It's been a good year for me as an Evil Overlord. Lots of princess kidnapping turned into sizable ransoms, several of my Doomsday Devices were sold for high prices at auction, and my Minions have been working harder than ever. In fact, I'd like to think I'm moving up the Evil Overlord ladder.
To capitalize on my profits and cement my position in the world, I've decided to construct an Evil Tower somewhere in the world. It's going to be ~500 meters tall, and I've got the blueprints more or less figured out, but it doesn't quite scream "evil". Yet.
To guarantee that everyone knows exactly who they're messing with, I'd like to have a persistent storm cloud hovering above my tower.
At least half a kilometer high and a kilometer in diameter, ideally floating a few hundred meters above the top of my tower
Must be stable and persistent under most weather conditions. This will depend on the suggested location, but the cloud should be present at least 90% of the time
The cloud does not need to be composed of the normal H2O molecules, but answers using more common and less environmentally disruptive substances will be preferred
Ideally, the cloud will not produce large quantities of rain but some precipitation (less than 1 meter per year) is acceptable
The cloud is dark grey or black (none of those cute sheep-looking cumulus clouds)
The cloud occasionally discharges lightning bolts to carefully positioned lightning rods in the top of my tower
How can I engineer my Evil Tower or the surrounding environment to fulfill the above requirements?