Much of the snowmelt season is characterized by a patchy surface; differential heating of the snow and snow-free surfaces results in a significant horizontal transport of energy that affects and contributes to the snowmelt. The calculation of the rate of energy advection requires some knowledge of the behaviour of the thermal boundary layer over the patches of snow and snow-free surfaces. We present the results from a series of field observations of the rate of growth of the thermal boundary layer over snow and snow-free patches. The results confirm that the boundary-layer growth can be described by a power function of the distance from the leading edge of the patch. For the case of the thermal boundary layer over a snow patch within a bare field, the boundary-layer growth is affected by the upwind surface roughness; the thermal boundary layer over a snow patch within a ‘rough’ field grows much more quickly than that in a ‘smooth’ field. Relationships are derived and presented for the parameterization of the boundary-layer growth as a function of distance and upwind surface roughness. Copyright © 2006 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.