<p>Long spark discharges of about one meter and natural lightning show a polarity asymmetry.&#160; While positive discharges propagate continuously, negative discharges propagate in a stepped manner. This stepped propagation is mediated by the so-called space stem, an isolated region in the streamer corona of depleted electron density and enhanced electric field. Kostinskiy et al. 2018 [1] reported the stepping of positive leaders under high humidity conditions and Malag&#243;n-Romero et al. 2019 [2] showed that positive leader steps, if they exist, would be shorter and thus harder to observe in experiments.&#160;</p><p>In this work we present the results of our simulations for the evolution of a space stem precursor [2] under dry and humid air conditions. These results show that the presence of water molecules enhances the electric field and the heating rate of the space stem, reaching 2000 K considerably faster than in dry air. This could make feasible the stepping of positive leader discharges under high humidity conditions as observed by Kostinskiy et al. 2018 [1].</p><p>&#160;</p><p>[1] Kostinskiy, A. Y., Syssoev, V. S., Bogatov, N. A., Mareev, E. A., Andreev, M. G., Bulatov, M. U., & Rakov, V. A. (2018). Abrupt elongation (stepping) of negative and positive leaders culminating in an intense corona streamer burst: Observations in long sparks and implications for lightning. Journal of Geophysical Research:&#160;Atmospheres, 123, 5360&#8211;5375. </p><p>[2] Malag&#243;n-Romero, A., & Luque, A. (2019). Spontaneous emergence of space stems ahead of negative leaders in lightning and long sparks. Geophysical Research Letters, 46, 4029&#8211;4038. https://doi.org/10.1029/ 2019GL082063</p>