It has long been a mystery why small Total Solar Irradiation (TSI) changes have significant effects on Earth’s climate. Solar cycle correlation studies abound but cannot conclusively point to a viable physical mechanism. Here, I show that geomagnetic storms have a profound terrestrial weather impact. Using 67 years of hourly Disturbance storm time (Dst) index and ERA5 reanalysis atmosphere data over North America, I find geomagnetic storm impacts up to two orders of magnitude larger than the long-term global mean surface temperature impact attributed to solar activity. The effects vary depending on region and season. Comparing with solar cycle studies shows that the solar cycle variations are more likely caused by storm-time solar activity bursts as opposed to slow continuous variations, such as the solar cycle scale TSI variations. As for the physical mechanisms, particle precipitation effects such as from cosmic rays, solar energetic particles, or magnetospheric electrons are least consistent with my results. In particular, the cosmic ray – cloudiness hypothesis is falsified. A top-down mechanism operating directly through the ionosphere or through stratospheric chemistry and the polar vortex appears to be more
likely.