What You Need to Know:
Name(s): Lennies; Lenticulars; UFO Clouds; Altocumulus Standing Lenticular (ACSL)
Common Altitudes of Formation: 2,000 to 5,500m (6,500 to 18,000 ft): mid-level
Signals: Usually lenticulars are associated with light precipitation within 12 to 36 hours.
Meaning of the Name: Altocumulus, meaning Altus (Latin for High) Cumulus (Latin for Heaped); Lenticularis (meaning shaped like lens)
Elaborated, more comprehensive information:
Altocumulus Lenticularis clouds have distinctive shaping and shading resembling lens, almonds or UFOs which can be seen up to 100km (60 miles) from the large object causing the formation’s downwind. They form orographically, which means that the usually stable airstream is forced up and over an obstacle such as a mountain or a hill, causing the water vapour coalesce into water droplets or ice crystals over or to the reverse side of the mountain that the wind is encountering (the lee), assuming that the air is moist enough. As if following the path of the obstacle, more lenticulars can form as the air evaporates down the mountain, leaving lenticular trails.
A pronounced attribute of the lenticular cloud is that it remains stationary (hence the name: Altocumulus Standing Lenticular (ACSL)) despite moderate wind. This is because the wind is actually passing through the cloud, creating water droplets which begin then end at the same place the lenticular begins and ends. Because the air flow speed is fixed, so is the point of evaporation and formation, meaning the ACSL is non-moving.
Lennies signal strong turbulence for pilots over or at the lee of the the mountain or object the lenticular forms over because of the way the wind travels over the mountain or hill.