When you are in the middle of a could everything is fine, you can still see the world through the semi transparent cloud color, that is until you step down a block. So basically, when you are standing 1 block less than the bottom of the cloud, or the bottom of the cloud is waist level on the player, the cloud becomes opaque and no longer semi transparent. You then can only see blocks within the cloud, and nothing below your feet, which makes it dangerous.