  We report the rate of cosmic ray air showers with multiplicities exceeding 15
muon tracks recorded in the NOvA Far Detector between May 2016 and May 2018.
The detector is located on the surface under an overburden of 3.6 meters water
equivalent. We observe a seasonal dependence in the rate of multiple-muon
showers, which varies in magnitude with multiplicity and zenith angle. During
this period, the effective atmospheric temperature and surface pressure ranged
between 210 K to 230 K and 940mbar to 990mbar, respectively; the shower rates
are anti-correlated with the variation in the effective temperature. The
variations are about 30% larger for the highest multiplicities than the lowest
multiplicities and 20% larger for showers near the horizon than vertical
showers.
