evanescent

evanescent \ev-uh-NES-unt\, adjective:

Liable to vanish or pass away like vapor; fleeting.

The Pen which gives. . . permanence to the evanescent thought of a moment.
-- Horace Smith, Tin Trumpet

Every tornado is a little different, and they are all capricious, evanescent and hard to get a fix on.
-- "Oklahoma Tornado Offers Hints of How a Killer Storm Is Born", New York Times, May 11, 1999

The accidentally famous. . . may write books, appear on talk shows, and, in so doing, attract even greater public attention. This type of celebrity status, of course, is brittle and evanescent.
-- Lawrence M. Friedman, The Horizontal Society

Evanescent is from Latin evanescere, "to vanish," from e-, "from, out of" + vanescere, "to disappear," from vanus, "empty."