboat move through the water.  Understanding how to best use the wind is a skill that every sailor should continuously practice.  There are several attributes that are important to know about the wind.  First, at any given moment, the wind has a true direction and a true speed.  Minute to minute and even second to second, the wind changes speed and direction with small gusts and calm periods.  Some days are especially gusty and it is important to notice that these gusts generally reflects a change in both speed and direction.  The Beaufort Scale (shown below) is a commonly used tool to describe wind speed over the water.

In addition to the true wind, there is also an apparent wind direction and an apparent wind speed.  The apparent wind takes into account both the true wind as well as the direction and speed of your boat.  For example, if you are steering your boat directly away from the wind at 3 knots and the wind is blowing 10 knots, your apparent wind is 7 knots (this is the wind speed you would feel onboard your boat).  The same concept is true with wind direction, the apparent wind direction not only accounts for the wind speed, but also the direction your boat is moving in relation to the direction of the wind.  When you are moving on the water it is often difficult to determine the true wind direction and speed.  To best determine the wind direction try using stationary objects, such as flags and trees on land.  If you are out of sight of land, observe the direction of the wind on the water: spray and even small wavelets will help you determine the true wind direction.

Source: BBC