Forecasters need to know how to use the nomogram for wave characteristic forecasts so we will introduce it here and refer to it in the cases.
The nomogram illustrates the relationship between wind speed, wind duration, fetch length, and wave growth. The X-axis indicates wind duration in hours; the Y-axis indicates wave height (m); and the contour lines represent fetch and wind speed.
If a wind blows over a 100-km fetch at 10 m/s over 12 hours, what’s the resulting wave height and period? Use the nomogram to determine this, then select the best answer below.
A 10 m/s wind speed and 100-km fetch length contours intersect at a point of about 9 hours duration. This gives a wave height of 1.8 m and wave period of 5 s, as shown in the nomogram below. Here wave growth is limited by the fetch. Even if the wind continued blowing for 12 hours, the wave would stop growing because it cannot exceed the 100-km fetch length over which the wind blows.
For more information about nomograms, see the COMET lesson Wave Life Cycle I: Generation.