Geoscience Reference
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(A)
(B)
Cumulus develop
and migrate
seaward
980mb
Counterflow
990mb
1000mb
LAND
AIR
SEA AIR
1010mb
Sea breeze
(C)
(D)
Coun
terflow
980mb
Bo
990mb
LAND AIR
1000mb
Land breeze
SEA AIR
Figure 6.11
Diurnal land and sea breezes. A and B: sea breeze circulation and pressure distribution in the early
afternoon during anticyclonic weather. C and D: land breeze circulation and pressure distribution at night during
anticyclonic weather.
Source: A and C after Oke (1978).
300m deep. In the early afternoon, a deeper
regional-scale circulation between the ocean and
the hot interior valleys generates a 1km-deep
onshore flow that persists until two to four hours
after sunset. Both the shallow and the deeper
breeze have maximum speeds of 6m s
-1
. A shallow
evening land breeze develops by 19:00 PST but is
indistinguishable from the gradient offshore flow.
The advancing cool sea air may form a distinct
line (or
front
; see Chapter 8D) marked by cumulus
cloud development, behind which there is a
distinct wind velocity maximum. This often
develops in summer, for example, along the Gulf
coast of Texas. On a smaller scale, such features
are observed in Britain, particularly along the
south and east coasts. The sea breeze has a depth
of about 1km, although it thins towards the
advancing edge. It may penetrate 50km or more
inland by 21:00 hours. Typical wind speeds in
such sea breezes are 4-7m s
-1
, although these may
be greatly increased where a well-marked low-
level temperature inversion produces a 'Venturi
effect' by constricting and accelerating the flow.
The much shallower land breezes are usually
weaker, about 2m s
-1
. Counter-currents aloft are
generally weak and may be obscured by the
regional airflow, but studies on the Oregon coast
suggest that under certain conditions this upper
return flow may be very closely related to the
lower sea breeze conditions, even to the extent of
mirroring the surges in the latter. In mid-latitudes
the Coriolis deflection causes turning of a well-
developed onshore sea breeze (clockwise in the
Northern Hemisphere) so that eventually it may
blow more or less parallel to the shore. Analogous
'lake breeze' systems develop adjacent to large
inland water bodies such as the Great Lakes and
even the Great Salt Lake in Utah.
Small-scale circulations can be generated by
local differences in albedo and thermal conduc-
tivity. Salt flats (playas) in the western deserts of
the United States and in Australia, for example,
cause on off-playa breeze by day and an on-playa
flow at night due to differential heating. The salt
