Tobacco dryers, also known as oasts, are machines used to dry tobacco leaves. They consist of a cylindrical kiln with a conical roof and a number of vents, which allow hot air to circulate through the tobacco leaves. The hot air is usually produced by burning wood or coal, and the drying process can take several days. Oasts are an important part of the tobacco production process, as they help to reduce the moisture content of the leaves and improve their flavor.