Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960-1990. However, after discounting an anomaly caused by the eruption of Mount Pinatubo in Nineteen ninety one, a very slight reversal in the overall trend has been observed. Global dimming is thought to have been caused by an increase in particulates such as sulfate aerosols in the atmosphere due to human action. It has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially counteracted the effect of greenhouse gases on global warming. It is thought that global dimming is probably due to the increased presence of aerosol particles in the atmosphere caused by human action. Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets. Water droplets in clouds coalesce around the particles. Increased pollution causes more particulates and thereby creates clouds consistiong of a greater number of smaller droplets. That is, the same amount of water is spread over more droplets. The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the Earth's surface. This same effect also reflects radiation from below, trapping it in the lower atmosphere. In models, these smaller droplets also decrease rainfall. Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location, and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect. At nights the Earth's heat loss gets slow. However, unlike in other areas of the world, in China the decrease in solar irradiance was not always accompanied by an increase in cloud cover and precipitation. It is believed that aerosols may play a critical role in the decrease of solar irradiance in China. British Broadcasting Corporation Horizon producer believes that many climate scientists regard the pan evaporation data as the most convincing evidences of solar dimming. Pan evaporation experiments are easy to reproduce wiht low-cost equipment. There are many pans used for agriculture all over the world and in many instances the data have been collected for nearly a half century. However, pan evaporation depends on factors besides net radiation from the sun. The other two major factors are vapor pressure deficit and wind speed. The ambient temperature turns out be a negligible factor. The pan evaporation data corroborates the data gathered by radiometer and fills in the gaps in the data obtained using a unique type of meter. With adjustments to these factors, pan evaporation data has been compared ot results of climate simulations.