[en] The thermal decay of an initially hot isothermal water body contained in a tank has been studied. Analytical solutions (including a group-theoretic solution) have been obtained for the one-dimensional heat conduction equation with heat loss from the sides of the tank. The convective heat transfer coefficient is assumed to vary with space and time rather than constant over the surface of the tank. A very good agreement is obtained between the group-theoretic solution and a numerical solution using the Crank-Nicholson finite difference scheme. The analytical results have been discussed in terms of underlying physical mechanisms. (author). 6 refs, 3 figs