Direct Answer
A revision cloud is a curved-line boundary drawn around changed areas on a revised drawing, making it easy to locate what changed between revision levels. When you receive a revised drawing with revision clouds, identify every clouded area, confirm you understand the change, and verify whether any already-fabricated material is affected before proceeding with production.
Full Explanation
Revision clouds are required by most drawing standards whenever a drawing is revised. They appear at each revision level with a triangle or delta callout (Δ1, Δ2) identifying which revision added the cloud. On complex revisions, multiple clouds may appear on a single sheet. The revision block in the title block lists each revision number, date, description, and authorizing initials. Reading the revision description before the drawing helps you understand the intent of the change before you look for the clouds.
What This Means for Your Shop
On fast-track projects, revision clouds help you quickly triage which pieces need rework. If all revision clouds are on connection details that haven't been fabricated yet, you may be able to proceed normally. If a cloud affects an already-cut member, flag it immediately for rework assessment.
Common Mistakes
Ignoring revision clouds because the drawing 'looks the same as before.' Even a small cloud (say, a bolt added to a connection) can mean the difference between an approved and a rejected connection.
Recommended Process
When a revised drawing arrives, read the revision description first. Then locate all revision clouds on the sheet. Compare to previously issued revision to confirm you've found every change. Update your fabrication status for all affected piece marks.