On the morning of Saturday, April 27, 2019 as I was arriving at Sibley Nature Center for our
annual Nature Festival, I saw to the north an altocumulus cloud with some very impressive
shafts of virgae. Altocumulus clouds are characterized by globular masses or rolls occurring in
layers or patches. Altocumulus is a middle-altitude cloud genus generally forming between
2,000 and 6,100 m (6,600-20,000 ft) above ground level. Altocumulus clouds signify
convection, or the transfer of heat, due to bulk movement of molecules within the atmosphere.
Altocumulus clouds often produce virga (virgae, plural), the streaks or shafts of falling
precipitation evaporating before reaching the ground. Virgae can affect the weather as liquid
rain vaporizes with a significant amount of heat lost resulting in downward microbursts of air.
(Virga of higher altitude altocumulus clouds are formed of ice crystals). Virgae can also assist in
seeding storm cells. Small particles from one cloud are blown into nearby areas of
supersaturated air and act as nucleation particles to build a storm cell.
I wish to thank meteorologist and Weather Director, Craig Stewart of CBS7, for the
identification of the altocumulus cloud.