  We revisit a method for determining the neutrino mass ordering by using
precision measurements of the atmospheric $\Delta m^2$'s in both electron
neutrino and muon neutrino disappearance channels, proposed by the authors in
2005 (hep-ph/0503283). The mass ordering is a very important outstanding
question for our understanding of the elusive neutrino and determination of the
mass ordering has consequences for other neutrino experiments. The JUNO reactor
experiment will start data taking this year, and the precision of the
atmospheric $\Delta m^2$'s from electron anti-neutrino measurements will
improve by a factor of three from Daya Bay's 2.4 % to 0.8 % within a year. This
measurement, when combined with the atmospheric $\Delta m^2$'s measurements
from T2K and NOvA for muon neutrino disappearance, will contribute
substantially to the $\Delta \chi^2$ between the two remaining neutrino mass
orderings. In this paper we derive a mass ordering sum rule that can be used to
address the possibility that JUNO's atmospheric $\Delta m^2$'s measurement,
when combined with other experiments in particular T2K and NOvA, can determine
the neutrino mass ordering at the 3 $\sigma$ confidence level within one year
of operation. For a confidence level of 5 $\sigma$ in a single experiment we
will have to wait until the middle of the next decade when the DUNE experiment
is operating.
