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Midwest Weather Warmer than Other Regions

In the Midwest, the weekend nights are hotter! Well, relative to the weekday nights, anyway.

A study published this week in the proceedings of the National Academy of Sciences found that the total range in daily temperature in cities across the country varies on a weekly cycle. The authors refer to this as a "weekend effect".

The authors analyzed more than forty years of data from stations around the country, and found that at many U.S. stations, particularly in the Midwest, the Southeast, and the Southwest, the average temperature range on Saturday, Sunday, and Monday differs by a few tenths of a degree Fahrenheit from the average temperature range on Wednesday, Thursday, and Friday.

The study asserts that the existence of this weekly cycling, regardless of the direction of the change, is strongly suggestive of a human influence on temperature.

Piers Forster is a research scientist at the National Oceanic and Atmospheric Administration and a co-author of the study.

Forster notes that while increased levels of carbon dioxide in the atmosphere, resulting from the burning of fossil fuels, may be one of the biggest human influences on climate, greenhouse gas emissions are not the cause of this effect. Fossil fuel emissions, he says, affect temperature over long timescales, but this effect is occurring over much shorter timescales.

The study suggests that one possible explanation for the weekend effect is the interaction of clouds with aerosols, tiny airborne particles that are a product of industrial pollution.

What effect aerosols have on clouds is dependent on the type of aerosol. aerosols can change both the lifetime of clouds, and the amount of precipitation from them. Cloud changes, Forster says, are a good candidate for this effect because the temperature changes resulting from them can be quite complex.

Determining the cause is also complicated because weekend effects observed at one station may not have originated there, but may be the result of the transport of pollutants downwind. This could explain why the data shows some of the largest effects far from major population centers, as well as the unexpectedly reversed weekend effect observed in the midwest.

The study's authors expected that less pollution on weekends would lead to colder nighttime temperatures. However, in the midwest, they found that, on average, temperatures were warmer at night on weekends.

While certain that the weekend effect itself is related to human influences, the authors stress that the direct causes are still uncertain.

Forster says the next step will be to collect more data, especially on cloud cover and aerosols in the atmosphere.