yes that would be the problem using a bag or tarpaulin etc.You may get away with it at 30 mph but defo not at 70mph .
Also anything like a tarp flapping for hours on end could wear your paint work away.
Unless it warms up a bit I wont bother anyway but if it does then cling film is getting a go.It wont increase the surface area of the bike at all .
Thanks for all the repluies .If I do it I will report back on how successful it was.(or not )