Wind as a Vector
1
mechanical
turbulence
thermal and
mechanical
turbulence
0
20
Time
40
60
-
0
FIGURE 5-1. Typical traces of a fast response wind sensor in conditions of pure
mechanical turbulence and thermal plus mechanical turbulence.
10 min
I
I
1
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I
the inner scale of turbulence. The outer scale is generally taken to be a
few meters and the inner scale a few millimeters.
This process of larger eddies breaking down into ever smaller ones is
expressed in a rhyme by L.F. Richardson, a scientist who is responsible
for much of the theory of atmospheric turbulence (Gifford, 1968):
Great whirls have little whirls
That feed on their velocity;
And little whirls have lesser whirls,
And so on to viscosity.
Gifford remarks that this parody of de Morgan's verse on fleas "may be
the only statement of a fundamental physical principle in doggerel."
5.2 Wind as a Vector
There is a fundamental difference between wind and the other environmental variables (temperature, vapor, and other gas concentrations)
discussed in earlier chapters. Wind is a vector quantity involving both
magnitude and direction, while the other environmental variables are
scalars, where only magnitude is specified. The wind velocity vector
is commonly divided into components along the axes of a rectangular
coordinate system. For convenience, the coordinate system is oriented
so that the x axis points in the direction of the mean wind. The velocity