What causes QRN?

Strictly speaking QRN means "noise from natural sources" as opposed to QRM which means "man-made noise". The abbreviation QRN is oftern used in in general terms to mean any noise that interferes with propagation or reception of transmissions.

Sources of QRN

 * 1) Atmospheric noise. This originates in the atmosphere. Lightning creates RF noise over a wide range of frequencies (usually between 100 kHz to 20 MHz) that affects radio transmissions over hundreds of km. Auroral discharges over polar regions generally create noise at lower frequencies (below 0.1 MHz).
 * 2) Geological activity. Increased RF noise has been recorded as a precursor to earthquakes, and near erupting volcanoes
 * 3) Cosmic noise. This comes from a source outside the Earth's atmosphere. The sun produces noise that reaches a maximum at 11 year intervals (the so-called solar cycle). The planet Jupiter produces large amounts of RF noise in the 16 to 24 MHz range. Stars and galaxies also contribute to cosmic noise.
 * 4) Snow Static yes, snow static! The link provides some detail about this unusual phenomenon