Everyone has watched a random wine snob swirling a glass, holding it up to the light before pontificating about the wine sticking to the glass known as “legs” which slowly (or quickly) fall back into the glass.
You hear comments like: “This is clearly an excellent, full-bodied wine.”
Or perhaps: “Not much viscosity with this wine portending a light body.”
And even: “These legs demonstrate a beautiful structure, promising a big bold wine.”
In truth, the “legs” in wine are a result of evaporation from the motion of swirling the wine in the barrel of the glass that releases aromatics in the wine allowing you to enjoy the “nose” or “bouquet” of the wine.
For those interested in physics – and those of us who are merely curious - a simple experiment demonstrates this principle which is called the Marangoni Effect. It was first explained in 1855 by the English physicist, James Thomson.
Put a very thin layer of water on a smooth plate. Then dribble a drop of rubbing alcohol onto the water. The water immediately rushes away from the droplet of alcohol.
This is what you are seeing as you swirl the wine in your glass. Wine is mostly water and alcohol, so as air hitting the wine causes it to mildly evaporate, it also causes the water within the wine to rush away from alcohol forming the voids you see. What remains briefly (the legs) is the alcohol in the wine sticking to the glass before it falls back into the glass due to gravity.
So while the “legs” of a wine cannot tell you whether the wine is of quality or even if it is full or medium bodied, it can point to the alcohol content of the wine.
One of the comments above was partially correct: you can detect the viscosity of a wine by the legs. Hence, heavier dessert wines have thicker, slow moving legs. But so does a high alcohol Zinfandel.
Wine lovers can become adept at determining the alcohol content of a wine just by viewing its legs. For example, if one is swirling a Barbera in the glass and sees very heavy, slow moving legs, deduction and experience dictate that this is not because the wine is viscous. Rather it points to a high alcohol content. Some oenophiles can correctly deduce the actual amount of alcohol within a given wine.
So grab a glass of wine without looking at the bottle to note the ABV percentage and have fun guessing the alcohol content. With practice, you too will become accustomed to accurately determining alcohol percentages in different varietals. You'll impress friends with your newly acquired skill and also be able to debunk one of the most common misconceptions of wine appreciation.