Cloud of the Day – Flumen
Flumen, from the Latin flumen, a noun meaning river, is another of the new cloud types included in the World Meteorological Organization’s fifth edition of their venerable Cloud Atlas, published in 2017. It’s the first new edition in thirty years so there aren’t many opportunities to get a new cloud in.
Flumen are described as bands of low clouds associated with a ‘supercell severe convective storm’ — a form of cumulonimbus — arranged parallel to the low level winds blowing toward the supercell. So the powerful convection of the supercell is drawing air into its base, and the high speed airflow causes a drop in air pressure with the concommitant drop in temperature, allowing condensation and cloud-forming. This explains why flumen form parallel to wind direction.
Flumen are associated with supercells, a particularly vigorous form of thunderstorm featuring a mesocyclone, a rotating updraft. These are the clouds that spawn tornadoes. Flumen are very likely to be associated with precipitation, and then some.