As the cloud becomes horizontally thick, transmission will decrease, darkening the cloud if you aren't in a position to observe scattering. It will appear darker if the sun is behind the cloud (transmission through the cloud).
Putting it together, as that white cumulus grows and becomes tens of thousands of feet tall, it will still appear white if the sun is on your side of the cloud. The presence of hydrometeors (rain drops, snow, hail) in a rain cloud will further decrease transmission and darken the cloud. If visible photons cannot arrive at your eye from the cloud, then the cloud will appear dark. Water emits in the infrared, so emission isn't an option. If there isn't a path from the sun to a surface of the cloud you can see, scattering isn't an option. This means that if the cloud size doesn't change but more and bigger rain drops are in the cloud, it will be optically thicker and appear darker.
Note that both the physical thickness and the density of the cloud will increase its optical thickness.
For an optically thick cloud of appreciable thickness, transmission isn't an option. These clouds are also optically thick, dense with cloudwater, rain, snow and ice.įor a cloud to appear white, photons representing the full visible spectrum must be able to arrive at your eye. A developed CB cloud also features an anvil. Even when there are many of them, they are not vertically developed and photons can still scatter off of them to you.Īs cumulus develop into towering cumulus and cumulonimbus they become horizontally widespread and quite high (the tops of severe convection overshoot into the stratosphere and the convection in the tropics can go even higher, owing to a higher tropopause). Fair weather cumulus clouds are white because there is a clear path for sunlight to reflect off the cloud and into your eye.
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