As mentioned, TAT is a much more useful indication of the actual icing potential on the airframe, as the TAT approaches zero with the SAT not too cold then airframe icing is probable. As an aside, during a departure in potential icing conditions it is often better to keep the speed up rather than reduce speed to get out of the cloud quicker (turb permitting). Often the higher speed will keep the TAT sufficiently high to stop airframe icing occuring during the climb and by the time the TAT is in the icing range the SAT is now sufficiently low that icing is unlikely anyway.
