Rain drops can symbolize heaven's tears and the accompanying LIGHTENING can be seen as heaven's anger (for example, thunderbolts were the favored weapon of Zeus, etc.). Rain is a symbol for tears, sorrow, anger, cleansing, renewal, forgiveness and more -- usually on a heavenly, worldly or very large sort of personal scale.
This is not a visual symbol for small sorrows or everyday events. Slang terms/phrases: "Don't rain on my parade" (rain as a spoiler of a big event), "Save it for a rainy day" (save something good to brighten up a gloomy day), "rain out" (a postponement or cancellation of an outdoor event because of rain) which leads straight to "rain check" (a promise for remittance at a later date, usually because product or time ran out) and "It's raining cats and dogs" (a very HEAVY downpour). The sound of rain, is considered extremely relaxing."