Total heating at the cloud surface without mechanical heating. The main characteristic of this plot is that above the diagonal, the contours are parallel straight lines; whereas below the diagonal (G₀ = n line), there is a break in the slope, indicating that photo-electric heating is no longer the dominant mechanism in that region. The minimum total heating is on the order of 10^-25.5 erg cm^-3 s^-1 at the lower left corner and 10^-15.2 erg cm^-3 s^-1 at the top right corner.