Minecraft wiki entry for Sun, associated text: Sunlight refers to the light, particularly sky light, which illuminates all blocks directly below the sky, and which do not have a shadow cast over them by an opaque block or by inclement weather. Clouds themselves do not cast shadows; however, during inclement weather (rain and snow), the sky darkens and reduces the amount of sunlight in the world. During these storms, the ambient light value during the day is reduced to 12 from the usual 15, and in thunderstorms the light level is reduced to 10 although treated as 5, allowing hostile mobs to spawn.