How cold has it been on Nantucket? Chilly enough to freeze waves.

Last Friday, Jonathan Nimerfroh, a photographer, arrived on the beach and saw an unusual sight: slow-moving waves of slush.

“I just noticed a really bizarre horizon,” said Mr. Nimerfroh, who is also a surfer. “The snow was up to my knees, getting to the water. I saw these crazy half-frozen waves. Usually on a summer day you can hear the waves crashing, but it was absolutely silent. It was like I had earplugs in my ears.”

It is not uncommon for the harbor to freeze, but even a fisherman he spoke with later said he had never seen anything quite like it.

Normally, water freezes at 32 degrees Fahrenheit. But salt in the ocean lowers the freezing temperature — basically by getting in the way of the water molecules — to about 28.4 degrees.