The visual impression of the giant planets from Earth is based on a two-dimensional view of their cloud tops. Atmospheres, though, are three-dimensional structures in which temperature, density, pressure, and even chemical composition vary with height and over horizontal distances. As a rule, atmospheric temperature, density, and pressure all decrease with increasing altitude, although temperature sometimes reverses itself at very high altitudes, as in Earth’s thermosphere. The stratospheres above the cloud tops of the giant planets appear relatively clear, but closer inspection shows that they contain layers of thin haze that show up best when seen in profile above the edges of the planets. The composition of the haze particles remains unknown, but it is likely that they are photochemical— smoglike—products created when ultraviolet sunlight acts on hydrocarbon gases such as methane.