Normally, a cloud-free day should be sunny and bright, but this doesn't happen in China, where the quantity of light reaching the earth's surface has been decreasing for fifty years.

An obvious question comes to one's mind: �Where does the rest of the light go?", and the answer is given by a team of researchers at the Department of Energy's Pacific Northwest National Laboratory.

It seems that although the number of overcast days has decreased, the pollution resulted from human activity has created a haze which absorbs and deflects the sun's rays.

In a way, such a phenomenon was to be expected considering that the fossil fuel emissions have increased nine times over the last fifty years.

For the study, lead author Yun Qian and colleagues surveyed records from more than 500 weather stations across China for the years 1954 to 2001. To remove the subjectivity and ambiguity of "partly cloudy" reports, they consulted only records of an either/or nature--cloud-free and overcast days.

Their statistical analysis noted a clear trend: overcast days decreased 0.78 percent each decade while cloud-free days increased 0.6 percent for the same period.

"The results strongly suggest that increasing aerosol concentrations (particles, mainly soot and sulfur, that pollute the air) in the past has produced a fog-like haze that has reduced solar radiation (surface heat from sunshine), despite more frequent clear days that should lead to increased solar radiation," , said co-author Ruby Leung.

One of the most serious consequences of reduced amount of solar radiations is the disrupting of the water cycle due to the small quantities of evaporated light.

In addition to this problem, the 'brown atmospheric clouds' are also responsible for the acid rains and the respiratory diseases.

Dust storm in Beijing in 2002. Visibility was reduced to 500 meters. Photo by Andrew Wong.