Charles Spurgeon Commentary 1 Samuel 12:18-19

"So Samuel called unto Jehovah; and Jehovah sent thunder and rain that day: and all the people greatly feared Jehovah and Samuel. And all the people said unto Samuel, Pray for thy servants unto Jehovah thy God, that we die not; for we have added unto all our sins [this] evil, to ask us a king." — 1 Samuel 12:18-19 (ASV)

That thunderstorm was a powerful preacher to them, and the raindrops, that fell so copiously, brought the teardrops into their eyes. The phenomena of nature frequently impress people with a sense of God's power, and prostrate them before Him.