Rain is difficult to capture. Painting every single rain drop is tedious for a painter, and the results are often distracting to the viewer. Raindrops present a conundrum of design, which is why most painters rely on visual cues to hint at rain.
A gloomy sky is the first and foremost indication of bad weather. When painting a rainy day, the best approach is dreary, rather than threatening. In most cases, the darker clouds get, the more severe the weather becomes. For simple rain, paint a light to medium gray sky, with an evenly distributed cloud cover. For more intense storms, clouds become darker, or a different color altogether. For example, many people who have seen tornadoes have recalled later that the clouds became faintly green.
If your painting includes people, they should be wearing rain slickers and rain boots, and carrying open umbrellas held above their heads. Vehicle headlights will be on and windshield wipers will be working. One or two people may be unprepared: Pulling a jacket or newspaper over their head is an obvious indication of rain.
In an urban scene, wet surfaces will reflect the lights of street lamps, headlights, neon signs and bright colors. If you are painting a nature scene, include puddles. Paint one or two puddles in the foreground and hint at the ripples of large drops falling into them. To add continuity, paint a few raindrops hanging from objects in the foreground, such as from a tree branch or lamp post.
Title your painting something that indicates it is a scene of rainy weather. "London in the Rain," "Rain on Monday Afternoon" or, simply, "Rain" are titles that will serve as confirmation to the viewer.
Some painters really can't resist the temptation to paint actual rain on the canvas. If you are one such painter, proceed carefully and patiently. Paint the rain after everything else is finished, making it the last thing you paint. Vary the size of the rain drops based on their distance from the front of the canvas.
One reason rain is hard to paint on the canvas is because it becomes tedious very fast. Painters tend to get annoyed and impatient. Proceed methodically and thoughtfully, and paint the rain at the same pace the entire time. Make tiny, thin streaks either down from the top or diagonally from the side. Remember to coordinate the direction of the rain and the direction of umbrellas in the painting.