QRN

QRN is "natural interference", as opposed to QRM - "man-made interference".

QRN is higher on the long wavelength bands (160, 80, 40), especially at night.

What causes QRN?

Propagation and radio wave theory
Propagation	Aurora * E-Skip * IPS * Lightning scatter * Meteor scatter * Satellites * Trans-Equatorial Propagation * Tropospheric ducting
Interference	QRM * QRN
Theory	Electromagnetic Waves * Frequency Wavelength and Period