Physics and Surfing :: physics surf surfing

When a wind begins to blow across a flat body of water ripples form almost immediately, then wavelets, and finally nearly fully developed waves, having almost definite wave lengths and shapes. Surface waves will also form, during calm wind, where opposing currents meet. This is known as current rips and can be dangerous for small or overloaded boats. A good estimate of the wind speed can be obtained by observing its effect on the water. For example, at 5 knots only small wavelets can be observed. A 10 knot wind speed will produce a few whitecaps. At 15 knots whitecaps can be seen up to 30% of the area. At 20 knots whitecaps can be seen up to 60% of the area. At 30 knots whitecaps and spray can be seen all over and foam from the breakers begins to form. At 40 knots streaks of foam can easily be seen. At 50 knots visibility is reduced and the sea begins to take on a white appearance from all the foam. Current directions and speed will greatly affect the areas covered by spray and breakers. Factors Affecting Wave Heights: 1. Wind. 2. Wind duration. 3. Length and width of wind fetch. (This is the distance over the water in which the wind blows in nearly the same direction.) 4. Depth of the water. 5. Direction and speed of the current. 6. Rate of precipitation. 7. Air temperature. 8. Amounts of flotsam, ice, or seaweed in the fetch area. 1. How high will the waves get? Maximum wave heights (in feet) will not get any higher than * the wind speed in knots. This is correct over 90% of the time. Note: Wave height begins to increase when the depth of the water is less than or equal to 1/4 of the wavelength because the wave starts to touch the sea floor. 2. What is the difference between waves and swells? Waves occur where the wind is blowing. Swells are waves that have moved away from the wind generating area and frequently give warnings of an approaching storm. 3. What is the significant wave height? Significant wave height (SWH) is the average height of 1/3 of the highest waves, or the height of the waves an observer is most likely to report. The significant wave height is used in the marine forecasts. Table 1.Wave heights in relationship to the significant wave height (SWH) Most frequent wave height .5 x SWH