Many local disturbances, weather systems or storms are superimposed on the regular atmospheric circulation patterns just described, in order to adopt the term we use on earth. The best known of these are large, oval shape, high pressure areas on Jupiter and Neptune.
The largest and most famous Jupiter storm is the Great Red Spot, a slowly changing reddish oval in the southern hemisphere; It was 25,000 kilometers long when Voyager arrived in 1979, but had shrunk to 20,000 kilometers by the end of the Galileo mission in 2000.
The giant storm has remained in Jupiter’s atmosphere since astronomers were able to observe it for the first time more than 300 years ago after the invention of the telescope; however, it continued to shrink, leading to speculation that we might see its end in a few decades.
In addition to its longevity, the red spot differs from terrestrial storms in that it is a high pressure area; Such storms are regions of lower pressure on our planet. The counter-clockwise rotation of the red spot lasts six days. In 1930, 3 similar but smaller dis-turbances (almost the size of the Earth) formed on Jupiter. They look like white ovals, and one is clearly visible below and to the right of the Great Red Spot. In 1998, the Galileo spacecraft observed two of these ovals collide & merge into one.
We don’t know what causes the Great Red Spot or White Ovals, but we have an idea how they can last long once they are formed. On Earth, the lifespan of a big ocean hurricane or typhoon is typically a few weeks or even less as it moves across the continents & encounters friction with land. Jupiter has no solid surface to slow down an atmospheric disturbance; In addition, the size of the disturbances gives them stability. We can calculate that on a planet with no solid surface, the lifespan of something as large as the red spot must be measured in centuries, while the lifespan of white ovals must be measured in decades, that is much of what we have observed.
Despite Neptune’s smaller size & different cloud composition, Voyager showed that it had an atmospheric feature similar to Jupiter’s Great Red Spot. The Great Dark Spot of Neptune was nearly 10,000 kilometers long. The giant storms formed on both planets at latitude 20 ° S, had the same shape and took up roughly the same fraction of the planet’s diameter. The Great Dark Spot rotated with a period of 17 days compared to 6 days for the Great Red Spot. However, when the Hubble Space Telescope examined Neptune in the mid-1990s, However, astronomers could not find any trace of the Great Dark Spot in their images.
While many details of the weather on jovian planets are still not understood, It’s clear that If you are a fan of dramatic weather then these worlds are place to see. We study the properties of these atmospheres not only because of what they teach us about the conditions on jovian-planet, but also because we hope that they can help us. understand the climate on earth a little better.