In general, heat and water vapor transfer in the atmosphere is accomplished through turbulent exchange, which is considerably faster than conduction.
However, surrounding the leaf and covering the surface of the soil is a thin skin of unperturbed air - the boundary layer. Heat must be transferred through this layer through molecular diffusion (conduction). The long timescale involved can be represented by a large resistance - the boundary layer resistance. The magnitude of this resistance depends mainly on the depth of the boundary layer and is proportional to (leaf size/wind speed).
See Goudriaan 1977 Eq 3.5.
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