From Glossary of Meteorology
Specific temperature anomalies given by the relation
where T is the actual temperature and Tp is the standard-atmosphere temperature at a point where the pressure altitude is zp.
S-values are usually expressed in terms of tenths of degrees per hundred degrees absolute temperature. S-value lines are drawn on 4D charts because they are related to D-values by
where ΔvD is the variation in D-value for a given Δzp variation in pressure altitude.