An ultraviolet image taken in 1979 by the U.S. Pioneer Venus spacecraft at a distance of 200,000 km from the planet showed the large, fast-moving cloud patterns resemble Earth's high-altitude jet stream more than the great cyclonic whirls characteristic of Earth's low-altitude clouds. The upper deck of clouds on Venus rotates around the planet in just 4 days—much faster than the planet itself.
The image was made by capturing solar ultraviolet radiation reflected from the planet's clouds, which are probably composed mostly of droplets of sulfuric acid (the highly corrosive acid used in car batteries).
Early spectroscopic studies of sunlight reflected from Venus's clouds revealed the presence of large amounts of carbon dioxide but provided little evidence for any other atmospheric gases. Until the 1950s, astronomers generally believed that observational difficulties alone prevented them from seeing other atmospheric components. The hope lingered that Venus's clouds were actually predominantly water vapor, like those on Earth, and that below the cloud cover Venus might be a habitable planet similar to our own.
Indeed, in the 1930s scientists had measured the temperature of the atmosphere spectroscopically at about 240 K, not much different from our own upper atmosphere. Calculations of the surface temperature—taking into account the cloud cover and Venus's proximity to the Sun, and assuming an atmosphere much like our own—suggested that Venus should have a surface temperature only 10 or 20 degrees higher than Earth's.
See this review of the Long-Distance Observations of Venus for more.