#arcus cloud

Shelf cloud

Known to meteorologists as arcus clouds, they form at the leading edge of a thunderstorm when descending air flows outwards from the storm front at the base of the cumulo-nimbus supercell. As the cool air hits the land, it spreads out as the powerful wind preceding the storm and pushes warmer air upwards, where the moisture within the column then condenses into the shelf cloud, that roils with the shearing air currents. As they pass over, the winds shift and change, shortly followed by violent rain or hail. They tend to be very low, highly turbulent, and obstruct the view of the storm in a dark and menacing manner while indicating an approaching squall.

They are often confused with wall clouds (which form at the trailing edge of storms), and most usually occur in lines of storms rather than solitary anvils. Shelf clouds often seem to rotate about a horizontal axis, while wall clouds will do so in the vertical one, and the former are associated with rain while the latter are a sign that the storm has passed.

Loz

Arcus Ominous….

An arcus cloud is a low, horizontal cloud formation, usually appearing beneath an already-overcast and roiling sky. This particular arcus cloud is called both a roll cloud and a tube cloud. 

These formations are quite unusual....and are usually associated with gust fronts out away from severe thunderstorms some distance away.

                                               Milangaz 4

This ominous scene was photographed at dawn by a Turkish commercial mariner as he arrived on the bridge of his liquid petroleum gas (LPG) vessel, Milangaz 4 -- far out to sea in the Black Sea.

Sure gets the morning bridge watch off to an exciting beginning!

                                          ______________________

Here is a gif of a tubular cloud formation along the Lake Michigan shoreline at Sleeping Bear Dunes National Lakeshore, in June (2017).

This phenomenon is a type of arcus cloud and is commonly (and regrettably) referred to as “The Morning Glory” cloud.

Despite being studied extensively, this cloud formation is not yet fully understood. However, it is determined that the main cause is probably due to mesoscale circulations.

In the front of the cloud, there is strong vertical motion that transports air up through the cloud and creates the rolling appearance, while the air in the middle and rear of the cloud becomes turbulent and sinks.

The formations can be up to 1000 kilometres long, 1 to 2 kilometres high, and can move at speeds up to 60 kilometres per hour. It is often accompanied by sudden wind gusts, intense low-level wind shear, a rapid increase in the vertical displacement of air parcels, and a sharp pressure jump at the surface, making these clouds problematic to air traffic.

-Jean