25th December 2005

Upslope Fog

Source: Excerpt from The Book " Weather "

Distribution: Worldwide, most common on hills and mountains near the sea. Height: 0 to 1000 feet AGL Cause: Gentle lifting of moist air, followed by condensation. Associated Weather: Drizzle or light snow. Hazard Warning: Restricted visibility. Upslope fog occurs when moist air is blown up a hillside or mountainside to a level where condensation occurs. The differences between this fog and Orographic Stratus are minor. Generally, stratus results from a significant wind, whereas the air currents that produce upslope fog are weak -- indeed, someone standing within the fog may be unaware of any air movement. Orographic stratus is more likely to form near the top of a peak, or just above it, whereas upslope fog usually begins farther down the mountain and covers a wider area.

Upslope fog is common in all mountain ranges. Good examples occur in North America during the winter months, when cold air from low-pressure systems drifts slowly westward in the wake of a cold front. When it encounters the eastern slopes of the Rocky Mountains, the air rises and condensation occurs, causing extensive areas of upslope fog, which may run for hundreds of miles in a north-south direction. A similar process takes place in eastern Australia, when maritime air from the Tasman Sea slides inland and is lifted by the gentle slopes of the Great Dividing Range, creating extensive fog on the eastern side of these mountains.

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Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.