coded symbols, letters, and numbers placed at definite points in relation
to the station dot. A sample model report, showing the amount of information possible to report on a chart, is shown in Figure 3-10. Not ail of
the data shown on this plot are included in each report, and not ail of
the data in the report are plotted on each map.
Figure 3-11 may be used to facilitate computation of the geostrophic
wind speed. A measure of the average pressure gradient over the area is
required. Most synoptic charts are drawn with either a 3- or 4-millibar
spacing. Sometimes when isobars are crowded, intermediate isobars are
omitted. Either of these standard spacings is adéquate as a measure of
the geographical distance between isobars. Using Figure 3-11, the distance
between isobars on a chart is measured in degrees of latitude (an average
spacing over a fetch is ordinarily used), and the latitude position of the
fetch is determined. Using the spacing as ordinate and location as abscissa
the plotted or interpolated slant line at the intersection of these two
values gives the geostrophic wind speed. For example, in Figure 3-9, a
chart with 3-millibar isobar spacing, the average isobar spacing (measured
normal to the isobars) over F2, located at 37°N. latitude, is 0.70° of
latitude. Using the scales on the bottom and right side of Figure 3-11,
a geostrophic wind of 67 knots is found.
Geostrophic wind speeds are generally higher than surface wind speeds.
The following instructions, U.S. Fleet Weather Facility Manual (1966), are
recommended for obtaining estimâtes of the surface wind speeds over the
open sea from the geostrophic wind speeds:
(a) For moderately curved to straight isobars - no correction is
applied.
(b) For great anticyclonic (clockwise movement about a high pressure
center in Northern Hemisphere and counter-clockwise in Southern Hemisphere)
curvature - add 10 percent to the geostrophic wind speed.
(c) For great cyclonic (counter-clockwise movement about a low
pressure center in Northern Hemisphere and clockwise in Southern Hemisphere)
curvature - subtract 10 percent from the geostrophic wind speed.
Frequently the curvature correction can be neglected since isobars
over a fetch are often relatively straight. The gradient wind can always
be computed if more refined computations are desired.
To correct for air mass stability, the sea-air température différence
must be computed. This can be done from ship reports in or near the fetch
area, aided by climatic charts of average monthly sea surface températures
when data are too scarce. The correction to be applied is given in Table
3-1. (U.S. Fleet Weather Facility Manual, 1966.)
Over océans, the surface winds generally cross the isobars toward low
pressure at an angle of 10° to 20°.
3-24
to the station dot. A sample model report, showing the amount of information possible to report on a chart, is shown in Figure 3-10. Not ail of
the data shown on this plot are included in each report, and not ail of
the data in the report are plotted on each map.
Figure 3-11 may be used to facilitate computation of the geostrophic
wind speed. A measure of the average pressure gradient over the area is
required. Most synoptic charts are drawn with either a 3- or 4-millibar
spacing. Sometimes when isobars are crowded, intermediate isobars are
omitted. Either of these standard spacings is adéquate as a measure of
the geographical distance between isobars. Using Figure 3-11, the distance
between isobars on a chart is measured in degrees of latitude (an average
spacing over a fetch is ordinarily used), and the latitude position of the
fetch is determined. Using the spacing as ordinate and location as abscissa
the plotted or interpolated slant line at the intersection of these two
values gives the geostrophic wind speed. For example, in Figure 3-9, a
chart with 3-millibar isobar spacing, the average isobar spacing (measured
normal to the isobars) over F2, located at 37°N. latitude, is 0.70° of
latitude. Using the scales on the bottom and right side of Figure 3-11,
a geostrophic wind of 67 knots is found.
Geostrophic wind speeds are generally higher than surface wind speeds.
The following instructions, U.S. Fleet Weather Facility Manual (1966), are
recommended for obtaining estimâtes of the surface wind speeds over the
open sea from the geostrophic wind speeds:
(a) For moderately curved to straight isobars - no correction is
applied.
(b) For great anticyclonic (clockwise movement about a high pressure
center in Northern Hemisphere and counter-clockwise in Southern Hemisphere)
curvature - add 10 percent to the geostrophic wind speed.
(c) For great cyclonic (counter-clockwise movement about a low
pressure center in Northern Hemisphere and clockwise in Southern Hemisphere)
curvature - subtract 10 percent from the geostrophic wind speed.
Frequently the curvature correction can be neglected since isobars
over a fetch are often relatively straight. The gradient wind can always
be computed if more refined computations are desired.
To correct for air mass stability, the sea-air température différence
must be computed. This can be done from ship reports in or near the fetch
area, aided by climatic charts of average monthly sea surface températures
when data are too scarce. The correction to be applied is given in Table
3-1. (U.S. Fleet Weather Facility Manual, 1966.)
Over océans, the surface winds generally cross the isobars toward low
pressure at an angle of 10° to 20°.
3-24