The plot below shows the passive microwave brightness temperature time series over Summit recorded between 1979 and 2011 (the ensemble of gray lines) and the time series recorded during 2012 (black thick line). The ensemble clearly shows that things are relatively stable at Summit, with relatively small interannual variability.
The plot also shows that the event of July 2012 s unique for the satellite era (1979 - to date) and it lasted for a few days. For those who are not expert in passive microwave remote sensing: as the liquid water appears, the recorded signal jumps to high values, suddenly. This kind of change (sudden and strong) can be related only to the appearance of liquid water within the snowpack as other changes in the physical properties of the snowpack would produce either a weaker signal or would have a different timescale. Note: this sensitivity of microwaves to water is the same physical principle through which we are able to cook our food using microwave ovens !