You can create a lot of impressive effects by stacking or layering photos, whether you do it in a darkroom or with Photoshop. The Harris shutter effect makes your photos super colorful, and double exposing or stacking negatives makes for some crazy looking portraits.
But, if you're looking for a way to add some flair to your cloud shots, this technique used by photographer Matt Molloy is perfect—it adds a brush stroke effect to long exposure photos by stacking them, giving them a look you usually only see in time-lapse videos.
Here's what Molloy told Petapixel about his process:
To make these 'photo stacks', I first shoot a timelapse, taking a photo every 5 seconds or so. (Settings differ depending on the subject and lighting conditions). I then merge several photos into one image using Photoshop. I start with the first image from the timelapse as a normal photo and then blend the rest of them with the 'lighten' blending mode. This only adds things that are brighter than what was in the first photo, and so you can see things like the paths of stars as they move across the sky. (The movement of the stars is actually from the earth's rotation).
It's a time-consuming process because it requires so many photos, but if you know your way around Photoshop, the end result is worth it. Be sure to share your results over in the Inspiration section if you decide to give it a try.