Taanis, Chapter 3, Mishnah 3. This mishnah deals with the question of how to relate to a situation that appears severe in one region while it does not exist in the neighboring one. The mishnah opens: "Ir shelo yarad bah geshem" - a city upon which no rain has fallen.
The severity of the situation:
The mishnah cites the verse in Amos, as it is written: "Vehimtarti al ir echas ve'al ir achas lo amtir" - I will cause rain to fall on one city, and on another city I will not cause rain to fall. One region receives rain while its neighbor is left without. A scenario of this sort teaches that there is open Divine providence here, a particular decree directed against that city. It is not a normal state of affairs that only one city should be stricken, and for that reason the matter is severe.
The order of the fast in the city and its surroundings:
That city itself: It must begin at once to fast and to sound the alarm, and to observe the most stringent form of fasting.
The surrounding cities: They participate to a certain degree - they fast, but they do not sound the alarm and they are not required to fast in the stringent manner.
The view of Rabbi Akiva:
Rabbi Akiva says "masri'in velo mis'anin" - they sound the alarm but do not fast. This is the reverse: the surrounding cities blow the shofar, but they do not fast.
Why should the neighboring cities be affected and participate at all? There are two reasons:
Brotherhood: They must show solidarity with the suffering city, their neighbor.
Concern for themselves: The economic effect is destined to reach them as well, for when the people of that city come to buy from them, there may not be enough to go around.
In summary: In this mishnah we learned that a situation in which one city does not receive rain while its neighbors do receive rain testifies to providence and a special decree directed at it, and therefore it fasts and sounds the alarm with stringency. The surrounding cities participate as well - according to the first opinion they fast but do not sound the alarm, and according to Rabbi Akiva they sound the alarm but do not fast - both out of brotherhood with the suffering city and out of concern for the economic consequences that will reach them too.