Cloud Visual Opacity
If clouds are present, we need some information on their opacity, or how opaque the clouds are. We use opacity rather than thickness because it means something a little different. When studying the radiative effects of clouds, we are interested in how much sunlight they let through, not in how much vertical space they take up. We use three somewhat subjective categories to describe this:
|Transparent (more examples) |
This describes thin clouds through which light passes easily, and through which you can even see blue sky. Note the milky bluish-whitish appearance of the cirrus clouds at left, especially those near the top of the photo.
|Translucent (more examples) |
This describes medium-thickness clouds that let some sunlight through; but through which you cannot see blue sky. There may be some milky bluish-white near the edges, and a very little bit of gray under the thickest parts; but these clouds are mostly a bright white.
|Opaque (more examples) |
This describes thick clouds which do not allow light to pass directly, although light can diffuse through them. Such thick clouds often look gray. When the sky is overcast, or when these clouds are in front of the Sun, it is impossible to tell where the Sun is.
We ask for an estimate of the opacity for the cloudy part of the sky. In most cases, the opacity of clouds will vary from cloud edge to cloud center, or from one cloud to another. We ask for the predominant cloud opacity present in the sky.
When multiple cloud layers are present, we would like this information for each cloud layer.