Average formation temperature across a spectral line. Left: normalized cumulative contribution function as a function of temperature at five points across the spectral line shown in the right panel. The dashed line indicates the location of T_1/2, i.e., where the contribution function is 50%. Right: line profile of one of the solar lines. The five points for which the contribution function is computed are indicated by large dots color-coded with the T_1/2values.