Towering altocumulus, known as altocumulus castellanus, frequently signals the development of thunderstorms later in the day, as it shows instability and convection in the middle levels of the troposphere (the lowest layer of the atmosphere), the area where towering cumulus clouds can turn into cumulonimbus.[3] It is therefore one of three warning clouds often recorded by the aviation industry, the other two being towering cumulus and cumulonimbus.[3] Altocumulus generally forms about 2,000 to 6,100 meters (6,600 to 20,000 ft) above ground level, a similar level to altostratus formations, and satellite photography has revealed that the two types of cloud can create formations that can stretch for thousands of square miles.[4] Extensive altocumulus formations, particularly if they take the form of undulatus are often referred to as altocumulus mackerel sky.