The brightest discrete radio source is the Sun (Figure 1.10), but the Sun is much less dominant than it is in visible light. The radio sky is dark even when the Sun is up because atmospheric molecules and dust particles don’t scatter radio waves whose wavelengths are much larger than these particles. Most radio observations can be made day or night. Clouds are also nearly transparent at wavelengths λ>2λ>2 cm, so long-wavelength radio observations can be made even when the sky is overcast.
Wait til you hear about radio astronomy: