Even if you're not Bob Ross, you've probably wanted to paint an accurate landscape at some point. Budding painters can rest assured, there are simple ways to make your skies and storm clouds look realistic that won't take years of training. A few simple steps will have your clouds looking like they're about to rain down on the rest of your scene. Not all of us can be famous artists, but all of us can be artists.
Create a solid sky with the colour of your choice, probably something in the middle range of tone. Stormy skies often display both light and darker colours that create a muddled effect. Make areas where you intend to add clouds slightly darker.
Apply some basic colour to the base of your clouds to get started. Use a light colour (blue or grey) and paint in wispy, overlapping strokes over your sky.
Mix several colours together in an empty can for a more blended effect. Try using a light or sky blue, a darker blue and grey or a small amount of black. Squeeze the colours together into the can and add a teaspoon of water before mixing thoroughly with a brush.
Use the same brush to apply the mixed paint. Paint in wispy, circular strokes again until you've filled in the area of the clouds.
Dab the corner of a wet sponge in the mixed paint. Dot the cloud with the corner of the sponge adding a layered effect. Hold it down momentarily on the canvas, then lift off.
Touch up the cloud with individual colours and a clean brush. Add white and grey flourishes (small lines) throughout the clouds. Darken the lower portions of the clouds by adding a solid grey colour.
Make bunches of converging clouds rather than the more traditional individual clouds popularised by children and novice artists. According to "Bob Ross' New Joy of Painting," you can "Reinforce the...light areas of the sky using Titanium white...using criss-cross strokes."