San Francisco is a few degrees south of Chicago’s latitude. Why are their temperatures so different?

Ask Tom Why

Dear Tom,
San Francisco is a few degrees south of Chicago’s latitude. Why are their temperatures so different?

Kathy Lingston, Elmhurst

Dear Kathy,
Both cities lie in the Earth’s mid-latitude zone of prevailing westerlies in which weather systems move mainly from west to east. The huge difference in the climates of the two cities is due to upstream surface features. Upstream of Chicago (to the northwest, west and southwest) is the Great Plains, a huge land surface that experiences dramatic summertime warming and wintertime cooling. Upstream of San Francisco is several thousand miles of water whose temperature remains mostly in the 50s. Chicago’s annual temperature range is 50 degrees: 23 in January to 74 in July. San Francisco’s annual temperature range is only 11 degrees, from 51 in January to 62 in September.

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