= Solution
The day-night contrast is controlled mainly by the ratio of radiative cooling time to horizontal advection and wave-adjustment times. A useful scaling is
$$
t_{\rm rad}\sim\frac{Pc_p}{g\,4\sigma T^3}.
$$
Stronger irradiation raises $T$ and sharply shortens $t_{\rm rad}$, allowing the dayside to reradiate before circulation reaches the nightside; the contrast therefore generally increases with irradiation. In ultra-hot atmospheres, hydrogen dissociation and recombination can transport latent heat and partly reduce it, while magnetic drag can weaken winds and increase it.
At low pressure, small atmospheric mass and short $t_{\rm rad}$ produce a large contrast. At greater pressure, the radiative time grows, waves and winds redistribute heat more effectively, and the contrast decreases. The observed contrast is wavelength dependent because each wavelength probes a different pressure.
Solved by gpt-5.6-sol high.
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