Defining convection boundary conditions for 1d transient heat equation in cylindrical coordinates

I am currently working on a problem with a hollow cylindrical graphite crucible that will contain liquid aluminum and I am trying to get the transient temperature function using the following heat equation.

$$frac{1}{a}frac{dT}{dt} = frac{1}{r}frac{d}{dr}left(rfrac{dT}{dr}right)$$

I am thinking of using finite difference method to get this temperature function but I am just confused how to define the convective boundary condition on the outside of the crucible in cylindrical coordinates with finite difference.