These are stationary lens-shaped clouds that are sometimes mistaken for UFOs because of their smooth saucer-like shape. Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. Lenticular clouds sometimes form at the crests of these waves.
Unfortunately we don't see these in Singapore because there are no high mountains nearby. I once did see some lens-shaped clouds while driving home from Tengah Airbase, though. I was driving at the time & didn't have a camera with me, but I just couldn't take my eyes off them. We do have some hills here & it is possible that the airflow over the hills may have produced those clouds.
I saw some lovely cloud formations when I was in NZ South Island. I think they are a truly unique & spectacular phenomenon. It's a pity I can't see them in real life more often.
PILEUS, OR CAP CLOUDS
A layer of cloud sometimes seen capping a cumulus cloud. The pileus layer forms when the cumulus cloud pushes a moist layer of air above it upwards, causing condensation in this layer. Eventually, the cumulus will grow into and penetrate the pileus cloud, after which the pileus may remain as a cloud skirt around the cumulus.
I have often seen these in Singapore. I get a kick out of spotting them, because they are transient & elusive. They are usually observed over towering cumulus clouds.