This will spawn an arrow rain spawner 10 blocks above the command block. You can change the ~10 at the beginning to higher or lower depending on where you want it.
Also, change the RequiredPlayerRange from 20 to whatever fits your needs (20 means the rain will only turn off once all players are over 20 blocks away from the spawner, so if you want it to be a trap, you change that to 1 or 2 blocks instead).
Lastly, if you don't want a constant rain, you can change the MinSpawnDelay to a larger number so that it's more of a random rain.
You've probably seen this before, but I had some fun killing unlucky cows with it.