The Memorial Day weekend forecast is looking more and more summery all the time. Not only in terms of temperatures (upper 70s Saturday rising well into the 80s by Monday,) but also in terms of the possibility of heat induced afternoon thunderstorms. A weak low pressure center is starting to drift towards the southern California coastline which will induce a slow southerly flow. Chances of thunderstorms start mostly south of Lake Tahoe on Saturday and then drift up farther north on Sunday. By Monday the flow dries out and sunnier and warmer weather moves back in.

So why do clouds “stick together?” Perhaps the easiest way to answer is to talk about why those kinds of clouds form in the first place. First, the cloud doesn’t really “stick together”; because if you look carefully at time-lapse imagery you will see parts of the clouds evaporate, while other parts seem to form out of clear air. So the make-up of the cloud itself doesn’t contain a single group of water molecules, but rather the actual cloud is always “recirculating” the water through it.

Cumulus form because small columns air rise and the water vapor condenses as the air cools. It is where this lifting occurs that the clouds seem to be sticking together, whereas in reality, it is a dynamic process of formation and dissipation. And it is not only the cumulus cloud that does this. The famous “Sierra Wave” (or flying saucer shaped) cloud is always forming cloud on the upwind side, and evaporating (disappearing) on the downwind side. Think of a standing wave that forms on a stream that flows over a rock. The wave itself seems to be “standing still.” But it is always utilizing “new water,” that only stays in the wave itself for a very brief time. To the eye on the ground, a Sierra Wave cloud seems to be motionless. But each water droplet inside is generally zipping through the cloud at over 35 miles per hour.

So in a nutshell, it’s not the water that is “sticking together”, but rather the physical processes of cloud formation that is contained in a definable volume.

Of course, it’s always fun when those physical processes end up looking like a UFO.