Do big cities make the weather worse?

Dear Cecil:

We were discussing weather the other day--specifically, crappy weather, and what causes it. I seem to recall reading somewhere that big cities were a major cause of bad weather, due to pollution and the like--and I don't mean just smog, I mean clouds and rain and things like that. Is this true? Is there any scientific proof?

Cecil replies:

Big cities do have a major impact on the weather, although the effect isn’t uniformly dismal. For one thing, big cities tend to be warmer, which is obviously useful in the winter. But they also seed the skies with a lot more dust, which gives rise to a lot more rain.

One of the most striking scientific demonstrations of this was a study of the so-called “La Porte weather anomaly” conducted by Stanley Changnon, an atmospheric scientist. La Porte is a town in northwest Indiana just east of Chicago that has notoriously bad weather–this in a region that is not exactly known for its balmy clime to begin with.

Changnon’s study surveyed weather conditions in La Porte from 1901-65 and showed a dramatic increase in unpleasant weather conditions starting around 1925, roughly corresponding with the growth in productivity of the Chicago-Gary iron and steel industry.

In 1920, for example, La Porte had about 175 inches of precipitation; in 1945, that figure had increased to about 290 inches, and fell to 245 inches in 1960. There was no correspondingly dramatic increase in rainfall for other areas near La Porte–statistically, at least, the “anomaly” appears to be fact. Other weather conditions that have been similarly affected are hail and thunderstorms.

The industrial activities on the south side of Chicago spill a lot of heat, water vapor, and various kinds of dust into the air. The heat can cause rising and expansion of air, thus cloud formation and precipitation. Dust provides nuclei for the condensation of water vapor. (Water vapor needs a surface to condense on–in the air, these surfaces are provided by particles called cloud nuclei, and an increase in them can stimulate rainfall. This is the basis for the practice of cloud seeding.) The influence of Lake Michigan on the general atmospheric circulation of the area seems to play some part in limiting the effect to La Porte.

Reminds me of the old joke about why the University of Notre Dame is located in South Bend, Indiana, not too far from La Porte. Seems the founders camped out in South Bend one night on their way to points west. Said their leader: “We’ll just stay till the weather clears.”

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