Xinhua indexes 云 under the 二 radical, picking the two horizontals at the top as the anchor. Originally the entire character was a side-on cloud spiral; in the modern form the top two strokes represent the sky level, with the curl beneath suggesting the rising vapour.
The lower curl preserves the original cloud-spiral shape from oracle bone forms. Stacked beneath the 二 sky-line, the curl pictures vapour rising and turning — the core 'cloud' meaning that the character carried before being borrowed for 'to say'.
