Cloud tops look white because sunlight illuminates them directly, while cloud bases often look gray because much less light reaches the underside. Water droplets scatter sunlight in many directions. In a thick cloud, much of the light is redirected before it can pass through to the bottom. A gray base does not, by itself, mean a storm is coming.
Why does water vapor make a white cloud?
Visible clouds are made mostly of countless tiny liquid droplets, ice crystals, or both—not invisible water vapor. The droplets are large enough to scatter different colors of visible sunlight relatively evenly. When the combined light reaches our eyes, the cloud usually looks white.
Why does the bottom look gray?
Imagine sunlight entering the top of a thick cloud. As it meets droplet after droplet, it scatters sideways and upward. The lower layers therefore receive less illumination. A thicker cloud often has a darker underside, but viewing angle, shadows and bright surroundings also affect the color you perceive.
Does a dark cloud always mean rain?
No. A dark base can mean the cloud is thick or strongly shaded; rain requires additional conditions that allow droplets or ice particles to grow large enough to fall. A flat dark bottom under a small cumulus cloud can be harmless. A rapidly rising tower, spreading anvil, thunder or a visible rain shaft deserves more attention.
How does sunset change cloud colors?
Near sunrise and sunset, sunlight travels through more atmosphere before reaching a cloud. More of its blue light is scattered out of the direct beam, leaving orange, pink or red hues to illuminate cloud edges. The shaded parts may remain gray or blue-purple.
Frequently asked questions
Are cloud tops always white?
Not necessarily. Clouds may appear gray when shaded and can glow orange, pink or red when lit by low-angle sunlight.
Why can the same cloud seem darker from the ground than from an airplane?
An airplane flying above a cloud deck often sees the bright sunlit top, while someone below sees its shaded underside. Different viewing angles and surrounding contrast change the appearance.
Sources: NOAA JetStream: The Color of Clouds and UK Met Office: Why Are Clouds White?.
Leave a Reply