Leviticus 26:4

Rain in due season. Palestine was peculiarly dependent on rain at the usual times, for prosperity and plenty. In Egypt, Israel had seen the regular overflow of the Nile, which watered the land and made it fruitful (see on Gen. 41:34). Palestine was “a land of hills and valleys,” and so not adapted to irrigation (Deut. 11:10-12). Israel had not been used to rain, for it seldom rains in Egypt. Now they were coming to a land where their very existence depended upon rain from heaven. To reassure them God promised to send the rains in their season, “the first rain and the latter rain, that thou mayest gather in thy corn, and thy wine, and thine oil” (Deut. 11:13, 14). It was not merely rain that was needed, but rain in “season.”

God warned them, however, that rain would not fall as a matter of course, that indeed there would “be no rain” (Deut. 11:17) if they turned from Him to worship idols. This was fulfilled during the days of Ahab (1 Kings 17:1).