I just heard on the radio that weather conditions in Sydney today are such that there’s a high probability of a lenticular cloud forming over the city late this afternoon, as the day cools. If it does, it will likely run from northwest to southeast, and should look fabulous from the east with the sun behind it as it sets.
Image Credit Rob Alexander: click image for full size
Lenticular clouds only form in conditions where the air forms pressure waves that lead to the cloud clumping and layering with clear air separating it from other clouds.
Normally, air moves much more horizontally than it does vertically. Sometimes, however, such as when wind comes off of a mountain or a hill, relatively strong vertical oscillations take place as the air stabilizes. The dry air at the top of an oscillation may be quite stratified in moisture content, and hence forms clouds at each layer where the air saturates with moisture. The result can be a lenticular cloud with a strongly layered appearance. [APOD]
Lenticular clouds also have a fabulous long technical name that is going to be the core of the next drinking game I play: altocumulus standing lenticularis. They are also an object of particular delight to glider pilots, as the same combination of winds and updrafts that allows them to form also provides a rapid lift and a long flight in a glider. Indeed, it was a glider pilot who rang the radio station to talk about the likelihood of a lenticular cloud forming today: he described lenticular clouds as like a cigar sliced in half, with the flat side underneath and the rounded side on top. I’ve read more elegant descriptions, but that’s a very vivid one. They have been mistaken for flying saucers as well when they are more rounded than cigar shaped.
Image Credit US Coast Guard via Splendid Pictures (Warning: Serious Procrastination Alert)
With Sydney’s low hills of the Blue Mountains escarpment being the catalyst for lenticular cloud stacking, they are unfortunately not high enough to produce the truly spectacular results in the pictures linked to above. But we do get formations like these below:
Image Credit: Oz Thunder
I hadn’t realised they were the undersides of lenticular clouds, but I’ve quite often seen leading edges like the second picture moving overhead. Cool. There’s lots more altocumulus clouds at OzThunder, and many more cloud types too.
Also mentioned on the radio was the term “Mackerel sky” which apparently involves lots of small spot clouds. I have to go pick up the kids soon, so I haven’t googled that one.