What causes the appearance of "legs" on a wine?

The appearance of "legs" on a wine, also known as "tears," is primarily caused by the wine's alcohol content. When wine is swirled in the glass, the alcohol evaporates more quickly than water due to its lower boiling point. This creates a gradient of viscosity, where the wine’s surface tension plays a role in how it clings to the sides of the glass.

As the alcohol evaporates, the remaining residual liquid, which is denser, starts to flow back down the glass, forming these visible streaks or "legs." Therefore, wines with higher alcohol content tend to produce more pronounced legs due to the greater difference in evaporation rates, enhancing the visual effect.

While aging in oak barrels, the grape variety used, and the temperature of the wine can affect various characteristics of the wine, they do not directly influence the formation of legs the way alcohol content does. Aging in oak might impart flavors and tannins, the grape variety contributes distinct attributes to the wine, and temperature can affect the wine's aroma and structure, but these factors are not responsible for the generation of the physical legs. Understanding this mechanism helps appreciate how alcohol concentration impacts not only the flavor profile but also the visual characteristics of the wine.