© Ottenby Fagelstation
Bird watchers walking along the beach on the Baltic island of Öland off Sweden's southeastern coast were puzzled by an unusual natural phenomenon recently when they stumbled across dozens of football-sized balls of ice lying on the shore.

A week before Christmas, Magnus Bladh of the Ottenby bird station, located on Öland's southern cape, was strolling along the beach with a colleague when he saw something he'd never seen before.

"Temperatures were below freezing and there was a light wind, but it was very cold! In the seaweed we noticed at least 200 large ice balls," he said in a report to Swedish meteorological agency SMHI.

"The balls varied in size but the biggest ones were quite large, some larger than a football."

What mystified Bladh was that the balls were resting on the west side of a bed of seaweed, even though the prevailing winds were from the east.

When Bladh and his colleagues later broke open one of the ice balls, they discovered that it consisted of a 2 to 5 centimetre thick shell of ice, which covered a core of soft, wet snow.

According to SMHI, the ice balls likely form when rolls of light snow are blown from the shore into water which is at or just below freezing, but fails to form uniform ice due to strong winds.

The rolls of snow are then tossed about in the chilly waters, where wave action eventually shapes them into balls of ice.

"It's hard to say just how common ice balls are, since we are reliant on witness reports," SMHI spokeswoman Alexandra Ohlsson told The Local.

A review by SMHI of weather conditions on southern Öland in the days leading up to Bladh's ice ball discovery revealed that temperatures in the area were generally below freezing, with snowfall, and winds from the north and northeast averaging 50 kilometres per hour.

According to SMHI, it was possible that rolls of snow near the shore remained soft due to warmth emanating up from the ground, which could have then been blown into the water by the strong winds.

Once formed, the balls likely came back to shore and, rather than floating out to open water, remained there due to a change in sea conditions in the days before the ice balls were discovered. Water levels sank several decimetres between December 17th and December 18th when Bladh and his colleagues found the ice balls lying on the shore.

Beside's Bladh's discovery, SMHI's website only mentions two other reported instances of ice balls being discovered in Sweden since the 1950s.