At 34,000 feet flying over Azerbaijan, the view out to the clouds as the sun starts to go down.
A cloud is a visible mass of condensed droplets, frozen crystals suspended in the atmosphere above the surface of the Earth.
The condensing substance is typically water vapor, which forms small droplets or ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths: they thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the cloud, hence the gray or even sometimes dark appearance of the clouds at their base.