So which is it: fog or low clouds?
Late in the day, a dense ribbon of cloud creeps into the Tehachapi Valley from the canyon to the west. The silent vapor bank moves slowly, enveloping the land and buildings in an opaque curtain of ash-colored mist as it glides eastward. . . Familiar surroundings become difficult to see, with even big trees and power poles looming fainting, barely visible as though a gray night had arrived.
At that same moment in other parts of our valleys, sometimes even on nearby Highline Road, the air is clear and visibility is still several miles. So you find yourself wondering “Is that fog, or just low clouds?”
That’s not necessarily an easy question to answer, but there are some things to keep in mind. To begin with, there is no difference between fog and clouds except for their altitude. If you happen to be in the midst of either of them, you can’t really tell them apart.
The official definition of fog is “visible moisture that starts below 50 feet in altitude.” Clouds are defined as “visible moisture that begins at or above 50 feet in altitude.”
So that would seem straightforward, except in mountainous terrain like ours, where clouds sometimes run aground on the elevated landscape like sailing schooners brushing the tops of a deep sandbar at low tide.
In Tehachapi, what often happens is that low clouds drift in from the west, from over the San Joaquin Valley. These clouds may be hundreds or even thousands of feet above ground at the valley floor, but when these clouds reach Tehachapi’s elevation of 4,000 feet and higher, the leading edge of these clouds may be at ground level, and residents experience these clouds as fog.
When it comes to actual fog,
there are two main common types: radiation fog and advection fog. Radiation fog is also known as ground fog or valley fog, and in the San Joaquin Valley, often referred to as tule fog.
Radiation fog generally forms at night as the Earth’s surface gets colder, and the air just above the ground cools as a result. If the air is moist, which is especially typical in damp and marshy areas like those able to sustain tules (bulrushes), the cooling may cause the air to reach its saturation point and ground fog, which is often very dense, will form as a result.
In years past, this radiation fog or tule fog would lead to two-hour school delays around Bakersfield, because it can be very hazardous to run school buses or have families dropping off their students in the poor visibility conditions. The delay was to allow time for the tule fog to burn off, or lift up and then vanish as the morning sun warmed the Earth’s surface.
I can remember hearing on the radio or on television news reports of Bakersfield-area school delays that went on day after day, with maybe a brief break of a few days, and then many more days of morning fog delays. These delays still happen, but they aren’t nearly as common as they used to be.
The other main type of common fog is advection fog, and this occurs when warm moist air moves horizontally over a cooler surface, and the air is cooled to below its dew-point temperature, resulting in fog. This often happens along the West Coast, and advection fog is the type most often experienced by coastal communities.
Most of the dense vapor we get in the Tehachapi Mountains and experience as fog, when we’re wandering around in it, or trying to navigate a car on streets whose lane lines are barely visible, is actually low clouds that have failed to clear the summit. Though if they are low streamers of cloud edges, lower than 50 feet, these conditions do meet the official definition of fog.
I love a foggy day, when the landscape fades into the mist, reappears, and then fades again into the pale haze, like distant childhood memories.
At those times I often think of a verse from a poem by Chia Tao, a Zen Buddhist monk who lived in China more than 1,200 years ago: “I asked the boy beneath the pines. He said: the Master’s gone alone. Herb picking somewhere on the mount. Cloud-hidden, whereabouts unknown. . .”
Have a good week.