Who will not have rain?
Radak: A land whose residents will not come to Yerushalayim [on Sukos], that year they will not have rain.
Malbim: Any family that will not ascend. A nation that will not be found there will not get rain of Brachah 1 .
Surely if no families of the nation ascend, they will not get rain! Perhaps Malbim adds that if the majority of a nation will not ascend, the families that ascend will get rain, but not of Brachah. (PF)
Why will they not have rain?
Rashi, based on Tosefta Sukah 3:7: The Mitzvos of Sukah are for rain - the four Minim, and Nisuch ha'Mayim [so we will be blessed with rain]. Sukos is the time of [judgment] for rain of the year. Those who doubt it will not have rain. Malbim - the influence of rain will be from Yerushalayim.
