Paint the sky pencil easy. Voluminous shapes of clouds is difficult to portray using conventional hatch.
However, if you follow a specific algorithm, you can get a good, fairly realistic result.
Draw some basic contours. Keep in mind that the cloud is very rarely have a circular shape. In addition, it depends on the specific type. cloud size may also vary. If you want to portray something really beautiful, take one big cloud, and many small.
Add fluffy contours. Some of the smallest clouds make feathery. That is, the wind has to literally carry them along a horizontal plane. The rest inject a little volume by increasing the rounded parts.
Circle the contours of the clouds thicker line. Designate specific areas of shading and drawing even more details.
Add the ground under the sky. This will make the image more realistic. Shading apply light and dark sides, adding volume. Keep in mind that the clouds are very far away, so the shadows should not have a clear outline.