The Shape Builder might be the best friend you never knew you had, here. It's a tool that allows you to add shapes together easily and quickly, even better than the Pathfinder does. Create a group of objects, select them, and select the Shape Builder tool (Shift+M). Drag to add shapes together and Alt-drag to exclude them.
In this cae, you could first set up a grid of horizontal lines, with lots of circle shapes repeated horizontally. Those would be the building blocks you can use to build your cloud shapes with: