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Even before my eyelids rose to greet the dawn, I knew fog had settled over the city. The repeated moan of the Victoria Harbour foghorn had been my distant town crier, spreading the news over the sleepy city. When I first looked out my window, the fog was not yet thick around me. But within the hour, just after the first muted daylight stole away the night, it closed around the neighbourhood, and trees and buildings only tens of metres distant were swallowed by the fog's embrace.
Fog comes in many flavours, depending on the prevailing large scale weather regime, location and topography. The one this morning would be classified as a radiation fog, a quite common variety across Canada and the USA, particularly in the Autumn season. Its cousins frontal fog and members of the advection fog family also find the year's last quarter conducive for a visit, but they will be left for other discussions. Today, my interest lies under a radiation fog. A light radiation fog forms over a small lake Fog is actually a cloud formed or lying on the ground -- even those patchy fogs which fill low spots or hollows in the terrain. Fogs form when the air becomes saturated (with its relative humidity at 100 percent), and the water vapour begins to condense on small particles in the air to form liquid cloud droplets. Particles of sea salt make excellent condensation nuclei as do smoke particles, soil and dust; many air pollutants and some naturally occurring chemicals emitted by trees and other plants also make good condensation nuclei. Air laden with the proper condensation nuclei can even form fog or clouds at a relative humidity just below 100 percent. Very clean air, on the other hand, may need supersaturation (relative humidity greater than 100%) before substantial condensation can begin. The air becomes saturated either by lower its temperature to its condensation point or by adding water vapour into the air. [For more on water vapour in the air, see my Science of the Sky article: Laying Some Groundwork: Humidity.] Cloud droplets that form fog are very small, only around 1 to 20 micrometres in diameter, and thus fall to the ground very, very slowly. This allows fog and clouds to seemingly hang suspended in the air. Warm the air slightly or mix it with drier air, however, and liquid water returns to the vapour state. Thus, cloud/fog edges are tenuous and ever changing. Radiation fog forms when the air near the surface cools to its saturation temperature due to nocturnal
The copyright of the article The Fog Rises in Meteorology is owned by Keith C. Heidorn. Permission to republish The Fog Rises in print or online must be granted by the author in writing.
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