Wine myths
The legs on the glass show how good the wine is
“Thick, slow legs running down the glass mean a rich, high-quality wine.”
Legs are a surface-tension effect driven mostly by alcohol content. They tell you something about strength, and nothing at all about quality.
The full answer
The physics is well understood and is called the Marangoni effect. Alcohol evaporates faster than water and has lower surface tension, so as it leaves the thin film on the glass wall the remaining liquid is pulled upward, collects, and eventually runs back down in drops.
The dominant variable is alcohol concentration. Glycerol and sugar contribute to viscosity and can modify the appearance, but alcohol is doing most of the work.
Nothing in that mechanism has any connection to balance, complexity, typicity or how much anybody enjoys the wine — which are what quality means.
It is also affected by things that have nothing to do with the wine: how clean the glass is, the temperature of the room, and the humidity.
It is not entirely uninformative. Pronounced legs are a reasonable hint that the wine is high in alcohol. That is genuinely all.
Legs really do correlate with alcoholic strength, and strength correlates loosely with body. The chain stops well short of quality.
Why the belief persists
It is the one thing a drinker can perform at a table that looks like expertise and requires no knowledge.
A biochemical or physical process that has been demonstrated experimentally and is not seriously contested — the sort of statement that appears in enology textbooks without hedging.
What would settle it: A controlled experiment failing to reproduce the process.
Sources
Sources consulted
- The Australian Wine Research Institute — AWRI
- The Oxford Companion to Wine — Oxford University Press