The feeling that clouds and rain seem more likely to occur on weekends may be more than just a myth. Weekend weather does differ from weekday weather in certain places, according to researchers who studied more than 40 years of daily temperature readings from 10,000 weather stations worldwide. In their study, the researchers focused on temperature differences between daytime highs and nighttime lows. This difference measurement is called the diurnal temperature range, or DTR. Part of the study involved 660 weather stations in the continental United States. At more than 230 of these sites, the researchers found that the average DTR for Saturday, Sunday, and Monday was different from the average DTR for Wednesday, Thursday, and Friday. Although the difference was small—only several tenths of a Celsius degree, the pattern was striking enough to make the scientists take notice. That is, the DTR changes on a 7-day cycle. This sort of weekly rise and fall doesn't line up with any natural cycles, the researchers say. Instead, it is the human activities—possibly air pollution caused by those activities—that are responsible for these weather effects. For example, tiny particles in the polluted air could effect the amount of cloud cover, which would in turn affect daily temperatures. Accordingly, tiny windborne particles from the western part of the United States, generated on weekdays, might first affect weather close to home in the southwest, and then later influence mid-western weather. So, stop feeling like the weekend weather is out to get us. It simply has a lot do with which way the wind blows and where it comes from.