Like an eyelid gently closing over the landscape – a weak shelf cloud in Swifts Creek, Victoria, Australia
Like most forbearers of bad weather, what we see from below is just the tip of the iceberg, as shelf clouds are attached to a much larger cloud system, usually bringing thunderstorms or cold fronts.
Nothing gentle here – a shelf cloud enveloping the Australian town of Wagga Wagga before bringing a thunderstorm
Roll clouds, on the other hand, are the gentle giants of the cloud world.
Composition in pink and blue – a shelf cloud bringing Moscow turbulent weather
Arcus cloud is the term used for a low, horizontal cloud formation associated with thunderstorm or cold front outflows. The two types of arcus clouds are shelf clouds and roll clouds; as their names might suggest they are vastly different in appearance and generation.
Parting the skies – shelf cloud over Enschede, Netherlands
Shelf clouds are attached to their parent cloud, usually a thunderstorm-bringing cumulonimbus cloud.
Here, a quite similar divide into good/bad weather zone
Unlike wall clouds, the cloud phenomenon they are often confused with, shelf clouds bring the storm along while wall clouds appear at the end of it.
This shelf cloud diagram shows with blue arrows how the cold air descending from a cumulonimbus meets the warm and moist air in the environment (red arrows). Because the cold pool is denser, the warmer, moist air is forced upward and its humidity condenses into a shelf cloud.
Shelf cloud over the Baltic Sea close to Øland, Sweden
In this image, the snaking shelf cloud looks almost ethereal in nature.
Is that an eagle’s face? Shelf cloud over Warsaw
Roll clouds are just as amazing a cloud phenomena. Unlike shelf clouds, they are completely detached from their parent cumulonimbus cloud. And unlike wall clouds, they are not liable to morph into tornadoes. Phew!
Textbook roll cloud over Las Olas Beach in Maldonado, Uruguay
Long cloud formations like roll clouds are rare as they require uniform cloud formation along an extended front.
Impressive roll cloud stretching over the fields of Albany, MO and into the horizon
Roll clouds also usually form near advancing cold fronts whose downdraft causes moist warm air to rise, then cool below its dew point and thus form a cloud.
Like a giant plantain? Roll cloud over a beach in Yucatan, Mexico
A bit of a cloud aberration is the Morning Glory cloud, an extremely long and rare roll cloud that has so far only been observed over Northern Australia’s Gulf of Carpentaria, the Mexican Sea of Cortez and Nova Scotia. It can reach a length of up to 1000 km and a height of 1 to 2 km.
Even cars speeding on Germany’s autobahn can’t escape this roll cloud
Often accompanied by sudden gusts of wind, the Morning Glory cloud can move at speeds of up to 60 km/h. Strong sea breezes and high humidity seem to further the formation of a Morning Glory cloud but despite being extensively studied, this meteorological phenomenon is not yet clearly understood.