Seasonal Variations of multiple-muon events with the NOvA Near Detector

Due to Temperature variations in the upper atmosphere, underground detectors measure more cosmic ray muons in the summer than the winter.~ MINOS unexpectedly measured the opposite situation for multiple-muon events. The underground NOvA Near Detector has confirmed the MINOS observation and studied the effect as a function of zenith angle, muon separation and multiplicity.~ Ideas to understand this effect will be presented.