Fuzziness defined around a point is a very commonly used concept. However in some situations, fuzziness defined around an interval looks more practical. For the Gaussian Plume Model of atmospheric dispersion for example, if certain parameters are assumed to be fuzzy, then the fuzziness in any individual case should actually be defined around an interval, and not around a point. In this article, we would discuss how to construct the membership function of a fuzzy number with unit possibility assigned to an interval.