  We analyze offshore wind speeds with a time resolution of one second over a
long period of 20 months for different heights above the sea level. Energy
spectra extending over more than seven decades give a comprehensive picture of
wind fluctuations, including intermittency effects at small length scales and
synoptic weather phenomena at large scales. The spectra $S(f)$ show a scaling
behavior consistent with three-dimensional turbulence at high frequencies $f$,
followed by a regime at lower frequencies, where $fS(f)$ varies weakly.
Lowering the frequency below a crossover frequency $f_{\rm\scriptscriptstyle
2D}$, a rapid rise of $fS(f)$ occurs. An analysis of the third-order structure
function $D_3(\tau)$ of wind speed differences for a given time lag $\tau$
shows a rapid change from negative to positive values of $D_3(\tau)$ at
$\tau\simeq 1/f_{\rm\scriptscriptstyle 2D}$. Remarkably, after applying
Taylor's hypothesis locally, we find the third-order structure function to
exhibit a behavior very similar to that obtained previously from aircraft
measurements at much higher altitudes in the atmosphere. In particular, the
third-order structure function grows linearly with the separation distance for
negative $D_3$, and with the third power for positive $D_3$. This allows us to
estimate energy and enstrophy dissipation rates for offshore wind. The
crossover from negative to positive values occurs at about the same separation
distance of 400 km as found from the aircraft measurements, suggesting that
this length is independent of the altitude in the atmosphere.
