i found this old latin rain spell a few years back in amongst my great grannys stuff. it works for me so give it a try-
elementum recolligo huic locus. Commodo mihi vestri vox. elementum unda ego dico vos. permissum pluit. is est meus nos sic vadum is exsisto.
roughly translated it means-
elements gather in this place. lend me your power. element water, i call you. let it rain. so mote it be.
let me know what you think and if it works for you.