Clouds absorb outgoing radiation emitted by earth and reradiate a portion of it back to the surface during

I would say the correct answer is nighttime. The clouds in the atmosphere would absorb the radiation that is emitted by the Earth and at at night 30% of these would be radiated back to the Earth's surface. Hope this answers the question. 

This is called, The Doppler effect.

In short terms, when the trains is farther away the wavelength of the whistles sound are longer and more separate from each other, and more farther away from you, as it gets closer to you, the wavelengths start getting shorter and more louder the closer it gets to you. Basically the wavelengths change because of the motion of the source.