Spotted large white weather balloon at airplane height

My husband and I spotted what looked like one large white weather balloon about 10,000-30,000 ft (airplane height) moving very slowly in sky at the speed of a slow cloud. Moving southeast. Object did not appear to emit light but reflected sun - was initially yellow orange from sunset, then white after sunset, then disappeared as it grew dark. Observed with naked eye and binoculars. Object did not photograph well with cell phone.