"Summer storms" have been observed on Uranus, but astronomers say they're late -- a full 7 years after the planet's closest "seasonal" approach to the sun -- and they've been trying to puzzle out why.

A succession of storms, including two giant tempests, has taken astronomers -- both amateur and professional -- by surprise.

The unusual activity was first spotted by Imke de Pater, a astronomer at the University of California, Berkeley.

She and her colleagues were using a Keck Observatory telescope in Hawaii in August to observe weather patterns on Uranus, something they'd been doing routinely for more than 10 years.

They were surprised to capture signs of eight giant storms, including one that was the brightest ever observed on the mysterious blue world.

"It was just by chance I was observing and noticed these incredible storms that we really had never seen before," says De Pater. "You always have to be looking, I guess, to see things like that -- so this was one of those lucky times."

The researchers said they might have expected such events in 2007, when Uranus experienced it equinox, with the sun shining directly over its equator at the height of its "summer," something that occurs only every 42 years in the planet's long journey around the sun.

However, 7 years on from that point is not when such storms should be happening, they said.

"People had predicted that the maximum in cloud activity would be around 2007 ... and indeed we did see cloud activity," De Pater says. "But it was nothing compared to what we saw in August."

Although happening long after Uranus passed its equinox, one of the recently observed storms, seen on Aug. 5 by the Keck telescope, might have a link to that event, says Larry Sromovsky, a planetary scientist at the University of Wisconsin, Madison.

"The colors and morphology of this cloud complex suggests that the storm may be tied to a vortex in the deeper atmosphere similar to two large cloud complexes seen during the equinox," he says.

Such vortices may be anchored deep in the atmosphere and extend over large vertical distances, as are similar vortices on Jupiter including its Great Red Spot, a storm that has persisted for centuries.

Uranus, around four times the diameter of Earth, has an atmosphere consisting mostly of helium and hydrogen, with just enough methane to create its icy blue color.