I am a keen disciple of St Ambrose, who advised, if you remember, Si fueris Romae, Romano vivito more - "When in Rome, do as the Romans do". It is mainly for this reason that during the current spell of warm, sunny, summer weather in the Rhine Valley, I resort at frequent intervals to the pleasurable pastime of contemplating the mysteries of meteorology through the medium of a rich, Teutonic brew. Some of my conclusions I have revealed to you on Saturday; let me continue this idyllic narrative.
An unopened bottle of beer contains a mixture of beer and carbon dioxide. While the contents remain sealed and under pressure, much of the CO2 is dissolved in the liquid; the remainder, in gaseous form, occupies the "empty" space just beneath the cap. Under such conditions the whole system is in equilibrium, and the liquid lies in perfect stillness in the bottle.
Removing the cap disturbs this equilibrium. The pressure of the mixture is now greatly reduced, and at this new pressure the beer contains more COs2 than it can handle. When the liquid is poured into a glass the excess carbon dioxide can be seen escaping from it as a multitude of pretty streams of rising bubbles. A glass like this can be thought of as an "inverted" cloud, since beer thus "supersaturated" with COs2 is similar to a volume of air which contains too much moisture. In the case of air, the excess moisture condenses into water droplets, which grow larger and eventually fall to Earth as drops of rain; they move downwards, because they are heavier than the air in which they are suspended.
Our glass of beer, on the other hand, is almost a mirror image of this process. The excess carbon dioxide forms little bubbles in the liquid, and they move upwards because they are many times lighter than the beer in which they find themselves afloat.
If you look at the bubbles in your glass of beer more closely, you will notice that they do not form everywhere; they emanate from a small number of sites, mainly on the sides or bottom of the glass. Here again there is a similarity to clouds.
In saturated air, the water droplets tend to form on tiny particles called condensation nuclei. Tiny imperfections on the surface of the glass perform a similar function in the case of beer; and any particles suspended in the liquid do the same, which is why you sometimes see a stream of bubbles rising for no apparent reason from some point right in the middle of your glass of beer.