Go outside or look at the sky through window and repeat this spell:
" god of the sky, hear my plea. Make it pouring so today is so boring. Mote it be, Mote it be!"
They have a spell that takes less time than 12 hrs in this site but you would do this one if you want to go outside but want it to rain later.
less longer rain spell:
Repeat this while looking at the clouds:
" Ancient gods and goddesses I envoke thee. Waters from the sky let it be. I command thee now to thee all. Listen to my desire, rain fall!