Most scientists agree that the temperature range between day and night in many localities has narrowed significantly in recent decades: now a previously unknown weekly cycle has been discovered within that trend.

According to a paper in this week's issue of the Proceedings of the National Academy of Sciences, the range of temperatures is smaller on weekdays, while a "weekend effect" boosts the gap between daily high and low temperatures from Saturday through to Monday.

The research, by Dr Piers Forster, of the National Oceanic and Atmospheric Administration Aeronomy Laboratory in the U.S, and Dr Susan Solomon, of the University of Reading, in Britain, may help scientists understand how much global temperature change is due to human activity.

The researchers found the phenomenon is widespread, being observed in places as different and dispersed as Mexico and China: "We conclude that the weekend effect is a real short time-scale and large spatial-scale geophysical phenomenon, which is necessarily human in origin," write the researchers, who note the effect operates on a seven-day cycle not found in nature.

"Observations have shown that DTR [diurnal temperature range, the difference between daytime maximum and nighttime minimum] has narrowed in many locations worldwide by up to 0.5 degrees ...per decade because nightly minimum temperatures have increased more than daytime values." The DTR difference between weekdays and weekends is also about 0.5 degrees.

The researchers gathered information on changes in DTR from about 10,000 surface monitoring stations around the world over the past 40 years. They found that in some areas, notably cities in the U.S, Mexico, Japan and China, the average DTR was larger from Saturday through to Monday than from Wednesday through to Friday.

Although the exact mechanism to explain the "weekend effect" is unclear, the researchers suggest that aerosols - small airborne particles - released into the atmosphere by industrial processes might affect cloud cover, in turn influencing temperature.

Global surface air temperatures have risen over the past century and the scientists say changes in DTR may help to unravel how much of that change is due to human activity.

"The weekend effect in DTR has the important strength of being a short-period relative measure, and hence insensitive to such issues as changes in instrument placement or slow changes in the environment such as urbanisation or land use," write Forster and Solomon. "A few studies, some of them controversial, have reported weekend effects in local meteorological parameters and even in urban temperatures. On the other hand, weekend effects caused by vehicular traffic practices are well documented in studies of urban pollution and atmospheric chemistry."
Urban heat island effects - the tendency of cities to hold more of the Sun's warmth overnight - have also been proposed as a possible cause of a weekly cycle in temperature over Melbourne, in Australia.

The new study found that the magnitude of the weekly cycle in Japan is smaller than at many U.S. stations and is not statistically significant enough to be detectable in Europe. But the evidence for it is strong enough to make it a useful element in studies aimed at identifying the "fingerprints" of climate change processes, and particularly the interactions between industrial processes, aerosols and cloud formation, the scientists conclude.