Answer to Question 1
Answer: Although the gases of the atmosphere are not effective at absorbing the wavelengths in the atmospheric window, clouds (even those of fairly modest thickness) readily absorb virtually all longwave radiation. When the evening sky is overcast, the cloud cover absorbs a large portion of the energy that otherwise would escape to space. Warmed by this energy, the clouds emit longwave radiation downward to the surface and lesser amounts upward to space. The clouds thus act something like a blanket, helping retain heat.
Answer to Question 2
Answer: Among others:
(i) Clouds inhibit solar energy from reaching the Earth's surface; therefore, cloudy days tend to be cooler than sunny days.
(ii) Clouds also inhibit radiation from the Earth's surface as it attempts to escape into space; therefore, cloudy nights tens to be warmer than clear nights.