JacobH wrote:Trying, desperately, to remember some maths: the volume of a barrel is proportional to the square of its radius, whilst the surface area is proportional to the radius alone?
Volume of a barrel is proportional to the cube of the radius, the surface area proportional to the square. So the ratio proportional to the radius (as you have it). It might be easier to think of it differently: there is a column(-ish) of liquid that evaporates through each square unit of exposed surface. The depth of that column is, approximately, proportional to the radius.
