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Map of Wisconsin with Lines Showing the Remarkable Effect of Lake Michigan in Elevating the Mean Temperature of January and Depressing that of July

Increase A. Lapham

A Moderating Influence
This is the earliest known climatological map showing the effects of Lake Michigan on air temperatures in Wisconsin.

Because of the tremendous amount of water they hold, the Great Lakes have a pronounced effect on the climate of surrounding lands. In summer, the waters are cooler than the land and help to cool off nearshore areas. In winter, the waters are warmer than the land and thus help to keep coastal areas warmer than interior lands. Increase Lapham was one of the first scientists to examine these effects.

Isotherms, lines of equal temperature, were first used by meteorologists in the mid-1800s. Assuming east-west isotherms in the absence of any lake effect, the black isotherms on this map illustrate how Lake Michigan elevates the mean air temperature of Wisconsin in January. The red isotherms show how the lake effect depresses average air temperatures in July.

For example, Lapham found the average monthly temperature in Milwaukee in July to be similar to that of Eau Claire, and Milwaukee’s average air temperature in January equaled that of Beloit. The moderating influence of the lake on temperatures in Door County makes it possible for this peninsula between Green Bay and Lake Michigan to sustain large cherry and apple orchards.

Lapham was renowned as a scientist, naturalist and conservationist who collected data and published maps and texts based on his analyses. While climatologists today understand a more complicated picture of Lake Michigan’s effects on Wisconsin’s weather, Lapham’s early analysis was correct in its general idea.