Aerosols are solid particles in the air. They can range from natural materials such as sand, dust, and salt, to man-made particles such as soot and smog. Once in the atmosphere, aerosols can have a variety of importance effects, heating the air (by absorbing sunlight), cooling land underneath (by blocking sunlight), and promoting or interfering with the formation of clouds, depending on the nature of the aerosol particles. Scientists have also noticed that many aerosols travel very great distances carried around the world by wind: dust storms in the Sahara rain down dust and sand from Africa on the Carribean and the Amazon basin in South America, smog from China's coal-powered electricity generation stations reaches the United States, while smog from the United States travels to Europe. These visualizations show two important characteristics of aerosols: their "optical thickness" (how much there is and how much they block light) and their "effective radius" (a precise measure of how large the individual particles are: natural aerosols like salt and sand tend to be larger than man-made fine soot).