Learning how to paint cumulus clouds is a necessary skill when doing digital landscapes. The reason is that these clouds are generally the first level of clouds most people see since it's so close to the earth.
Usually hovering under 6500 feet, these cotton shaped clouds are the precursors for bigger clouds like the cumulonimbus.
A cumulus cloud can exist alone or grouped up with other clusters. In terms of painting it, you will need to use a sharp digital brush in order to capture the edges of these cotton ball clouds.
So first thing is first, we need to set the proper foundation.
To get in the right mind frame for painting these types of clouds, we need to know where these clouds are found. Like I said, since it exists very close to the earth, you need a background that will reflect that.
That is, it's preferable to have a background that both focus on the land and the sky. In your digital art program, create a layer gradient.

What I have done is create two different colors: blue for the sky and green for the land. The separation of these two colors is gives you the horizon for when you begin to paint cumulus clouds.
In this step, you will begin to define you cloud shapes. With your digital brush, set the opacity to about 25% to 50%, and start to paint in a circular fashion.
The goal here is just to create cotton shaped clouds. While you are doing that, consider the horizon and the perspective point.
Basically, any clouds that appear closer to you are larger and are higher up. On the other hand, clouds that are far away are smaller and there are a lot of them. That means you can group as many as you fee right.

There is no limitation as to how many you need to add. It's up to you to think when you have enough. Just paint cumulus clouds while following the perspective rules and you'll be fine.
The next step in learning how to paint cumulus clouds is to figure out where the light hits the clouds. It's not as hard as it seems.
Due to the low altitudes of these clouds, sunlight will most likely hit the top of it. So with your digital brush, begin to paint near the top and the edges of the cotton shaped balls. You can even start adding color if you wish.
Another thing to note is to try and leave the bottom of the clouds as a darker shade of grey or color that you would like the clouds to be due to the lack of sunlight penetration.

If you ever have any doubt as to where the sunlight hits the clouds, what you can do is use the smudge tool. Reason: it's been known you see form better from squinting. Smudging your cotton clouds has the same effect!
Of course, once you have smudged the clouds a bit, reapply where the sunlight should be. Continue to repeat this process when you feel the clouds have already correctly capture how the light will hit it.
The final touches of your cloud painting involve highlighting the light even further. Since the clouds can exist by itself or in groups, you will need to emphasize its individuality.
That means sharpening the edges so it stands out better.
Not only that, you're also emphasizing the cotton shapes. To do this, you need to set your digital brush at higher opacity levels and work with a smaller brush to get the details.
Obviously, you also need to think about color. How do the clouds communicate with the color of the sky? Use simple color adjustments as you paint over the grey with colors that you need.

Use color selection tools to get the right color rather than guessing. This means you will be switching between the brush tool and the eyedropper to get the right colors.
Moreover, if anything feels wrong, just smudge that particular area and repaint over it. I can't stress how important it is to be patient. Just like anything, the more you work on it, the better you will become.
In the long run, you will be working on larger and larger canvases. The advantage is that you can put in incredible details just with coarse painting techniques. The disadvantage is that you will still need patience to cover the whole digital canvas.
So, continue to paint cumulus clouds on your own time and keep up the good work!
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