The interval between values is not interpretable in an ordinal measure. In interval measurement the distance between attributes does have meaning. The interval between values is interpretable. Because of this, it makes sense to compute an average of an interval variable, where it doesn't make sense to do so for ordinal scales. Finally, in ratio measurement there is always an absolute zero that is meaningful.
Weight is a ratio variable.
It's important to recognize that there is a hierarchy implied in the level of measurement idea. At lower levels of measurement, assumptions tend to be less restrictive and data analyses tend to be less sensitive. At each level up the hierarchy, the current level includes all of the qualities of the one below it and adds something new. Purchase the complete printed text of the Knowledge Base online Research Methods Knowledge Base, 2nd edition.