# What exactly is breakdown voltage of air?

Most of the online sources say that the breakdown voltage of air is 30kV/cm. I've made a Cockcroft Walton multiplier and I can see the air breaking in between 17-18kV/cm. Only in a discussion at one online forum : , someone claimed it to be 17kV/cm.

What exactly is breakdown voltage of air at STP?

• A lot depends upon the relative humidity. In Michigan I've gotten breakdown as low as 12 kV/cm in the humid summers, even in an air conditioned lab. The dryer the air, the higher the breakdown voltage. Commented Apr 16, 2016 at 19:55
• There is no such thing as "a breakdown voltage". What is relevant is the electric field at which a cascade develops in a small volume element. Since the electric field depends on the shape of the electrodes, one can cause gas discharges around sharp tips and edges at a lower voltage than for perfectly flat or spherical electrodes. Indeed, it is hard to get anywhere close to the max. breakdown voltage, that requires very clean and polished surfaces that do not cause local increases in the field that are above average. Commented Apr 16, 2016 at 20:54
• breakdown ocurrs when an electron "impacts" a gas molecule at some speed and calculating the required speed requires quantum physics. Breakdown electric field is just an aproximation and depends on a lot of factors. The breakdown voltage is the breakdown electric field multiplied by the distance. You are maybe asking about a range of values of the dielectric strength of air at STP Commented Jun 26, 2019 at 1:50