S-values

Specific temperature anomalies given by the relation

where T is the actual temperature and T_p is the standard-atmosphere temperature at a point where the pressure altitude is z_p.
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 Δ_vD is the variation in D-value for a given Δz_p variation in pressure altitude.