Stratocumulus clouds reflect much of the incoming shortwave radiation but also re-emit large amounts of longwave radiation. Their cloud albedo forcing is larger than their cloud greenhouse forcing, resulting in a net cooling of the Earth.
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The longwave radiation emitted downward from the base of a stratocumulus cloud tends to warm the surface and the thin layer of air in between, but the preponderant cloud albedo forcing shields the surface from enough solar radiation that the net effect of these clouds is to cool the surface.
