ESD and Humidity

[Phrak]

Why does triboelectric charge accumulation increase with a decrease in humidity?

Is the inhumid air less conductive and allow a greater charge difference, or do dissimilar materials when rubbed against each other or pulled apart develop a greater potential difference than when they are in a dryer atmosphere?

[gnurf]

This is what Henry Ott has to say about static discharge (ch 15.3) in his book Electromagnetic Compatibility Engineering:
Charge accumulated on an object leaves the object by one of two ways, leakage or arcing. Because it is better to avoid arcing, leakage is the preferred way to discharge an object. Charge can leak off an object through the air, because of humidity. The higher humidity, the faster the charge will leak off the object.

He then goes on to define the decay time (τ) it takes for the charge to be reduced to 37% of its initial value

τ = ε/σ = ερ

where ε is the dielectric constant of the material, σ is the conductivity and ρ is the surface resistivity. So, at least your first suggestion regarding humidity and conduction seems to be correct.

[sparkey]

ok, a more low key answer to your question. consider first what static IS and how it builds; the whole friction and transfer of electrons blah blah.

in high humidity situations, the air has a higher density of molecules and such bouncing off each other. the more interaction, the higher the chance to transfer electrons and create static charge.