Events began at 2105 GMT on December 5, when ground-based telescopes detected a "barely visible white point" on a normally unblemished and hazy part of Saturn's northern hemisphere, at around latitude 35 degrees north.
At the same time, Cassini turned its "ears" towards the target, listening in to radio emissions from the storm via an onboard plasma-wave instrument.
These signals are the telltales of lightning. Lightning cannot be seen visually on Saturn at night because of interference from sunlight scattered from the planet's ring system, which comprises billions of shiny particles.
Within a few weeks, the point had ballooned into a storm system that was some 10,000 kilometers (6,000 miles) across, roughly comparable with the diameter of the Earth, and after two months the clouds had almost encircled the entire planet.
Analysis of the data suggests that the "spot" is in fact a cluster of super-storms, produced by upwelling of heat, moisture and ammonia from water clouds from lower down in the Saturnian atmosphere, where the pressure is high.
As this mix rises into a cooler atmospheric layer called the tropopause, bright, white clouds of ammonia start to spread horizontally into a tail, sculpted by eastward jets of wind.
Every 29 years or so as Saturn completes its journey around the sun, the planet's summer solstice is a time of violent storms. This time around the storms are particularly fierce, and they're giving astronomers quite a show.
Very cool.
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