“Transparency” is the architectural watchword of the era, almost guaranteed to appear in any reference to contemporary glass-clad towers.

But real buildings usually look a lot different than the clear plastic models and crystalline renderings shown at news conferences and ribbon cuttings.

Once in a while, however, when the light and the angles are just right, a skyscraper can come close to vanishing.

That happened last Thursday in New York, when the 52 floors of 7 World Trade Center faded into the cloud-flecked blue of a late summer afternoon. Its masonry neighbors (140 West Street and 90 West Street) stood out in contrast.