Objects weigh less in air than in a vacuum, because they displace air, and the buoyant force lifts them upward. If there is air present, the buoyant force will reduce the scale reading by an amount equal to the volume of air displaced. This would be an important consideration when, say, one makes a precision weight measurement of a very low-density material (such as a sample of foam, or an aerogel).
Work Step by Step
This is discussed on page 271.