When a layer of cloud rises in distinct turrets with bumpy tops that resemble crenellations, it is of the species known as castellanus. This cloud can give an early indication of unsettled weather to come later in the day. While several types of cloud have castellanus species, it is when these jagged crenellations appear in Altocumulus clouds, like the ones shown here, that they best forecast unsettled weather. The vigorous turrets show that the air at the mid-cloud level is unstable. Any that start to develop on thermals will, upon reaching this layer, continue growing with extra vigour, and quite possibly develop into tall, stormy Cumulonimbus clouds.