Adam Clarke on Amos 4:7

When there were yet three months to the harvest - St. Jerome says, from the end of April, when the latter rain falls, until harvest, there are three months, May, June, and July, in which no rain falls in Judea. The rain, therefore, that God had withheld from them, was that which was usual in the spring months, particularly in April.

I caused it to rain upon one city - To prove to them that this rain did not come fortuitously or of necessity, God was pleased to make these most evident distinctions. One city had rain and could fill all its tanks or cisterns, while a neighboring city had none. One farm or field was well watered, and abundant in its crops, while one contiguous to it had not a shower. In these instances a particular providence was most evident. "And yet, they did not return to the Lord."