EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
Figure II-2-2. Duration of the fastest-mile wind speed as a
function of wind speed (for open terrain conditions)
b. General structure of winds in the atmosphere.
(1) The earth's atmosphere extends to heights in excess of 100 km. Considerable layering in the vertical
structure of the atmosphere occurs away from the earth's surface. The layering is primarily due to the
absorption of specific bands of radiation in vertically localized regions. Absorbed radiation creates
substantial warming in these regions which, in turn, produces inversion layers that inhibit local mixing.
Processes essential to coastal engineering occur in the troposphere, which extends from the earth's surface
up to an average altitude of 11 km. Most of the meteorological information used in estimating surface winds
in marine areas falls within the troposphere. The lower portion of the troposphere is called the atmospheric
or planetary boundary layer, within which winds are influenced by the presence of the earth's surface. The
boundary layer typically reaches up to an altitude of 2 km or less.
(2) Figure II-2-4 shows an idealized relationship for an extended wind profile in a spatially homogeneous
marine area (i.e. away from any land). The lowest portion is sometimes termed the constant stress layer, since
there is essentially a constant flux of momentum through this layer. In this bottom layer, the time scale of
momentum transfer is so short that there is little or no Coriolis effect; hence, the wind direction remains
approximately constant. Above this layer is a region that is sometimes termed the Ekman layer. In this
region, the influence of Coriolis becomes more pronounced and wind direction can vary significantly with
Meteorology and Wave Climate
II-2-5
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