  In the past years, large particle-physics experiments have shown that muon
rate variations detected in underground laboratories are sensitive to regional,
middle-atmosphere temperature variations. Potential applications include
tracking short-term atmosphere dynamics, such as Sudden Stratospheric Warmings.
We report here that such sensitivity is not only limited to large surface
detectors under high-opacity conditions. We use a portable muon detector
conceived for muon tomography for geophysical applications and we study muon
rate variations observed over one year of measurements at the Mont Terri
Underground Rock Laboratory, Switzerland (opacity of ~700 meter water
equivalent). We observe a direct correlation between middle-atmosphere seasonal
temperature variations and muon rate. Muon rate variations are also sensitive
to the abnormal atmosphere heating in January-February 2017, associated to a
Sudden Stratospheric Warming. Estimates of the effective temperature
coefficient for our particular case agree with theoretical models and with
those calculated from large neutrino experiments under comparable conditions.
Thus, portable muon detectors may be useful to 1) study seasonal and short-term
middle atmosphere dynamics, especially in locations where data is lacking such
as mid-latitudes; and 2) improve the calibration of the effective temperature
coefficient for different opacity conditions. Furthermore, we highlight the
importance of assessing the impact of temperature on muon rate variations when
considering geophysical applications. Depending on latitude and opacity
conditions, this effect may be large enough to hide subsurface density
variations due to changes in groundwater content, and should therefore be
removed from the time-series.
