An unusual cloud formation emerged above Tulsa, Oklahoma, after a severe thunderstorm swept through the county yesterday.
The formation, called mammatus, features pouch-like bulges from the base of a cloud that can extend across the sky, remaining visible for several minutes at a time.
Mammatus clouds usually occur in association with large cumulonimbus clouds, which are formed due to vigorous rising and overturning of warm, moist and unstable air.
Turbulence within the cumulonimbus cloud will typically cause mammatus to form, especially on the underside of the cloud, as it rapidly descends to lower levels, according to the UK's Met Office.
This reverses the usual cloud-forming process of upward growth, making for an uneven cloud base.
"Their striking appearance is most visible when the sun is low in the sky and their pouches are framed by the sunlight. This supplementary feature is a firm favourite with many meteorologists as well as cloud and photography enthusiasts," the UK Met Office said on its website.
The timelapse, taken by David Foster, shows mammatus clouds rolling above Tulsa during the sunset yesterday.
