Ullage (wine)

The headspace of air is a mixture mostly of alcohol and water vapors with carbon dioxide that is a by-product of the fermentation process.

[1] At the winery, the natural process of evaporation creates ullage in the barrel by causing some of the alcohol and water particles to escape as vapors, a loss sometimes referred to as the "angel's share".

To prevent this possible oxidation and spoilage wineries will regularly "top up" the barrels by replacing the lost liquid with new wine.

As a cork is not a completely airtight sealant, some wine is lost through the process of evaporation and diffusion.

[1] Generally the greater the amount of ullage, the more potential that the wine has been exposed to harmful levels of oxidation.

The ullage level can also give insight as to the type of care and storage condition that the wine was kept in.

The fill levels descriptions are different for Bordeaux and Burgundy wines due to the different shape of the bottles from those two regions.

Ullage is the headspace of air between the wine and the top of the container (such as this barrel).
The "red band" seen on many wine barrels is from wine spill. Some of this spillage comes from the topping off process when the bung is reinserted into the barrel.
An assortment of aged Bordeaux wine with various ullage levels
Commonly used terms for ullage of Bordeaux bottles (Capsule line appears to indicate depth of cork.)