As new cold air front swept into Taiwan from the north, residents of Miaoli County and Taichung City yesterday (March 5) witnessed a rare "roll cloud" formation ominously approach like a scene from an apocalyptic disaster film.
On his Facebook page, Cheng wrote, "This was a leading cloud of a cold air mass which is has appeared in the form of a long, solitary line referred to as a 'roll cloud' with a relatively large width.
"Roll clouds" are a type of arcus cloud which are rarely seen and are formed from the downdraft of a thunderstorm.
Your course or sail selection should be influenced by sea state, anticipated wind shift (southern hemisphere almost always goes left), approaching roll cloud
, ability to manage or conserve sails, prior experience or local knowledge.
Others include the volutus or roll cloud
, the hole-punch cavum cloud, the wave-shaped Fluctus cloud, as well as a few special clouds.
The special roll cloud
occurs in the months that the north of Queensland is changing between the dry season and the wet season.
But it was actually a harmless type of cloud called a roll cloud.
Don't worry: Roll clouds don't create damaging winds like a tornado.
From drifting westward, toward sunset, it built further, developed a bow-wave (roll cloud
) and moved north while a line of similar development formed westward to the horizon.
Though a roll cloud
may not be seen, strong chinooks are often accompanied by spectacular multiple-layered standing lenticular clouds ("altocumulus standing lenticular" or ACSL) and a chinook arch, a broad mid-level altocumulus or cirrus cloud extending leeward from the mountains.
These are known as gravity waves, and may appear visually in the form of features like roll clouds
or rotor clouds.