Box 14, Folder 4: Climate

On the Climate of the Country bordering upon the Great North American Lakes.

I. A. Lapham.

902

TE

902

TE

(Wis Authors)

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ART. II. On the Climate of the Country bordering upon the great North American Lakes. By I. A. LAPHAM, LL. D., of Milwaukee, Wis.

Among the more important results deduced from the great mass of meteorological observations accumulated within the last thirty or forty years, in the United States, none are more interesting than those that show the effect of local causes upon the climate of particular districts. Thus, in the valley of the Hudson river, in the State of New York, the prevailing winds are from the north or south; while in the valley of its tributary, the Mohawk, they are from the east or west-these being the directions of the valleys.1 We often see indications of a strong current of air passing over deep valleys, while it is quite calm below. On the borders of our great lakes, atmospheric currents frequently flow in opposite directions at the top of steeples, and on the ground.2 It is to represent one of the most striking of these local phenomena that the accompanying map (Plate X) of the State of Wisconsin, has been constructed, on which the points of equal mean temperature for the month of January are connected with black lines, 902 and those for July with red. It will be seen that the former TE approach Lake Michigan with a north-easterly direction, while the latter have a direction towards the south-east, thus cutting each other at a considerable angle, and showing the remarkable effect of that lake in elevating the mean temperature of winter, and depressing that of summer. To show these modifications of temperature in their maximum, the coldest and warmest months have been selected; but if we contrast in like manner the mean temperature of the three winter months with that of the summer months, the same phenomena would be seen, though less in degree. The map is based upon observations made at various points, and continued during several years. Longer-continued observations may require some modification of the exact position of these several lines, but the

1 HOUGH, Results of Meteor. Obs., &c., p. xi, Albany, 1855.

2 KIRTLAND, Am. Jour. of Science, 2d Ser. XIII (1852), p. 217.

[4]

59 CHICAGO ACADEMY OF SCIENCES.

general phenomena intended to be represented will remain substantially the same.

The well known property of water in large bodies, by which it imparts heat more slowly to the air above in winter, and absorbs it more slowly in summer, than the adjacent land, may be stated as the chief cause of this remarkable deflection of the isothermal lines as they approach Lake Michigan, a body of water about three hundred miles in length, and fifty miles in average breadth. The water varies less above or below the mean annual temperature, than the land.

But it would be wrong to attribute the whole of this remarkable effect upon the climate of the lake region to the slow-conducting power of water; there is another cause, heretofore overlooked, which materially aids in producing the result under consideration.

When the horticulturalist wishes to protect his tender plants from the severity of winter cold, he directs a wall to be built along the northern border; and the enterprising farmers on the prairies of Illinois plant trees on their northern boundary lines for a similar purpose. It is upon the same principle that the elevated mountain range, extending from a little west of Lake Superior, through the British Possessions to the coast of Labrador, protects the whole lake region from the northern blasts of winter which would otherwise sweep over it. The elevation of this range north and north-west of Lake Superior, is from 1,035 to 1,900 feet above the sea level; in Labrador it is 2,240 feet.3 Of the intermediate portion little is accurately known; but as it is said, at some points, to reach the limits of perpetual snow, we may infer that this great garden wall protects the lake region in no inconsiderable degree from the excessive cold of winter. It is the absence of this mountain system west of Lake Superior, that allows the cold winds from the arctic regions in the water to spread over the country west of the Mississippi river, depressing the tempera ture in a very remarkable manner on and about the 95th degree of west longitude.4

3 H. Y. Hind's Report, &c.-Toronto, 1860.

4 Prof. Joseph Henry, Patent Office Agricultural Report, 1856, p. 485, and map, pl. iv.

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It would be interesting to pursue this subject further, and show the effect of this modified climate upon the distribution of the native vegetation, and of forest trees, on the shores of the lake; upon the products of agriculture; the growth and ripening of fruits; and upon the comfort and health of the inhabitants;-but these are matters that must be left for other hands.

TRANS. CHICAGO ACADEMY OF SCIENCES. VOL. I. PL. X.

[color illustration]

A MAP OF WISCONSIN

With lines showing the remarkable effect of LAKE MICHIGAN in elevating the mean temperature of JANUARY and depressing that of JULY

By I.A. Lapham L L.D.

Milwaukee

1867