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As expected, the dominant constituents of the atmosphere are hydrogen and helium. But the amount of helium -- about 15 percent -- was considerably less than the 40 percent that had been suggested by some Earth-based studies. Methane, acetylene and other hydrocarbons exist in much smaller quantities. Methane in the upper atmosphere absorbs red light, giving Uranus its blue-green color.

Voyager images showed that the atmosphere is arranged into clouds running at constant latitudes, similar to the orientation to the more vivid latitudinal bands seen on Jupiter and Saturn. Winds at mid-latitudes on Uranus blow in the same direction as the planet rotates, just as on Earth, Jupiter and Saturn. These winds blow at velocities of 40 to 160 meters per second (90 to 360 miles per hour); on Earth, jet streams in the atmosphere blow at about 50 meters per second (110 mph). Radio science experiments found winds of about 100 meters per second blowing in the opposite direction at the equator.

A high layer of haze -- photochemical smog -- was detected around the sunlit pole.

The sunlit hemisphere also was found to radiate large amounts of ultraviolet light, a phenomenon that Voyager scientists have dubbed "dayglow."

The average temperature on Uranus is about 60 Kelvin (- 350 degrees Fahrenheit). The minimum near the tropopause is 52 K (-366 F) at the 0.1-bar pressure level. (The tropopause is the boundary between the stratosphere and the troposphere, the lowest level of atmosphere, comparable to the region on Earth where life abounds. One bar is the average pressure at sea level on Earth.)

Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the same temperature below the tropopause. Voyager instruments did detect a somewhat colder band between 15 and 40 degrees latitude, where temperatures are about 2 to 3 K lower. The temperatures rise with increasing altitude, reaching 150 K (-190 F) in the rarified upper atmosphere. Below this level, temperatures increase steadily to thousands of degrees in the interior.