AMHERST, Mass. - Under threatening skies and amid chilly temperatures, the atmosphere was festive and upbeat at the 131st Commencement of the University of Massachusetts. About 4,000 undergraduates from 34 states and 21 countries, ranging in age from 16 to 57, received their bachelor''s degrees in nearly 100 majors during this morning''s ceremonies at Warren P. McGuirk Alumni Stadium. About 18,000 family members and friends attended. Chancellor David K. Scott presided over the ceremonies; President William Bulger conferred the degrees.