The skin effect in power transmission occurs with AC signals. The outer surface – the “skin” of the wire/trace is used more than the inner part of the conductor to carry current. This causes an increase in the effective resistance of the wire.The depth into the conductor at which the charge carrier current density falls to l/e, or 37% of its value along the surface, is known as the skin depth and is a function of the frequency, the permeability and conductivity of the medium.Skin depth is inversely proportional to frequency. Skin depth also varies with the type of material.The skin effect causes high frequency ( HF) signals to be concentrated near the surface, the “skin”, of a conductor. For HF signals braided copper wires are used, which have a larger combined surface.At very high frequencies, all the current goes through the “skin”. The fundamental formula that is used to calculate skin depth is:

Here :

ω = angular frequency (rads/sec)
μ = permeability of the medium(Henries/meter)
σ = conductivity of the trace/wire ( Siemens/meter)
Data on material properties can be found at the two links below:
http://en.wikipedia.org/wiki/Permeability_(electromagnetism)
http://en.wikipedia.org/wiki/Electrical_conductivity

A script is available to calculate skin effect and can be found here.