Relief Shading

Previously we have seen how light shining through clouds can give rise to an amazing play of colored brightness and shadow. However, quite frequently something else happens: The low sun illuminates the cloud from below, and this gives rise to an equally impressive display of color.

Cloud relief illuminated by low light.

In contrast to the 'gold lining' effect, the variation of light and shadow appears more muted - what is bright is not glowing from the Mie forward scattering enhancement but merely illuminated, what is dark is not blocking light but merely not illuminated - but at the same time, if the layer shows sufficient complexity and is not simply a flat, featureless water vapour blob, the amount of visible detail is often simply amazing - Altocumulus clouds generally look very pretty when illuminated from below.

Visuals for different cloud types

The light has to come in at just the right angle to bring out the relief in the cloud - if the Sun is too high, the light does not reach underneath the layer, if the Sun is too low, it may not reach the cloud at all - or illuminate everything as the illuminating angle gets larger than the typical angle of the cloud structures.

Here is an example for a somewhat less pronounced relief - still pretty, but not as detailed:

Somewhat less pronounced relief.

If the cloud bottom is rather diffuse, so is the color variation between light and shadow:

Very weakly pronounced relief.

In essence, what we observe here is a cloud casting a shadow onto itself - the parts of the relief that are more prominent have an illuminated sunward slope and a darker slope away from the sun (anyone who does 3d rendering will recognize the action of a normal map here...), and filaments that 'stick out' cast a shadow across the rest of the cloud.

Time dependence

As the Sun moves, lighting conditions for relief shading can change rapidly. Even in the high north where the Sun sinks slowly, the difference between a nicely illuminated layer and fading colors is just two minutes in time.

A relief-shaded cloud layer.

Basically the light on the layer fades rapidly and the color display is no longer impressive.

The same layer just two minutes later.

To capture relief shading on film, one really has to work quickly - the most stunning visuals only last a few minutes. The visuals of light shining through translucent clouds are by far less time-critical to observe.

Continue with Cloud Shadows.

Back to main index     Back to science     Back to sunsets

Created by Thorsten Renk 2022 - see the disclaimer, privacy statement and contact information.