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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.
American cartoon continuity
The Transformers cartoon
On Megatron's orders, a trio of Rainmakers seeded the red clouds over Cybertron, disabling Bumblebee, Ironhide, 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.