MIL-HDBK-1011/2
APPENDIX A (continued)
Additional belts downwind of each other
have been found to have
slightly decreasing effect, presumably due to the
increased turbulence in the
lee of the first belts. Similarly, the sheltered
zone leeward of a wide
shelterbelt or forest is less extensive than that
behind a single permeable
windbreak.
2.2.4.3
Local Winds. In addition to the synoptic winds caused by large
scale weather patterns, there are predictable "local winds" induced by
features of the terrain. The differential heating of land and water cause sea
and land breezes in many coastal locations. The sea breeze tends to move
inshore around midday as the land warms and the pressure differences increase.
Figure A-5 shows the pressure distribution and flow causing the
daytime sea breeze and night land breeze. Frictional resistance of the
surface often causes the incoming air to dam up and form a small scale front
which progresses inland throughout the afternoon. In locations where there is
not a great temperature difference between land and water, the sea breeze
layer will be shallow and the velocities weak. Tall buildings along a
waterfront can completely block such a breeze. On the other hand, the strong
San Francisco sea breeze is over 660 ft (200 m) deep, predictably exceeds 22
mph (10 m/sec) in the city throughout summer afternoons, and extends 37 miles
(60 km) inland. At night the flow is reversed, but velocities seldom exceed
4.4 mph (2 m/sec).
Slope winds (Figure A-6) are caused by the radiant heating and
cooling of inclined surfaces, which cause temperature differences between the
air over the inclined surface and air at the same level some distance from the
slope. This causes heated air to rise along hillsides in daytime and cool air
to descend ("drain") down slopes at night. Measurements on slopes surrounding
the Inn Valley, Austria, found upward velocities parallel to the slope between
4.4 and 8.8 mph (2 and 4 m/sec) in the daytime, and somewhat lower downward
velocities at night. The vertical extent of the wind layer was 330 to 660 ft
(100 to 200 m).
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