This NASA Hubble Space Telescope image of the ringed planet Saturn shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The east-west extent of this storm is equal to the diameter of the Earth (about 12 700 kilometres). Saturn's prevailing winds are shown as a dark 'wedge' that eats into the western (left) side of the bright central cloud. The planet's strongest eastward winds are at the latitude of the wedge. To the north of this arrowhead-shaped feature, the winds decrease so that the storm centre is moving eastward relative to the local flow. The storm's white clouds are ammonia ice crystals that form when an upward flow of warmer gases shoves its way through Saturn's frigid cloud tops to even colder levels.