Hi Marcoz,
We've been having a lot of humidity here today. Luckily my Van de Graaff machine is fairly immune. The ionization going on inside the Gray tube could be affected.

I did play around with my ignition coil power supply and my big 4kV, 4uF and 5kV, 5uF oil capacitors today. All the circuit values were pretty much the same as in Aaron's video except for the grid was a perforated metal instead of mesh. However, I let C1 get to around 300V (measured) and then shut off the power supply. When I discharged C1, the arc I got was scary enough. I don't think I want to use these capacitors as C1 and C2. If I found 300V at 4uF that scary, I'd hate to work with 1000V in C2 at 4uF. I'll leave that to others more courageous and stick to my Van de Graaff machine and my 340 picofarad homemade C2.
-Steve