Taanis 3:2-3

Taanis 3:2

If rain that is good for vegetation fell but not rain that is good for trees, or rain that is good for trees but not rain that is good for vegetation, or if both of these fell but not rain to fill the wells, cisterns and caves, in all of these cases, the shofar is blown immediately.

Taanis 3:3

If no rain fell over a particular city, similar to Amos 4:7, “I caused it to rain on one city and not to rain on another city, one piece was rained upon…,” then the inhabitants of that city fast and blow shofar. The residents of neighboring cities fast but do not blow shofar; Rabbi Akiva says they blow shofar but do not fast.