In the channel sharing problems associated with broadcast planning at VHF and UHF it is usually assumed that interference from co-channel stations occurs randomly in time. This implies that there is equal probability of occurrence of the meteorological conditions giving rise to abnormal tropospheric propagation at any time of day or any season of the year. In this report the results of previous measurements are re-analysed to assess the validity of this assumption. It is deduced that both diurnal and seasonal trends occur, and that these trends are different for propagation over land and sea paths.