Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 480

-- exposed ridges and mountain summits in areas of strong upper-air winds;
-- exposed coastal sites in areas of strong upper-air winds or strong thermal
pressure gradients.

Features generally indicative of low mean wind speeds are as follows:

-- valleys perpendicular to the prevailing winds aloft;

-- sheltered basins;

-- short and/or narrow valleys and canyons;

-- areas of high surface roughness ( e.g. , forested, hilly terrain).

Evidence of strong, persistent winds can be found from wind-deformed vegetation
[Putnam 1948]. Hewson et al. [1978, 1979] suggest methods by which mean wind speeds
can be deduced from the extent of wind deformation on trees and shrubs. It should be
noted that although wind-flagged trees ( i.e. , trees with stunted branches on the side toward
the prevailing wind) may indicate that the annual average wind speed is stronger than 4 m/s,
unflagged trees do not necessarily indicate that the winds are light. These trees may be
exposed to strong winds from all directions, with insufficient persistence in any one
direction to cause flagging. Methods of identifying areas of wind-deformed vegetation may
include aerial surveys, ground investigations, and public questionnaires.
The removal and deposition of surface materials by the wind to form playas, sand
dunes, and other types of eolian landforms indicate strong winds from a nearly constant
direction. However, correlation of the characteristics of eolian terrain features with annual
average wind speeds has proven difficult [Marrs and Gaylord 1979].

Boundary Layer Similarity and Surface Pressure Methodology

A team of Danish scientists devoted much time and effort over several years to de-
velop a wind atlas for Denmark based on a wind resource assessment method that uses
boundary layer similarity theory in combination with surface pressure measurements
[Petersen et al. 1981]. The basic data used to map wind speeds at a given elevation over
terrain of a specified roughness are ordinary surface ambient pressure readings from
synoptic observations. Existing direct wind measurements were used to validate this
procedure, and the best agreement between measurements and predictions was found where
the terrain is the least complicated. It is reasonable to conclude that this methodology has
promise for estimating wind power classes with sufficient accuracy for identifying areas
worthy of more detailed assessment.