Fig. 7. Two situations relevant to pitviper biology present particularly strong surface temperature contrasts. (A) A rodent burrow below a desert shrub at high noon, scaled to an 80° view angle with the burrow 28 cm distant. Conditions are warm but not extreme, with air temperature of 33°C and peak ground temperatures near 50°C. (B–D) Pit membrane temperature contrasts visualized using the indicated _i. The burrow could be easily identified from a distance by a snake leaving the shade of a bush to seek underground shelter. (E) An American cardinal Cardinalis cardinalis viewed against a clear sky, scaled to an 80° view angle with the cardinal 31 cm distant. Note that the thermal imager does not record radiant sky temperature properly, and so the sky has been rescaled to a radiant temperature of 5°C on the basis of Swinbank's formula (Swinbank, 1963). (F–H) Pit membrane temperature contrasts visualized using the _i indicated for that row. The temperature contrast on the pit membrane in B–D and F–H are indicated by color steps of 0.001°C.