What are the chances for Storm Nimbus's effects?

What are the chances for Storm Nimbus's effects?

It's been said in a Steam Discussion:

"X% chance of one of the following" in this game means equal chance of the following which add up to X%. For example if it says "30% chance of one of the following" and there are 3 lines below it, the actual chance would be 10% for each line. If there is no "X%", then X=100%.

So, each time I attack with anything, there's a 100% chance of some effect, and a 50% chance of it being Ice or Lightning Damage added to my attack? Since there's two effects and at least one of them must happen?

While we cannot answer the question definitively without a developer reply, or diving into the actual code, you are correct in assuming that there is always a 100% chance of some effect.

I think it's safe to assume that either effect will have an equal chance of appearing, as that seems to be the consensus among the player-base. Either sub-effect will have a 50/50 chance of appearing.