662 LOCAL CIRCULATIONS 
weak maximum that barely reaches a quarter of the 
intensity of the land and sea breezes near the surface. 
Theoretically, further circulations exist above this circu- 
the oblique position of the flow ellipse relative to the 
perpendicular to the coast, in good agreement with 
observations (see Fig. 2). 
Tasue III. Crrcunation CHARACTERISTICS COMPUTED FOR VARIOUS VALUES OF 
Friction COEFFICIENT AND CORIOLIS PARAMETER 
Da u w 
5 c A x i , Upper countercurrent 
es es Amp. Amp. Alt. of Max. Alt. of 
(sec) (sec) hase aiaee z Ehase wince. peatisie: Alt. of Tea pouaion Peat 
(hr) (m sec7) (hr) (m sec") (m) Max.amp. | jax. wind “) (cm sec) (m) 
(m sec™?) (mn) 
0.0 0 12 M 16.7 §.438M 320 1.64 650 16.6 4.21M 320 
0.5 0 12 M 1155, 11 4.46M 340 1.11 670 15.1 3.45M 340 
1.0 0 12 M 14.1 3.6817 365 0.801 700 14.1 2.86M 365 
2.5 0 12 M 13.4 1.70M 500 0.261 920 13.6 1.37 500 
2.5 1.031 12 M 13.1 1.84 500* 0.251 920* 13.1 1.76M 500* 
Vv 
2.5 1.031 12 M 2.2 0.72M 500* 0.11 920* 
*Approximate. 
Above the zero layer there is a countercurrent whose 
vertical extent exceeds that of the land- and sea-breeze 
layer by a factor of 4 to 5. This countercurrent has a 
u 
1310=MAXIMUM u 
1200/}1400 
1000; 
1230/4 1430 f 
1030 fea i 
1730 H 
0830 / 
1830 / 
0730 08 00; 
1600 
04 Oo: 7 2200 
eS ee) 
0.2 4 6 8 10 
WA. M SEC! 
0200f 2400 
Fie. 6.—Theoretically calculated flow ellipse of the land 
and sea breeze under the influence of friction (o = 2.5 X 107% 
sec‘) and the Coriolis force (f = 20 sin ¢ = 1.03 X 104 sec*}; 
¢ = 45°) when the maximum temperature difference between 
land and sea is at 1200 LMT. The vector diagram at the left 
shows the mean winds during the sea-breeze period (0730-1830 
EST) at Logan Airport, Boston, Mass. (based on 40 cases). 
(After Defant [18].) 
lation cell of land and sea breezes and its countercur- 
rent. However, their intensities are so low as to be 
negligible for all practical purposes. Thus, in contrast 
to others, this theory fixes a definite upper boundary 
to the land- and sea-breeze circulation. This upper 
boundary is, according to the theory, independent of 
the intensity of the land-water temperature contrast 
and a function only of the structure of the atmosphere, 
the Coriolis force, and the turbulent heat transfer. 
Friction tends to raise this boundary, while the Coriolis 
force tends to lower it. 
MOUNTAIN AND VALLEY WINDS 
The Mountain- Wind Circulation and Its Components. 
As in the case of coastal areas, local temperature and 
wind conditions occur in the vicinity of large mountain 
ranges that are often so strong in their effect that, 
locally, they modify or even obscure the general weather 
conditions. Here, thermal differences create a circula- 
tion system which, in daytime, consists of a lower cur- 
rent toward the mountains and an upper current in the 
opposite direction. In the region of the Huropean Alps, 
Burger and Ekhart [12] have actually shown that this 
upper compensation current flows away radially from 
the mountains toward the neighboring plains in day- 
time. A corresponding flow system was found on the 
east slope of the Rocky Mountains by Wagner [71] 
and Ekhart [23]. This upper compensation flow is natu- 
rally less pronounced (speeds of the order of 15 cm 
sec |) than the lower current, since it is more strongly 
affected by the wind system determined by the general 
pressure distribution, the influence of which is difficult 
to separate from the local effect. Figure 7 shows these 
conditions schematically. 
The Thermal Slope Wind. The difference in tem- 
perature between the air heated over the inclined 
mountain slopes and the air at the same altitude 
over the center of the valley causes the phenomenon 
of air rising in daytime along the slopes of moun- 
