Problem solved--at least to my satisfaction.
I had been doing a series of practice flights in Quick Combat but got fed up with a run of bad weather days and switched over to Weather Override, selecting Broken Saturated Cumulus Clouds and that is when the dark bar appeared.
In my stupidity I suspected everything except my cloud selection and in total panic tried everything I could think of to no avail.Finally I tried another cloud type and Eureka---no dark bar.I tested every other cloud type and no dark bar.
Broken Saturated Cumulus is the only cloud type that shows the bar.
I can live happily with that.
Many thanks to all for your interest.