3.41 ESTIMATING THE WIND CHARACTERISTICS
To predict wave properties from meteorological data by any of the
simplified techniques, it is necessary to:
(a) Estimate the mean surface wind speed and direction, as discussed in Section 3.4, Wind Information Needed for Wave Prédiction;
(b) delineate a fetch over which the wind is reasonably constant
in speed and direction, and measure the fetch length, and
(c)
estimate wind duration over the fetch.
These déterminations may be made in many ways depending on the location and the type of meterological data available. For restricted bodies
of water, such as lakes, the fetch length is often the distance from the
forecasting point to the opposite shore measured along the wind direction.
There is no decay distance, and it is often possible to use observational
data to détermine wind speeds and durations.
When forecasting for océans or other large bodies of water, the most
common form of meteorological data used is the synoptic surface weather
chart. {Synoptic means that the charts are drawn by analysis of many
individual items of meteorological data obtained simultaneously over a
wide area.) These charts depict lines of equal atmospheric pressure,
called isobars. Wind estimâtes at sea, based on an analysis of the sealevel atmoshperic pressure are generally more reliable than wind observations because pressure, unlike wind, can be measured accurately on a
moving ship. Pressures are recorded in millibars, 1,000 dynes per square
centimeter. One thousand millibars (a bar) equals 29.53 inches of mercury
and is 98.7 percent of normal atmoshperic pressure.
A simplified surface chart for the Pacific Océan is shown in Figure
3-9, which is drawn for 27 October 1950 at 0030Z (0030 Greenwich mean
time). Note the area labelled L in the right center of the chart, and
the area labelled H in the lower left corner of the chart. These are
low- and high-pressure areas; the pressures increase moving out from L
(isobars 972, 975, etc.) and decrease moving out from H (isobars 1026,
1023, etc.).
Scattered about the chart are small arrow shafts with a varying number of feathers or barbs. The direction of a shaft shows the direction
of the wind; each one-half feather represents a unit of 5 knots (2.5
meters/second) in wind speed. Thus, in Figure 3-9 near the point 35°N.
latitude, 135°W. longitude, there are three such arrows, two with 3^>
feathers which indicate a wind force of 31 to 35 knots (15 to 17.5 meters/
second) , and one with 3 feathers indicating a force of 26 to 30 knots
(13 to 15 meters/second).
On an actual chart, much more meteorological data than wind speed and
direction are shown for each station. This is accomplished by the use of
3-22
To predict wave properties from meteorological data by any of the
simplified techniques, it is necessary to:
(a) Estimate the mean surface wind speed and direction, as discussed in Section 3.4, Wind Information Needed for Wave Prédiction;
(b) delineate a fetch over which the wind is reasonably constant
in speed and direction, and measure the fetch length, and
(c)
estimate wind duration over the fetch.
These déterminations may be made in many ways depending on the location and the type of meterological data available. For restricted bodies
of water, such as lakes, the fetch length is often the distance from the
forecasting point to the opposite shore measured along the wind direction.
There is no decay distance, and it is often possible to use observational
data to détermine wind speeds and durations.
When forecasting for océans or other large bodies of water, the most
common form of meteorological data used is the synoptic surface weather
chart. {Synoptic means that the charts are drawn by analysis of many
individual items of meteorological data obtained simultaneously over a
wide area.) These charts depict lines of equal atmospheric pressure,
called isobars. Wind estimâtes at sea, based on an analysis of the sealevel atmoshperic pressure are generally more reliable than wind observations because pressure, unlike wind, can be measured accurately on a
moving ship. Pressures are recorded in millibars, 1,000 dynes per square
centimeter. One thousand millibars (a bar) equals 29.53 inches of mercury
and is 98.7 percent of normal atmoshperic pressure.
A simplified surface chart for the Pacific Océan is shown in Figure
3-9, which is drawn for 27 October 1950 at 0030Z (0030 Greenwich mean
time). Note the area labelled L in the right center of the chart, and
the area labelled H in the lower left corner of the chart. These are
low- and high-pressure areas; the pressures increase moving out from L
(isobars 972, 975, etc.) and decrease moving out from H (isobars 1026,
1023, etc.).
Scattered about the chart are small arrow shafts with a varying number of feathers or barbs. The direction of a shaft shows the direction
of the wind; each one-half feather represents a unit of 5 knots (2.5
meters/second) in wind speed. Thus, in Figure 3-9 near the point 35°N.
latitude, 135°W. longitude, there are three such arrows, two with 3^>
feathers which indicate a wind force of 31 to 35 knots (15 to 17.5 meters/
second) , and one with 3 feathers indicating a force of 26 to 30 knots
(13 to 15 meters/second).
On an actual chart, much more meteorological data than wind speed and
direction are shown for each station. This is accomplished by the use of
3-22