Diurnal and semidiurnal variations in surface winds and wind divergence ove
r the globe (50 degrees S-70 degrees N) are documented using 3-hourly wind
observations from similar to 10,000 land stations and available marine repo
rts during 1976-1997. A strong diurnal cycle in surface winds is found over
land areas (strongest over high terrain and in summer) with an amplitude o
f 0.6-1.1 m/s for wind speed and 0.5-0.7 m/s for the zonal and meridional w
ind components. Surface wind speed peaks in the early afternoon over most o
f the globe. It is suggested that increased downward turbulent mixing of mo
mentum during the day could be one of the main causes for the early afterno
on maximum of surface wind speed. The diurnal anomalies of surface wind div
ergence tend to be out of phase in adjacent regions. In particular, land ar
eas (except for extreme inland locations) exhibit maximum divergence around
dawn (0600-0800 local solar time(LST)), while nearby oceanic regions have
their maximum divergence in the evening (1700-1900 LST). Thus, there is evi
dence for a large-scale diurnal circulation in which surface air rises and
converges over the continents and sinks and diverges over nearby oceans in
the afternoon and early evening, and the opposite occurs in the early morni
ng. Over the tropical Atlantic and Pacific Oceans (10 degrees S-10 degrees
N), a zonally coherent pattern of maximum convergence (divergence) north (s
outh) of the equator around 0900-1200 LST is generally similar to the latit
udinal profile of the mean daily divergence, indicative of an enhancement o
f the local Hadley cell around 1030 LST relative to the daily mean. Another
zonally coherent north-south dipole occurs over the North Pacific, with ma
ximum surface wind divergence (convergence) around 0600-0800 LST in the sub
tropics (midlatitudes). This phase pattern correlates with cloud cover over
the two regions. Over the United States, surface divergence peaks around 0
600 LST in the west and the east and around 2000 LST in the center.