Jeff Liebermann wrote:

This might help:
http://en.wikipedia.org/wiki/Atmospheric_noise

If you extend the red line showing man made noise, at greater than
about 30Mhz, the man made noise (ignition noise, motor noise, etc)
predominates over atmospheric noise.

In my last couple of postings, I was lumping man-made and atmospheric
noise together as "atmospheric noise". Both enter the system between the
transmit and receive antenna, so improving the receive antenna
efficiency won't help the ratio of signal to either atmospheric or man
made noise. The referenced graph doesn't show receiver noise at all,
which dominates at VHF and above.

It can be useful, however, to distinguish between atmospheric noise and
*local* man-made noise, since the latter can sometimes be reduced by
using techniques such as feedline decoupling and using horizontally
polarized antennas.

Roy Lewallen, W7EL