  In December 2010, a major storm erupted in Saturn's northern hemisphere near
37 degree planetographic latitude. This rather surprising event, occurring at
an unexpected latitude and time, is the sixth "Great White Spot" (GWS) storm
observed over the last century and a half. Such GWS events are planetary-scale
atmospheric phenomena that dramatically change the typically bland appearance
of the planet. Occurring while the Cassini mission was on-orbit at Saturn, the
Great Storm of 2010-2011 was well-suited for intense scrutiny by the suite of
sophisticated instruments onboard the Cassini spacecraft as well by modern
instrumentation on ground-based telescopes and onboard the Hubble Space
Telescope. This GWS erupted on December 5th and generated a major dynamical
disturbance that affected the whole latitude band from 25 deg to 48 deg N.
Lightning events were prominent and detected as outbursts and flashes at both
optical and radio wavelengths. The activity of the head ceased after about
seven months, leaving the cloud structure and ambient winds perturbed. The tops
of the optically dense clouds of the storm's head reached the 300 mbar altitude
level where a mixture of ices was detected. The energetics of the frequency and
power of lightning, as well as the estimated power generated by the latent heat
released in the water-based convection, both indicate that the power released
for the storm was a significant fraction of Saturn's total radiated power. The
effects of the storm propagated into the stratosphere forming two warm
airmasses at the 0.5-5 mbar pressure level altitude. Related to the
stratospheric disturbance, hydrocarbon composition excesses were found. The
decades-long interval between storms is probably related to the insolation
cycle and the long radiative time constant of Saturn's atmosphere, and several
theories for temporarily storing energy have been proposed.
