Why, you may ask, is he showing us a photo of a bicycle bell? Well, I noticed it this morning while locking up my bike and I thought it was interesting that the condensation was predominantly on the side facing forward, into the breeze. That's all. The bike (not mine) was likely stored outside last night and the bell got really cold. Then, as it was being ridden, more condensation than evaporation occurred on the forward facing side and more evaporation than condensation occurred on the rear facing side.

The balance between condensation and evaporation is pretty much what clouds are about. Clouds are not static fluffy things floating in the sky. Rather, they are dynamic parcels of atmosphere where the instantaneous, moment-to-moment equilibrium between evaporation and condensation determines what we see. Clear air simply has the balance on the evaporation side, while clouds, (e.g. fog, if you want to think about a cloud you know intimately) has the balance on the condensation side. Every state in between the extremes is possible leading to clouds that appear briefly, persist, or don't appear at all.

A great way to think about this is with aircraft contrails. A contrail is formed when the exhaust from a jet engine pushes local conditions over to the condensing side of the balance. Sometimes you'll see a jet fly past with no contrail though. That happens when conditions are simply to dry or too warm for condensation to occur even with the aid of the moisture and CCNs (Cloud Condensation Nuclei) in the exhaust. Sometimes a contrail is short-lived and sometimes it persists and even grows larger. Each of these cases is frequently observed and illustrate how the local equilibrium forms clouds.

Another example occurred to me just now. Sometimes when you exhale you can "see your breath". This is a mixing cloud like the jet contrail. In my experience, it's only rarely that conditions are right for your breath to form a persistent cloud. Usually, the small amount of water vapour you add to the atmosphere briefly pushes the local environment over to the condensing side. This extra vapour, now in the form of droplets of water quickly evaporates again as your breath mixes with more of the surrounding air and the cloud disappears. Occasionally, I saw it recently here in Victoria, it is so humid out that the cloud from your breath persists for a lot longer than usual. In the case I experienced the air was clearly saturated with moisture already as I was in a light fog.

So, that's why I liked the little red bell.

[Edit: Many of our daily cloud videos show this kind of change of equilibrium nicely. For example, watch the first few (daylight) minutes of this one 2012-03-21 View North from UVic (University of Victoria) ]
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