Climate of North America. 133 
miles east of Chicago, in Lake Michigan, and at Chicago, with a view to de- 
termining the relation of the wind at the two stations. It appeared that the 
mean hourly direction of the wind at Chicago, during July of that year, in- 
dicated a change from almost due east at ı P. M., through southeast to almost 
south at ı0o P.M. 
The subject of continental winds now concerns us. The warming of 
the atmospheric strata over an extended land surface is influential in the pro- 
duction of winds. The ascending currents of warm air and descending cur- 
rents of cool air are established and there results a continuous interchange of 
air between the upper and lower strata. This play of air currents is inter- 
rupted at night, when there is a rapid cooling of the surface of the ground 
by radiation, while the higher strata of the atmosphere are not correspond- 
ingly cooled. A difference of atmospheric pressure results and a movement 
of air from the ocean toward the continent takes place in a direction opposite 
to the movement aloft. The air flows from the region of higher pressure to 
that of lower pressure and deflected in consequence of the earth’s rotation, 
we may expect the following winds around the margins of the continents in 
summer’). | 
West Coast. | North Coast. East Coast. | South Coast. 
Northern Hemisphere. . N.W. N.E. S.E. S.W. 
Southern Hemisphere. . S.W. N.W. N.E. S.E. 
Such a circulation of air, as we have described for the larger continents, is 
known as cyclonic. Cyclonic circulation briefly consists then in the formation 
of two eddies, an up-draught eddy, the center of low barometric pressures 
into which and about which the winds circulate spirally, the focus of storm 
phenomena, called in consequence of the inward motion of its winds, the cy- 
elone and on the other hand, a down-draught eddy, the center of high baro- 
metric pressures from out of which and about which the winds move spirally, 
the focus of clear-weather phenomena, called in contradistinction to the cyclo- 
nic eddy, the anti-cyclone. When it is remembered that by reason of cosmic 
causes the permanent relations of the atmospheric circulation arrange themselves 
more or less in the form of a belt of anti-cyclones Iying over the oceans on 
or near the tropics, a belt of cyclones near the poles, and in the middle 
latitudes an eastward drift of migratory cyclones and anti-cyclones, changing 
in latitude and intensity with the seasons, some idea of the mechanism of the 
eirculation that produces weather and conditions of climate is given. The 
only additional important fact to bear in mind is that the winds of summer 
are different from those in winter in consequence of the fact that the land is 
colder than the ocean, and that the air from the oceans flows in aloft towards 
the continents. The socalled cold and hot waves of the United States are a result 
of the cyclonic and anticyclonic movements of the atmosphere. For example, the 
ı) Hann, Jurius: Handbook of Climatology. Engl. transl. 1903: 165. 
