Volts. Lots and lots of volts. Also, humid air conducts electricity better than dry air, and maybe the raindrops themselves could help. In any case, once a little current flows there is a path of ions which improves the conductivity allowing yet more current to flow.
Electricity can only pass through air if it is in the neighborhood of 50KV. That 's 50,000 Volts . Lighning produces a voltage that is always above 50,000 Volt