If there was light rain which helps the crops but not the trees or heavy rains which are good for trees but not crops or if there wasn’t enough for the wells, we start fasting right away.

Taanis 3:3

If only one city didn’t have rain and the surrounding cities had rain, that city fasts and sounds the shofar. The cities around should fast but not blow the shofar. R’ Akiva says the surrounding cities should blow the shofar but not fast.