  Space-charge dominated streamer discharges can emerge in free space from
single electrons. We reinvestigate the Raether-Meek criterion and show that
streamer emergence depends not only on ionization and attachment rates and gap
length, but also on electron diffusion. Motivated by simulation results, we
derive an explicit quantitative criterion for the avalanche-to-streamer
transition both for pure non-attaching gases and for air, under the assumption
that the avalanche emerges from a single free electron and evolves in a
homogenous field.
