A capacitor in an electric circuit, Wolgang tells us in Understanding Basic Electronics, is basically an insulating device with two plates and an insulating gap. Due to the insulation between the two plates, electrons are temporarily held-up at the first plate. After a time, the pent-up energy of the traffic-jammed electrons becomes powerful enough to burst through the insulating gap and move to the other plate, where the electrons can continue their journey around the circuit. With the release of this burst of electrons, the electric pressure is relieved. However, the first plate will immediately begin backing-up again with electrons until the electronic force once more becomes so powerful that the electrons force their way through the gap. This cycle can continue indefinitely.
There are three main factors determining how strong a capacitor is (meaning, how much electronic force it can hold back). First, the wider the plate surface area, the more electrons it can contain, and the more dispersed will be their energy. Due to this wider dispersion, the capacitor can hold back more electronic force before giving-in. Second, the greater the distance between the plates, the harder it will be for electrons to make the leap, and so this also makes for a stronger capacitor. And third, the type of insulating material in the gap between plates will also determine how resistant the capacitor can be to the arriving current.
Speaking of types of insulation, air can act as an insulator between plates. It takes 30,000 volts, Wolfgang tells us, for one centimeter of air to be transgressed by an electric current. This isn’t bad, but it’s nowhere near the insulating value of glass, which requires one million volts to push a current through one centimeter of it.
Clouds under the right conditions can build up enough electric power to cross even a large swath of insulator-behaving air. When this happens, the cloud is like the first plate of a capacitor, with the air forming the resistance-gap, and some other object forming the second, receiving plate– say, a church steeple. Lighting is merely a powerful electric current overcoming the resistance-gap and crossing through it to the other “plate.”
This is why lightning has been observed to strike the highest object around. There is a smaller resistance-gap (less air-space) between the cloud and the closet object than there is between the cloud and objects farther away, and the lightning simply takes the path of least resistance.