  We investigate the effect of humidity on the propagation of streamers in air.
We present a minimal set of chemical reactions that takes into account the
presence of water in a nonthermal air plasma and considers ionization,
attachment, detachment, recombination and ion conversion including water
cluster formation. We find differences in streamer propagation between dry and
humid air that we attribute mostly to an enhanced effective attachment rate in
humid air, leading to higher breakdown electric field and threshold field for
propagation. This higher effective attachment rate in humid conditions leads to
a faster decay of the conductivity in the streamer channel, which hinders the
accumulation of charge in the streamer head. In some cases a propagating
streamer solution still exists at the expense of a smaller radius and lower
velocity. In other cases a high humidity leads to the stagnation of the
streamer. We finally discuss how all these statements may affect streamer
branching and the dimensions and lifetime of a streamer corona.
