(~27 versus ~30 years). The high-latitude GWSs tend to be much less prominent than their equatorial counterparts. Based on these apparent regularities, in 1992 Kidger forecasted (incorrectly, given the 2010-11 storm) that the next GWS would occur in the North Temperate Zone in 2016, and would probably be less spectacular than the 1990 GWS. Characteristics and causes The Great White Spot typically begins as discrete "spots", but then rapidly expands in longitude, as the 1933 and 1990 GWSs did; in fact, the latter eventually lengthened enough to encircle the planet. Though computer modeling had by the early 1990s suggested these massive atmospheric upwellings were caused by thermal instability, in 2015 two Caltech planetary scientists proposed a more detailed mechanism. The theory is that as Saturn's upper atmosphere undergoes seasonal cooling, it first gets less dense as the heavier water rains out, passes a density minimum, and then gets more dense as the remaining hydrogen and helium continue to cool. Low-density upper-layer gases tend to suppress convection, but high-density upper layers are unstable and cause a thunderstorm when they break into lower layers. The theory is that the storms are significantly delayed from the winter solstice due to the time it takes for the very large atmosphere to cool. The team proposes that similar storms are not seen on Jupiter because that planet has less water vapor in its upper atmosphere. Saturn's rings block the view of the northern hemisphere from Earth during the winter solstice, so historical data on the GWS is unavailable during this season, but the Cassini space probe has been able to observe the whole planet since it arrived shortly after the winter solstice in 2004. See also Great Dark Spot Great Red Spot Dragon Storm Extraterrestrial cyclone Kármán vortex street References Notes Article on Saturn's Northern Electrostatic Disturbance on Sky and Telescope 1990/1 Hubble Space Telescope image 2006: observed with a 12" telescope by amateurs near Paris. Volunteers Help NASA Track Return of the Dragon External links The Great White Spot at ESA/Hubble Christopher Go's Saturn Website Pictures of Saturn's Northern Electrostatic Disturbance (2011) Saturn Planetary spots Storms 1876 in science