EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
be considered. When the observed wind is given in terms of the fastest mile, Figure II-2-2 can be used to
convert to an equivalent averaging time.
(c) Overland or overwater. When the observation was collected overwater (within the marine boundary
layer), this adjustment is not needed. When the observation was collected overland and the fetch is long
enough for full development of a marine boundary layer (longer than about 16 km or 10 miles), the observed
wind speed should be adjusted to an overwater wind speed using Figure II-2-7 (see Example Problem II-2-4).
Otherwise (for overland winds and fetches less than 16 km), wave growth occurs in a transitional atmospheric
boundary layer, which has not fully adjusted to the overwater regime. In this case, wind speeds observed
overland must be increased to better represent overwater wind speeds. A factor of 1.2 is suggested here, but
no simple method can accurately represent this complex case. In relation to all of these adjustments, the term
overland implies a measurement site that is predominantly characterized as inland. If a measurement site is
directly adjacent to the water body, it may, for some wind directions, be equivalent to overwater.
(d) Stability. For fetches longer than 16 km, an adjustment for stability of the boundary layer may also
be needed. If the air-sea temperature difference is known, Figure II-2-8 can be used to make the adjustment.
When only general knowledge of the condition of the atmospheric boundary layer is available, it should be
categorized as stable, neutral, or unstable according to the following:
Stable - when the air is warmer than the water, the water cools air just above it and decreases mixing in
the air column (RT = 0.9).
Neutral - when the air and water have the same temperature, the water temperature does not affect mixing
in the air column (RT = 1.0).
Unstable - when the air is colder than the water, the water warms the air, causing air near the water
surface to rise, increasing mixing in the air column (RT = 1.1).
When the boundary layer condition is unknown, an unstable condition, RT = 1.1 , should be assumed.
(4) Procedure for adjusting winds from synoptic weather charts. As discussed earlier, synoptic weather
charts are maps depicting isobars at sea level. The free air, or geostrophic, wind speed is estimated from these
sea level pressure charts. Adjustments or corrections are then made to the geostrophic wind speed. Pressure
chart estimations should be used only for large areas, and the estimated values should be compared with
observations, if possible, to verify their accuracy.
(a) Geostrophic wind speed. To estimate geostrophic wind speed, Equation II-2-10 or Figure II-2-12
should be used (see Example Problem II-2-5).
(b) Level and stability. Wind speed at the 10-m level should be estimated from the geostrophic wind
speed using Figure II-2-13. The resulting speed should then be adjusted for stability effects as needed using
Figure II-2-8.
(c) Duration. Wind duration estimates are also needed. Since synoptic weather charts are prepared only
at 6-hr intervals, it may be necessary to use interpolation to determine duration. Linear interpolation is
adequate for most cases. Interpolation should not be used if short-duration phenomena, such as frontal
passages or thunderstorms, are present.
(5) Procedure for estimating fetch. Fetch is defined as a region in which the wind speed and direction
are reasonably constant. Fetch should be defined so that wind direction variations do not exceed 15 deg and
wind speed variations do not exceed 2.5 m/s (5 knots) from the mean. A coastline upwind from the point of
interest always limits the fetch. An upwind limit to the fetch may also be provided by curvature, or spreading,
of the isobars or by a definite shift in wind direction. Frequently the discontinuity at a weather front will limit
fetch.
II-2-36
Meteorology and Wave Climate
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