Complex sentence: It is probably due to the same principle that the drops of rain are so much larger in thunderstorms than in ordinary showers: electric charge on the clouds probably causes the drops of water to cohere into large ones.
Split into simpler sentences: It is probably due to the same principle that the drops of rain are so much larger in thunderstorms than in ordinary showers. Electric charge of the clouds probably cause the drops of water to cohere into large ones.