Optical thickness
(Section 2.7.6)The optical thickness of a cloud is the degree to which the cloud prevents light from passing through it. Optical thickness depends on the physical properties and the cloud dimensions. The observer should record the optical thickness, and indicate the direction in which the clouds or cloud layers have the greatest thickness. Table 13 provides a numerical scale for optical thickness.
Table 13. Scale for optical thickness of clouds
|
Scale |
Description |
Comment |
|
1 |
Very weak |
Blue of the sky is discernible through the cloud |
|
2 |
Weak |
The cloud hides the blue of the sky, but does not prevent the Sun from casting shadows; such a cloud is usually white but may be light grey |
|
3 |
Moderate |
The cloud has a good general luminance, but noticeable shading in places; when present as an extensive sheet or layer, the cloud is light grey |
|
4 |
Strong |
The cloud is strongly shaded; when present as an extensive sheet or layer, the cloud appears dark grey; when the layer is discontinuous or formed of scattered elements, the parts directly exposed to the Sun are white and fairly brilliant |
|
5 |
Very strong |
The cloud is dark, except for the parts exposed to the Sun, which are brilliantly white; the cloud has a threatening appearance |