Ionized acid rain is a glowing red liquid (presumably acidic) that falls from clouds on Cybertron and some other planets. Unlike Earth's acid rain, this type of acid rain comes from space, and is usually blocked by Cybertron's upper atmosphere. 
If acid rain comes in contact with a Transformer's circuits, it will severely disable their functions and power flow, rendering them weak and barely able to move. Prolonged exposure can lead to death. However, if a Transformer can get to safety, standard self-repair protocols can clear up the ill effects in short order.
The Transformers cartoon
On Megatron's orders, a trio of Rainmakers seeded the red clouds over Cybertron, disabling Bumblebee, Trailbreaker and Bluestreak. Chip Chase harangued them until Trailbreaker remembered that having a force field means they don't actually have to just lie there and die, allowing Bluestreak to fire special missiles that dispersed the clouds.
The nature of the red clouds is unknown.
Since the Cybertronian "acid rain" seems to affect Transformers but not humans, it seems more likely that it's some sort of electromagnetically-charged rain that's causing the problems.
- ↑ "...the energies released had ripped a hole in the atmosphere, leaving the area to be torn and shredded by acid rain from outer space,..." - The Magnificent Six telling the origin of the Acid Wastes.
| This page uses content from Teletraan I: The Transformers Wiki. The original article was at Acid rain.