Past exam of the atmospheric physics course of the University of Cambridge / 2014 / Paper 1 / 1 / d

Thick low clouds usually reflect much sunlight and emit infrared at temperatures close to the surface, so their daytime shortwave cooling is strong and their longwave greenhouse contrast relatively small. Cold high clouds can strongly reduce outgoing longwave radiation; thin high clouds reflect less sunlight. Thickness and particle properties matter as well as height.

At night there is no solar term. Warm low-cloud tops commonly cool radiatively, promoting condensation and cloud-driven turbulence that can sustain the layer. A cold, optically thin high cloud can instead absorb more infrared from below than it emits, warming and sublimating and thus shortening its lifetime. A gray-layer diagnostic is absorption minus emission, $ϵ(F_{\uparrow }+F_{\downarrow }-2\sigma T_c^4)$. This is a tendency, not a universal lifetime ranking: thick cirrus can cool at its top, and ascent, entrainment, humidity and precipitation may dominate.