If strong updrafts of wind make that altitude separation big enough, an electric current flows to cancel out the difference in charge. (sciencedaily.com)
Every once in a while, though, they can form in this strange shape, usually when a horizontal spinning air current is deformed by updraft from a cumulus cloud. (discovermagazine.com)
As for economic viability of solar updraft towers compared to solar PV systems is very difficult to estimate. (centerforcarbonremoval.org)