Altocumulus

• Altocumulus clouds range in appearance from thick to thin, white to gray, and are organized to random in distribution. These commonly occur worldwide when the air in the middle of the atmosphere is moist. When unstable conditions are present, the air tends to rise in currents and allow cumuliform turrets to evolve, which suggest vertical turbulence within the cloud itself. Altocumulus clouds can actually form from dissipating thunderstorms in the morning and redevelop later the same day if unstable conditions remain. Because altocumulus clouds can form in multiple layers and in conjunction with other types of clouds, they often develop within the structure of thunderstorm-producing cumulonimbus clouds.
  
  

Lenticular Altocumulus

• Lenticular altocumulus clouds appear in the shape of a lens; very smooth and flat, often in multiple layers. These clouds gain their shape due to an air flow wave effect and normally appear around mountain ranges on windy days. This wave effect causes air to rise above condensation level, allowing lenticular altocumulus clouds to form. Because this stable, dry air moves in a horizontal rather than vertical flow for lenticular altocumulus cloud establishment, these are not typically associated with precipitation.
  
  

Altostratus

• The thick veils of gray clouds seen extending across the entire sky are altostratus clouds. Their presence indicates an approaching warm front and precede the nimbostratus clouds. Light rain, snow or ice may occur with altostratus clouds, which appear chiefly over the central United States after severe thunderstorms. These layered clouds are comprised primarily of liquid water and occasionally appear pale blue in color. They take shape when the middle layers of the atmosphere are moist and gradually elevated.
  
  

Nimbostratus

• Nimbostratus clouds result in persistent, light to moderate rain or showers. These light gray or white sheet clouds can occasionally produce moderate to heavy precipitation if strong low clouds and cold fronts are also present. Nimbostratus clouds can develop from altostratus clouds that are thick enough to produce precipitation. The same weather conditions that produce nimbostratus clouds in the middle atmospheric layers also lead to higher level cloud development.