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According to my understanding, lightning happens because the clouds get charged due to friction from the moving clouds and the ground gets polarized by the nearby charged clouds. And the two opposite charges cause a huge electric field in between them that's strong enough to break down an insulator like air itself which we call lightning.

My question is: When it starts raining (since water is a conductor), won't the water droplets start forming a circuit between the cloud and the ground carrying current between them? If so, won't this rain start draining charge from the clouds reducing the potential difference in air, therefore preventing lightning? And does that mean the chances of lightning decrease with heavy rain and increase with light drizzles? And since our bodies can tolerate small amounts of current, is this current in rain going unnoticed by us?

I have heard people argue on the internet that corona discharges from lightning rods can prevent lightning. But the answers on Stack Exchange declared that the effect is negligible and that the rods help only in attracting lightning by providing a shorter route to ground. Is the same true for rain? In the case of rods, it's just one rod, but thats not the case with rain. So is the effect still negligible? Because in my experience I have never seen lightnings disappear once the rain starts pouring. Can someone explain why?

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  • $\begingroup$ If you pay attention during a strong thunderstorm, the lightning tends to occur on the boundaries of the cell more than in the center where the rain is heaviest. Pure water is an insulator, but rain is not pure water so yes it can conduct electricity. However, rain is not a river or continuous parcel of fluid. It's a lot of little drops separated in space. Despite rain water not being pure water, it still acts as a sort of insulator against lighting when the rain is heavy. $\endgroup$ Oct 28, 2021 at 13:48

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Lightning generally initiates within a cloud or similar structure, due to charge buildup on its various constituents. Many lightning bolts pass harmlessly across the sky. The ground-sky static field can augment the local field and draw the leader downwards until it meets the ground.

Rain has little effect. Consider scattering iron filings loosely on a sheet of paper and connecting a battery either side. The filings cannot conduct electricity across the sheet because they are not touching. Nor are they close enough together to arc across. Nor do they make an effective dielectric to turn the contacts into a capacitor, or anything like that. So it is with raindrops in the sky. What rain can do is to carry charge down to the ground and so, during the storm, slowly reduce the ground-sky potential a little. But again, the only effect of that is to reduce the downward attraction of bolts already under way.

Similarly, corona discharges on lighting rods are at the wrong end of the lightning bolt and have no effect on its initiation. The rod itself does intensify the field, sometimes enough to create a corona discharge, and can attract any lightning that is already on its way down and very close by, but it does not affect initiation. In general the current to ground is so vast that the earthing strip offers little protection. It is better not to have a rod or strip, so as not to attract the lightning towards the target in the first place.

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  • $\begingroup$ Does your last paragraph imply that Franklin's work was harmful? $\endgroup$
    – Ruslan
    Jul 22, 2021 at 16:13
  • $\begingroup$ I don't think this answer is complete. Rain droplets can and do carry net currents to ground. When there is no rain, there tends not to be lightning (there are exceptions, but that is my experience from living in a temperate climate). Theories for the generation of charge separation in clouds even use the current carried by rain to explain how charges separate in clouds. So why does rain, which carries current, tend to increase the charge separation that makes lightning rather than cancel it? $\endgroup$
    – HTNW
    Jul 22, 2021 at 16:36
  • $\begingroup$ As a reference, from measurements in this paper (the June storm), I calculate rain areal current $-230\,\mathrm{pA}/\mathrm{m}^2$ into the ground. For comparison, using $-15\,\mathrm{C}$ to ground per bolt (Wikipedia) and $0.02\,\mathrm{bolt}\,\mathrm{km}^{-2}\mathrm{storm}^{-1}\mathrm{dy}^{-1}$ (doi.org/10.1175/2010MWR3517.1), I get lightning areal current $-3.5\,\mathrm{pA}/\mathrm{m}^2$. So perhaps we should treat the rain current as significant! $\endgroup$
    – HTNW
    Jul 22, 2021 at 17:19
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    $\begingroup$ Yes. There are two ways to transport a parcel. One, you can directly go to the destination yourself carrying the parcel or Two you can pass it to your friend and ask him to pass it along. The water droplets in this scenario belongs to the first case as all droplets originate at the source - clouds and all droplets reach the destination - ground, they are personally delivering the electrons themselves instead of having to connect with each other like iron filings. $\endgroup$
    – Vignesh Sk
    Jul 23, 2021 at 6:19
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    $\begingroup$ In response to the above, my answer does point out that "What rain can do is to carry charge down to the ground". That this constitutes a current seems too obvious to need mentioning. It also points out that all this makes little difference to the presence of lightning, as the latter is initiated by air-to-air potential differences and not air-to-ground. $\endgroup$ Jul 23, 2021 at 7:30

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