During streams, this number can get as high as several hundreds. The worst one is sporadic-E or Es, consisting of conduction clouds in the high atmosphere that make permanent reception of remote stations possible during minutes to tens of minutes. The sporadic-E season is from May to July in large parts of the northern hemisphere. There are also tropospheric influences. A temperature inversion can also cause reception over wider ranges than normal.
Thunderstorms cause very sharp peaks.
However, DO NOT connect your antenna to your receiver during thunderstorms or when you are absent for longer time. Lightning strikes have ruined several radio shacks. Special care has to be taken to shield well the computer, power supplies, add filters etc., or the computer signal will be more or less noticed in the receiver. Optocouplers can be used to separate the grounds. However, some sensitivity and linearity will be lost in using them.