The dielectric strength of air (ie. the maximum electric field that the material can withstand under ideal conditions without undergoing electrical breakdown and becoming electrically conductive) is 3 MV/m (according to wikipedia). I have read many articles where, to protect some facilities from lightning as in the Kennedy Space Center, the electric field's magnitude is measured by instruments called electric field mills, and where lightning is considered to be possible from a certain threshold. This threshold is generally taken between 1kV/m and 3kV/m (see page 4 of this article), which is not even close to the 3 MV/m needed to make air electrically conductive. Is there any reason for this apparent contradiction?
Thanks.
Dielectric Breakdown Voltage – Air vs Electric Field During a Thunderstorm
dielectricelectricityelectrostaticslightningvoltage
Best Answer
I would like to start with the fact that your question is directly related to the main unsolved problem of atmospheric electricity, i.e., lightning initiation. Specifically, as you mentioned, while electric breakdown of air requires a huge electric field, decades of balloon measurements of the electric field in thunderclouds, e.g., Dwyer & Uman, The physics of lightning, Physics Reports, 534, 147–241, 2014, p. 167 have failed to find electric field strengths large enough to make the electric discharge we routinely observe inside the thunderclouds in the form of lightning. The range of electric fields measured is generally 1-2 orders of magnitude less than the breakdown field. The important distinction to make here is the fact that by the time the thundercloud electric field is close to the breakdown field, the (lightning) discharge has most probably already started. Lightning occurs in various steps, one of them being the streamer to leader transition. Streamers are very small nonthermal gas discharges that are present ahead of the lightning leader (the huge branches of the lightning flash that we observe exiting the clouds or traveling through the clouds). Its been already demonstrated that streamers can propagate in electric fields below the breakdown electric field and therefore, may contribute to generation of lightning.
Another way to understand the lightning initiation process is very similar to the amplification of the background electric field on the surface of a conductor. We know from electromagnetic theory that the electric field on the conductor may be $\sim 3$ times the background field. One of the ideas for solving the lightning initiation problem considers hydrometeors, i.e., liquid or ice particles in clouds. The idea is that the presence of hydrometeors locally enhances the field near their surfaces, both due to charges residing on them and due to their polarization. The local increase of field then can lead to electrical breakdown (e.g., Jansky and Pasko, 2020).
I hope this provides the answer to your question. Please let me know if anything is unclear or you have other questions.