Lightning is less likely to strike flying creatures since lightning seeks the shortest route to ground. (Flying creatures are not connected to ground)
This is a little biased imo. Lighning has a nasty habit to hit objects which stand higher than the rest and, furthermore, airplanes are known to have (or had) problems with lightning strikes.
However, the truth is that ground objects are more usual targets of such an electric discharge. As a result, dealing extra damage to non-airbornes (or even being able to hit burrowed targets) makes sense imo.
Finally, I think it needs a cost increase of at least 1
![Gravity :gravity](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/gravity18x18.png)
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