The first Mishna in Massekhet Ta’anit opens with a discussion of the prayer for rain, distinguishing between two different parts of the Amida prayer. Towards the beginning of the Amida we recite gevurot geshamim (mashiv ha-rua’h u’morid ha-geshem – He who makes the winds blow and brings down the rain) in the blessing of mehayye ha-metim (He who revives the dead). In the middle of the Amida, in the blessing of mevarekh ha-shanim (He who blesses the years) we also add a specific request for rain. Although Rabbi Eliezer and Rabbi Yehoshua disagree whether we should begin praising God for His deliverance of rain at the beginning of Sukkot, it becomes clear in the Gemara that all are in agreement that the request for rain should wait until after the Sukkot holiday is over. According to Rabbi Eliezer, it is appropriate to praise God for His works at any time, while according to Rabbi Yehoshua, since rain is not wanted during Sukkot it would be inappropriate to mention it in any way until after the holiday.
The idea that rain during Sukkot is a siman kelalah – a curse – is explained by Rashi to refer to the Gemara in Massekhet Sukka (28b), which teaches that a person who becomes uncomfortable in his sukka because of the rain is permitted to leave his sukka. A parable is told in which a person who is forced to leave his sukka because of the rain is compared to a servant who pours a cup of wine for his master and then has the wine flung in his face by the master, who clearly rejects his service. Thus, rain on Sukkot is a siman kelalah because a Jew forced out of his sukka by rain experiences the rejection of his desire to serve God by means of the sukka. The Me’iri suggests a much simpler explanation, pointing out that simply missing out on the opportunity to perform a mitzva is, itself, indicative of a siman kelalah.
Placement of gevurot geshamim in the blessing of mehayye ha-metim is understood by the Ritva as signifying the revival that the rainy season offers the land after a dry summer. Moreover it is a reminder to us of God’s power and His ability to change the reality of the world based on His establishment of a natural cycle.