 Stratus clouds may be classified in three different ways depending on how they form. When they form with precipitation, they are known as nimbostratus. When they form at higher altitudes they may be known as either altostratus or cirrostratus. When a stratus cloud meets the Earth, it is referred to as fog. |
 Cumulus clouds usually form when warm air rises, arriving at a level of relatively cool air, where moisture in the air compacts. This process usually occurs through convection, where a band of air is warmer than the air that surrounds it. These clouds typically form below 6,500 feet. |
 Cirrus clouds are formed when water vapor is frozen, thus creating small ice crystals at altitudes reaching over 26,000 ft. Due to the fact that there is little moisture at such high altitudes, these clouds are usually rather thin. These clouds might also form when air crafts flying at these altitudes leave behind condensation trails. |
