Different Altitude, different Light

Often there is not just a single cloud layer in the sky but different layers at different altitudes. Consider the following picture:

Different lighting for lower and upper cloud layer.

Here we see three layers - a low dissolving Cumulus layer in yellow-orange color when illuminated and dark shade when not, a higher Altocumulus layer which is slightly yellow tinted and always translucent and a very high Cirrus layer in the upper left corner which is near white.

Why low light is position-dependent

Evidently the light reaching the lower and higher clouds via the illuminating ray is not the same - and from what has been said so far we can deduce that the amount of Rayleigh scattering along the ray must be higher for lower clouds.

Why is this so?

Consider the following sketch:

We can have a situation in which the sun is below the horizon for the observer - the observer is inside the shadow cast byu Earth's curvature. The air above the observer however is still partially in light, i.e. the sky is blue and not yet dark, and clouds at different altitudes are illuminated.

Comparing the path of the light rays to the lower and upper cloud, it becomes clear that the pathlength through the atmosphere is larger for the lower cloud. What is more, the atmosphere is much denser close to the ground, so there is less scattering happening when the higher clouds are illuminated - and yet more, most sources of dry haze are contained inside the Troposphere, i.e. the lowest 10 km of the atmosphere. Very few dry haze scattering takes place in the Stratosphere above.

As a result of these factors, higher clouds generally receive not only light when the ground is already in shadow, but also the light they receive is less red-shifted.

In principle also clouds located towards the sun are less affected by Rayleigh scattering than those above or behind the observer, but that effect is often more difficult to see than the altitude dependence (see example below).

This position dependence of the light color can lead to very beautiful color displays when there are different cloud layers in the sky.

Polar Stratospheric Clouds

One of the most stunning illustrations of the altitude dependence are Polar Stratospheric Clouds (PSC). They occur much higher in the atmosphere than nearly every other cloud type - around 25 km in altitude. And as a result they can still receive full light when most of the air column above the observer is already in shadow, aka when the sky (including the highest Cirrus clouds) is significantly darkened.

Polar Stratospheric Clouds.

The whole scene looks eerie, because it seems impossible from our experience that clouds can still be that bright while the sky around is dark and the sun has vanished way beneath the horizon already. And yet - there they are in nearly full daylight.

Yet somewhat later, the Sun has gone so low that the path through the atmosphere is long even to 25 km altitude - and the clouds appear red.

Polar Stratospheric Clouds, somewhat later.

Except those seen in the distance - they illustrate the principle that objects sunward from the observer receive in general less-altered light as stated above.

Continue with Relief Shading.

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