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Folks who do not live around the ocean or large bodies of water probably have never thought about what determines the size of waves. There are actually 3 main factors that determine wave size. We all know that when it gets windy, bodies of water gets rough. So wind speed is the most obvious factor in determining wave height. The harder it blows, the bigger the waves get. When you have constant winds of about 17 knots you will begin to see whitecaps on the water. The very tops of the waves begin to break and curl upon themselves when the wind blows at this speed.
The second factor in the wave size formula is the amount of time that the winds have been blowing. When higher winds first start up the waves are do not immediately get big. It takes some time of hard wind blowing across a body of water for the waves to come up. So Time is the second factor.
The third major factor in determining wave height is a little more difficult to know intuitively. It is just not quite as obvious as Wind Speed and Time. The third factor is called ‘Fetch’. This is the Distance over which the water that the wind has been blowing. If you are on a small lake or a channel the fetch is never big, there just is not enough distance for the wind to blow to build up big waves. No matter how hard it blows there is not enough time for the pressures of the wind exerted on the water for it to build up. Out in the ocean it is a different story. The fetch is huge out here, the wind blows for hundreds of miles over the surface of the ocean. There is plenty of time and distance for the wind to do it’s thing to the water and make some very impressive waves.
I have simplified wave creation with this explanation and there are many more factors that affect waves but these are the major forces involved. Sometimes at sea you will experience large sets of big rolling waves come in. Usually these are caused by distant storms or low pressure systems. They seem to come at somewhat regular intervals. Surfers also wait for these sets of waves rolling in to a beach because they create the best surfing opportunities.
The weather reports that we receive while sailing give us the big pictures of where the high and low pressure systems are located and general feel or which way they are traveling. By looking at these pressure charts you can determine the wind direction and get a feel for what the waves will be doing by using the “left-hand rule”. If you see a low pressure system on a map you point your left thumb down in the center of the low and wiggle your fingers. The direction of the wiggle is the direction the winds will be blowing in a circular pattern around the center. The reverse is true for high pressure systems. If you see a high pressure center on the map, you point your thumb up from the center of the high and wiggle your fingers. The wind will be blowing in the direction of the wiggle. So as pressure systems move around your location you can get a feel for what winds you will be experiencing based on how close you are to the center of the pressure system and the direction of the wiggle of your left-hand fingers. Of course the left hand rule only works in the Northern hemisphere. If we were to cross the equator to the southern hemisphere we would have to switch to the right-hand rule.