Icy fog is an occasional visitor

It's a fact of nature that the ability of air to contain, or "hold," water in the gaseous state is dependent upon temperature. When the atmosphere is cooled, that capacity is reduced. Air is said to be saturated when it can't "carry" any more water vapor. This is the process by which clouds are born.

Normally, clouds form at some height above the ground - but not always. Clouds that like to hang out on the ground we call fog. And this form of condensation imposes a whole set of challenges to safe transportation because of reduced visibility.

Fog is a frequent visitor to the coasts of Oregon and Washington, not only during the winter but sometimes the summer months, too. Persistent northerly winds during the summer tend to promote strong upwelling just off the coast. When warm, moist air moves over this colder water, surface fog can often result. Then, if interior regions of the two states heats up, the fog and low clouds are sucked onto the coastal areas that sometimes can penetrate inland for several miles.

Mid-to-late autumn is the ideal season for the formation of fog in the western valleys. Before the progression of winter storms normally kicks in, high pressure tends to settle in over the Pacific Northwest, producing sunny days but cool, clear and longer nights. As the ground radiates heat into space, it chills the air above it. If the cooling is sufficient, fog will form before sunrise the next morning.

Potentially the most impactful fog is the subfreezing variety that sometimes fills the Columbia Basin of eastern Washington and north-central Oregon during late fall/early winter. It takes a special set of atmospheric conditions to produce it, and it can be hard to get rid of once it forms; hanging around for up to two weeks in some cases.

The primary ingredient in concocting this foggy brew is the development of a strong ridge aloft along the western U.S. that serves two purposes. First, it causes the jet stream that carries storms across the Pacific Ocean to be diverted far to the north and east, thus bypassing the Northwest. This results in mostly clear skies, at least initially, allowing ground heat to be radiated into space during the long nights.

The second consequence of the ridge is to produce a strong inversion that suppresses convection, the vertical motion of air. So the cold air that's being chilled by the ground stays put, capped by the warmer air aloft.

Eventually, if the air isn't too dry, it will cool sufficiently for condensation to occur. As the fog overspreads the basin and thickens, the weak daytime sun is insufficient to burn it off. If the temperature is at freezing or below, rime ice will begin to collect on exposed surfaces.

Several high-voltage electrical transmission lines and even more lower-voltage utility lines in the Columbia River Basin are susceptible to this type of ice damage. When ice accretion becomes great enough, towers buckle or collapse and current-carrying wires break and fall to the ground, interrupting electrical service.

A related, but less catastrophic, phenomenon called "red-belt" sometimes occurs when the freezing fog moves into Central Oregon.

Evergreen trees that are on a hill that pokes above the fog layer are vulnerable.

The top of the fog deck acts like a mirror in reflecting the low angle sunlight onto the needles of south-facing trees just above the fog.

As this weather condition persists, the concentrated energy from the sun dries out the needles, destroying their cell structure.

The damage isn't evident until several months later when temperatures warm and the sap starts flowing again.

A narrow reddish band of dead needles is left behind on the trees slightly above where the top of the fog had been.

The trees are not permanently affected and recover in a year or two.

Local naturalist Jim Anderson has studied occurrences of red-belting on Black Butte, and elsewhere. His article on the subject is found in The Nugget Newspaper, March 11, 2009.

The last time power lines were damaged by rime ice in the Columbia Basin of central Washington was in January of 2009 when a very strong bubble of warm air settled over the region, trapping freezing fog there for a period of about 10 days. That was also the last time red-belt has been seen on Black Butte.