One of the things I love about Jewish practice is how it attunes us to the seasons and through that, the flow of life. Of course you have to realize that the seasons of record are those of the Middle East, but that works well for us Californians.....
Sukkot is the fall harvest festival and one of the three Jewish pilgrimage festivals--the other two being Pesach and Shavuot. At these times, all who could traveled to Jerusalem for the celebrations. On the eighth day of Sukkot, Shemini Atzeret--the last day of the festival, the service includes the Prayer for Rain, T'fillat Geshem. From that time until Pesach we add the line "Mashiv Haruah Umorid HaGashem" in the Amidah. It's a time we acknowledge that Adonai, the Transcendent, is the one who "causes the wind to blow and the rain to fall." It is, after all, the rainy season and if there is no rain then there will likely be no rain that year and the crops will be lost.
However, we don't add the request for rain that appears in the Amidah within the blessing for abundance until December 4th (December 5th in a Hebrew year divisible by four--for reasons too complicated for me to explain, you can read about it here).
After Shemini Azteret, it was time for the pilgrims to travel home. Although the rain is needed to start the growing cycle once again, it would be nice if it held off until most travelers were off the road. In the Mishna, it was Rabbi Gamliel's opinion that you wait "fifteen days after the festival [of Sukkot] so that the last Jew [returning home from the festival] could reach the River Euphrates". Outside of Israel, the request for rain is added to the Amidah 60 days after the fall holiday season, not 15. This is from the Talmud, where Rabbi Chananiah said, "In the Diaspora [we do not begin to pray] until the sixtieth day after the [Tishrei] cycle." His reason was that the need for rain is not yet urgent enough to officially request it. (I got the Mishna and Talmud info here at askmoses.com - open for your questions 24/6 :)
Whether it's to symbolize the safe return of the pilgrims or the waiting for urgent need, the time separation between the addition of the two very similar prayers reminds me that seasons are about process, not quick change. Our lives are a series of seasons. We don't change from one day to the next. But as we walk on a path, as we arrive at different points where needs are greater than before, we need to make the room to add the blessings that will give us the strengths that we need.