JP 3-50 National Search and Rescue Manual Vol I - US Navy

The first uses Figure 5-6, entering with wind history and
forecasts for the incident area. The second uses historical
data or local knowledge of wind currents. Wind currents need
not be computed for coastal, lake, river, or harbor areas, but
should be determined for water depths greater than 100 feet
(30 meters) and distances greater than 20 miles (32
kilometers) from shore.
2. Wind observations should be obtained beginning 48 hours before
the assumed time the distressed craft began drifting.
Accuracy of calculation is less with shorter wind histories.
Forecasts should be used through the time period containing
datum. On scene wind data should agree with the general
circulation shown on area surface weather charts; if it does
not, it should be either confirmed or disregarded. Generally,
surface winds are directed 20 degrees toward the low pressure
side of isobars on surface weather charts.
a. Wind history is wind speed and direction near datum
for the previous 48 hours, divided into 6-hour periods.
Period 1 is the most recent 6 hours, period 2 the next most
recent, and so on. Because wind observations are often
available only at normal synoptic hours (0000Z, 0600Z, 1200Z,
and 1800Z), it is best to select the wind interval that begins
and ends midway between the synoptic hours bracketing the
reported wind time. All other wind intervals also begin and
end midway between normal synoptic hours.
b. For each period, wind speed and direction are
determined by using observed wind for the midpoint of the time
period. When hourly winds are available, average wind speed
over the time period is used. Wind speeds must be averaged
vectorially.
3. Wind current computation involves the following
considerations:
a. Wind current should be calculated in 48-hour periods
made up of subperiods of 6 hours
5-8