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I wanna learn to paint clouds, so I checked some tutorials and noticed that usually people highlight the top part and shade the bottom part of each "cotton ball" a cloud is constructed from.

Then I took a picture of a real cloud and noticed that the top parts of each "cotton ball" are dark, while the bottom part is white.

Why is that so?

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The why of it is that clouds, being made of tiny water vapor (or other) particles, both reflect and transmit light. So in addition to the standard directional highlighting on the "surface" of the cloud there is internal reflection and retransmission of the light, ie "subsurface scattering". The clouds become self-illuminating and thereby self-shadowing depending on how that light is bouncing around. It is why they are so amazing looking and do not render quite as simply as actual balls of cotton.

So clouds can take on a huge variety of appearances depending on their altitude, their positional relationship to where the sun or moon or whatever light source may be illuminating them, the density of the air, etc. As far as capturing that complex play of light and shadow you are starting with the right approach: careful observation. Next start experimenting with different techniques (material, shading, form) to express what you have observed.

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There is never just one way to highlight an object. It always depends on where the source of light is supposed to be and what you want to express.

If you want to paint as realistically as possible, you'll have to decide where the sun is. If it's behind you as the observer of the picture, all parts of the cloud that are most in the foreground should be highlighted because they receive the most light. If the sun is behind the canvas and therefore behind the cloud, the parts of the cloud that are furthest back should be the lightest and the parts in the foreground should be darker, just like your photo shows it.

However, most people don't actually care for a super-realistic representation of clouds in paintings. Clouds are a means to create a certain mood. White, fluffy clouds for a light mood and dark stormy clouds for a dark mood. Red or orange clouds for a sunset scene can also create a dreamy mood.

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Gathering Point 24 x 30" oil on linen ©2010 Katherine Kean
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The dark clouds create a stormy mood and an impression of cold winds. The eye is drawn into the light center of the canvas.

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6"x6" Mini #8 - Billowing Cloudscape Painting by Mya Bessette
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Despite the dark sky the clouds create a light mood and an impression of a warm summer day.

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