EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
Figure II-2-8. Amplification RT ratio of Wc (wind speed accounting for effects of
air-sea temperature difference) to Ww (wind speed over water without
(f) Empirical relationship. A rough empirical relationship between overwater wind speeds and land
measurements is discussed in Part III-4-2-b. This highly simplified relationship is based on several restrictive
assumptions including land measurements over flat, open terrain near the coast; and wind direction is within
45 deg of shore-normal. The approach may be helpful where wind measurements are available over both land
and sea at a site, but the specific relationship of Equation III-4-12 is not recommended for general
(2) Wind estimates based on information from pressure fields and weather maps. A primary driving
force of synoptic-scale winds above the boundary layer is produced by horizontal pressure gradients.
Figure II-2-9 is a simplified surface chart for the north Pacific Ocean. The area labeled L in the right center
of the chart and the area labeled H in the lower left corner of the chart are low- and high-pressure areas. The
pressures increase moving outward from L (isobars 972, 975, etc.) and decrease moving outward from H
(isobars 1026, 1023, etc.). Synoptic-scale winds at latitudes above about 20 deg tend to blow parallel to the
isobars, with the magnitude of the wind speed being inversely proportional to the spacing between the isobars.
Scattered about the chart are small arrow shafts with a varying number of feathers. The direction of a shaft
shows the direction of the wind, with each one-half feather representing a unit of 5 kt (2.5 m/s) in wind speed.
(b) Figure II-2-10 shows a sequence of weather maps with isobars (lines of equal pressure) for the
Halloween Storm of 1991. An intense extratropical storm (extratropical cyclone) is located off the coast of
Nova Scotia. Other information available on this weather map besides observed wind speeds and directions
includes air temperatures, cloud cover, precipitation, and many other parameters that may be of interest.
Figure II-2-11 provides a key to decode the information.
(c) Historical pressure charts are available for many oceanic areas back to the end of the 1800's. This
is a valuable source of wind information when the pressure fields and available wind observations can be used
to create marine wind fields. However, the approach for linking pressure fields to winds can be complex, as
discussed in the following paragraphs.
Meteorology and Wave Climate