A cloud is a visible mass of condensed droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body.
Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals.
On Earth, the condensing substance is water vapor, which forms small droplets of water or ice crystals, typically 0.01 mm in diameter.
When surrounded with billions of other droplets or crystals, they are 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 very efficiently, so that the intensity of the solar radiation decreases with depth into the cloud, hence the grey or even sometimes dark appearance of the clouds at their base.
Thin clouds may appear to have acquired the color of their environment or background, and clouds illuminated by non-white light, such as during sunrise or sunset, may be colored accordingly.
In the near-infrared range, however, clouds would appear very dark because the water that constitutes the cloud droplets strongly absorbs solar radiation at these wavelengths.