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Wind, Continued
Visual
Estimation of
Wind Speed
True wind direction may be observed by noting the direction from which
ripples, small waves, and sea spray are coming. The direction is most
easily found by sighting along the wave crests and turning 90° to face
the advancing waves. The observer is then facing the true wind
direction. You may estimate the true wind speed by noting the sea
condition and referring to table 10-1, which is based upon the following
assumptions and should be considered in arriving at an estimated true
wind speed:
1. The wind has been blowing at a relatively constant speed and
direction for the time indicated by table 10-1.
2. The fetch area (an area where waves are being generated by the
wind) is unlimited.
Some factors that cause the speed estimation of the wind to be too low
are as follows:
1. Winds have increased rapidly.
2. Offshore winds within the sight of land.
3. Moderate or heavy precipitation smoothing the sea surface.
4. Swell waves from varying directions.
Some factors that will cause the speed estimation of the wind to be to
high are as follows:
1. Waves running into shallow water.
2. A decreasing wind speed. The relative wind speed and direction can
be estimated by observing the ship’s flag, smoke, and rigging on the
windward side of the ship. Table 10-1 should be used when you are
using this method. Notice that this method gives you the relative wind
and should be used only when the surface of the sea cannot be observed.
10-23
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