LOCAL WINDS 
HEIGHT (KM) 
2400 
NIGHT 
0600 
SUNSET 
LOCAL MEAN TIME 
Fig. 3.—Velocity isopleths for the land and sea breeze in 
Batavia. (After van Bemmelen [70].) 
the water are governed also by apparent conduction or 
turbulent mixing. In contrast with the strong heating 
of the air over the coastal region, the air over the strip 
of water offshore is only mildly warmed, and, as a 
result, a temperature difference between land and water 
develops. This difference diminishes toward sunset and 
reverses during the night. 
Temperature Differences, Pressure Differences, and 
Pressure Distribution during Land and Sea Breezes. 
The maximum temperature differences (AZ) are given 
in the literature as follows: Kaiser [46] gives a range of 
At from 1.6C to 10.9C over a distance of 130 km be- 
tween Wiistrow and Adlergrund Lightship (anchored 
in the middle of the Baltic Sea, about 100 km out of 
Swinemiinde) as averages of a period of twenty summer 
days. Grenander [31] found differences in temperature 
between 3.6C and 7.6C at 1400, and between 1.4C and 
3.1C at 0700 and 2100. These measurements, taken 
on the Swedish east coast, involved a distance between 
land and sea stations of 115 km. However, maximum 
values were as high as 10C or more; on the other hand, 
very small temperature differences occurred on some 
sea-breeze days. Measurements of At at lakes, as for 
instance at the Lake of Constance, likewise show a large 
range of temperature differences, namely values be- 
tween 0.9C and 4C at 1400. We may conclude from 
these few measurements that At covers a wide range of 
values, and it is certain that a temperature gradient 
from sea to land exists in the morning hours. During 
forenoon a reversal takes place, and in the afternoon an 
increasing land-sea temperature gradient develops 
which is the driving force in the formation of the sea 
breeze. Over inland lakes, where opposite shores show 
this same behavior, the center of the lake must be a 
neutral zone. In general it may be assumed that the 
heating over land during daytime can reach a value 
five times that over water. Such temperature differences 
force the development of a pressure gradient and cir- 
culation system. 
657 
At the beginning of the day, the air pressure at higher 
altitudes over the land rises, while there is only a negli- 
gible increase over the water. As a consequence a 
drainage of the upper air from the land toward the sea 
takes place, and during forenoon the pressure close to 
the surface of the sea begins to rise, while it starts to 
fall over the land. This developing sea-land pressure 
gradient is accompanied by an air current in the same 
direction, that is, the sea breeze. A countercurrent, 
blowing toward land, is established at upper levels 
above the surface sea breeze. The circulation in day- 
time is completed by cumulus-forming convection over 
land and cloud-dissolving subsidence over water. In 
the evening the land and the overlying air cool faster 
than the sea and its overlying air, and a reverse noc- 
turnal circulation develops. This circulation must be 
of smaller intensity and vertical extent because of the 
lack of instability and convection. 
The land-sea pressure gradient near the surface at 
night and toward morning, as compared to a sea-land 
gradient during the day, is clearly discernible in Fig. 4. 
LOCAL MEAN TIME 
0000 0600 1200 1800 2400 
764 
= SSE 
= eq ~ 
~ 763 att +s 
3 ae Sx 
> N 
a 7 Vs 
eee $ Sa = 
eS eee 
761 
LAND BREEZE SEA BREEZE 
Fig. 4.—Average daily period of the air pressure on twenty 
sea-breeze days at the Baltic Sea. The solid line refers to 
Swinemiinde; the dashed line, to Adlergrund Lightship. 
(After Kaiser [46].) 
It is to be expected that the air current, according to 
the pressure distribution, is, at first, nearly at right 
angles to the coast line; only later, during the after- 
noon, a gradual change in the direction of the sea breeze 
appears because of the effect of the Coriolis force. How- 
ever, the influence of the Coriolis force remains slight 
because of the short distances that these winds travel. 
Conditions are somewhat complicated by the addition 
of a gradient wind associated with the over-all weather 
situation. A gradient wind blowing parallel to the coast 
line has no particular influence, since the pressure 
gradient causing it also acts normal to the coast line and 
thus only weakens or strengthens the pressure gradient 
associated with the land or sea breeze. However, a 
pressure gradient parallel to the coast line with, for 
example, an offshore gradient wind causes a mass trans- 
fer of air seaward. In that case a kink appears in the 
isobars where they protrude over the sea. Then the 
condition of gradient force plus Coriolis force equal 
to zero, which was valid in the previously discussed 
case, is no longer fulfilled, and unbalanced shoreward 
components of the gradient wind create a possibility 
for the development of a strengthened sea breeze. In 
simple schematic cases these conditions can also be 
