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We simulate short positive and negative streamers in air at standardtemperature and pressure. They evolve in homogeneous electric fields or emergefrom needle electrodes with voltages of 10 to 20 kV. The streamer velocity atgiven streamer length depends only weakly on the initial ionization seed,except in the case of negative streamers in homogeneous fields. We characterizethe streamers by length, head radius, head charge and field enhancement. Weshow that the velocity of positive streamers is mainly determined by theirradius and in quantitative agreement with recent experimental results both forradius and velocity. The velocity of negative streamers is dominated byelectron drift in the enhanced field; in the low local fields of the presentsimulations, it is little influenced by photo-ionization. Though negativestreamer fronts always move at least with the electron drift velocity in thelocal field, this drift motion broadens the streamer head, decreases the fieldenhancement and ultimately leads to slower propagation or even extinction ofthe negative streamer.

Positive and negative streamers in ambient air: modeling evolution and velocities