Radio noise is caused by both internal and external factors. Internally, there is “thermal” EMF noise caused by random electron movements in components, as well as harmonic noise generated by the RF circuitry. External sources are natural and man-made.
Radiation from the sun (“solar flares”) noise across the radio spectrum, with variable effect at different times. Lightning is another natural source of noise. All electrical and electronic equipment produces radio frequency (RF) harmonics, however the noise amplitude of electrical power equipment at the frequencies used for industrial wireless products is extremely small and can be ignored. Harmonics from other radio sources such as television, broadcast radio services, wireless cellular telephones etc. cannot be ignored and is a significant noise source.
“Radio Interference” is noise caused by other wireless transmissions on the same channel or on nearby channels.
The sum of all noise sources is a volatile mix of varying amplitudes. Over a short period of time, the noise level on any radio channel will have peaks and troughs about an average level, with the amplitude at any instant following a random probability function. Over longer periods of time, the noise pattern will be the same, however the average noise level will change according to what is happening in the surrounding environment.