Why can't you listen to distant MF and HF radio stations in winter?

Answer 1

MF (20 meter/40 meter) and HF differ from VHF and UHF in that they "bounce" lower in the atmosphere. Winter atmosphere is heavier than Summer atmosphere because the colder weather puts the molecules closer together causing this bounce to become even lower than normal thus keeping it from going as far. I hope this helps.

Answer 2

Elle is not a physicist, that's fer sure. Actually, if one has a decent antenna and pre-amp, one can listen to HF, or shortwave, broadcasts any time of year. The reduced propagation from ionospheric reflection, or "bounce" that is sometimes noticed in wintertime is usually significant only with very low power transmissions or very inefficient receiving equipment. To us ham radio fanatics who operate QRP, or very low power, that's a welcome challenge. It is possible to "work the world" on less than a watt of output power...