Special Effect Clouds

Something unusual appeared above the city this evening. Thick rain clouds that looked like lumps of dark cotton were covering the sky. Sunlight penetrates at its wrinkle like the sky would crack into pieces. The clouds were rolling like burning thick smoke, look so similar to the special effect in movie; imagine a giant monster or an alien spaceship could come out from the cloud. Few minutes later, there was no monster or alien falling from sky but only rain.