La Crosse sees optical effect around sun

LA CROSSE, Wis. - An optical effect of the sun known as a 22-degree circular halo was briefly visible in the La Crosse area this week.

The ring around the sun - seen for at least 10 minutes on Monday morning - is visible only under certain weather conditions, Dan Baumgardt, science and operations officer for the La Crosse office of the National Weather Service, told the La Crosse Tribune. The optical effect occurs when a certain orientation of ice crystals in cirrus clouds refracts light, he said.

Halos are more common in the winter because that's when there's more ice in the thin, wispy clouds that sit about 5 or 6 miles above the ground. Moonlight can also create the effect.

Other optical effects, such as rainbows, are usually seen during warmer times of the year, Baumgardt said.

Sun dogs, another type of halo, are formed the same way as 22-degree circular halos. But the ice crystal orientation doesn't allow a full halo to form, Baumgardt said, and sun dogs instead look like patches of light usually to the left and right of the sun.

Halos don't indicate precipitation but they can occur when an approaching weather system brings the high cirrus clouds.

The halo seen in La Crosse will travel with the sun until the physical conditions change, Baumgardt said.

The 22-degree name refers to the measurement of the sun to the halo.