Pogonip: a rare weather condition comes calling on cold nights

This winter and spring we've had the good fortune to have an average amount of precipitation, about 11.5 inches worth of rain and snow. And though we have had snow, it was a pretty mild winter without exceptionally low temperatures, so we had almost no examples of an unusual weather condition called pogonip.

The word pogonip is derived from a Shoshoni word for this phenomena, and it refers to what is basically frozen fog that forms on exposed surfaces during very cold, windy nights. The result is tree branches, fences, shrubs and other surfaces covered with a coating of white ice, even though it didn't snow.

Many people use the term "hoarfrost" or "glaze ice" to refer to this transformative spectacle, but there are a few differences in true pogonip that are worth noting.

For example, pogonip in our area is always accompanied by strong winds, so the ice coating is much heavier on the windward side of trees and other objects, and may be absent entirely on the leeward side. On mountaintops where the winds tend to swirl more, the coating may be more uniform, but it is most areas it is decidedly one-sided. Because our cold winds blow predominantly from the west, pogonip generally appears heaviest on the west side of objects.

Hoarfrost, on the other hand, can occur on totally clear, windless nights and present an even coating on exposed surfaces.

Another indicating factor of pogonip is the presence of visible water vapor in the form of clouds or fog. Generally, pogonip in our area occurs in the upper mountains on frigid nights when the west wind blows supercooled clouds through the conifers, depositing layer after layer of ice crystals on the pine needles and fir branches.

Pogonip results from a process scientists call sublimation. As water moves through its three stages, the typical sequence is vapor-liquid-solid, as it freezes, or solid-liquid-vapor as it melts and evaporates.

In the case of sublimation, however, the liquid phase is skipped and water moves directly from a vapor form (fog) to a solid form (ice). As the wind continues to push the water vapor in clouds or fog against subfreezing exposed objects, layer after layer of ice builds up.

Jon Hammond.

A big Jeffrey pine and an oak by Tehachapi Mountain Park are coated with pogonip after a cold night, but it clearly didn't snow – look at the bare ground beneath the trees.

And the next morning when temperatures rise even slightly, this "frozen fog" rains down in small chunks of flattened ice. This spectacle is little-noticed in the sparsely-habited mountaintops, but once in a great while, on an exceptionally cold and windy night, pogonip will occur on the floor of local valleys and even in town.

This happened one memorable Sunday night on January 9, 2011, when pogonip coated trees, power lines and other objects with a thick layer of frozen fog that then rained down on Monday morning like a strange shower of rectangular-shaped hailstones or ice chips.

By dawn on January 10, many leafless trees had turned ghostly, encased in a shimmering layer of ice. A few hours later, gusting breezes and slightly warming temperatures had caused these trees to shed their frozen coating and the ground beneath them was littered with strange white wafers of ice. Monday's steady patter of falling ice sounded like an unseasonal hailstorm as the pieces pummeled rooftops and cars.

But it was over quickly: by noon no trees remained coated, and there were only white skirts of ice particles beneath trees to give a hint of the earlier winter spectacle.

There are usually examples of pogonip in the mountains two to four times a year, often up near the tops of Bear Mountain and Tehachapi Peak, but incidents like the one on January 09, 2011, in the town of Tehachapi only happen once a decade, and I've never seen more ice than we had on that Monday morning.

Even more unusually, our pogonip episode that cold night was caused by low-level fog and the mountains were unaffected – it was entirely a valley event. Even the higher locations in West Golden Hills, up on Country Club Drive and the surrounding streets had no pogonip. Residents living there were above the fog and consequently had no pogonip.

This past winter wasn't cold enough to bring much pogonip, even in the mountains, but it will return again on some very cold and windy night. . . .

Keep enjoying the beauty of life in the Tehachapi Mountains.

Jon Hammond is a fourth generation Kern County resident who has photographed and written about the Tehachapi Mountains for 38 years. He lives on a farm his family started in 1921, and is a speaker of Nuwä, the Tehachapi Indian language. He can be reached at [email protected].