An attempt has been made to derive the envelope amplitude probability distributions (APD) for atmospheric radio noise near thunderstorms. Formulas for APD's have been derived based upon the model for atmospheric radio noise near thunderstorms: First, elementary formulas for APD's are derived for the case when impulsive noise sources are continuously distributed over a limited range. Next, by taking into account the characteristics of the return stroke and K change pulses, APD curves at VLF are calculated using these formulas for atmospheric radio noise near a thunderstorm. Good agreement is shown between the calculated and measured APD curves. Further calculations indicate that the changes of the measurement frequency, and of the shortest distance to a thunderstorm influence the characteristics of the APD's. At last, similar procedure in analysis is applied to derive APD's for automotive radio noise, and the resulting calculated APD curves are shown for (1) a single vehicle and (2) a large number of vehicles running on a roadway, respectively.