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Credit: E. Wiebe

This picture illustrates two common causes of fog. The distant patch that rises higher in the atmosphere is caused by air rising over a pool of relatively warmer water. Standing anywhere on the shore of the lake makes this really clear as you can watch the air containing whisps of fog streaming (slowly) toward the centre of the lake. This is commonly called steam fog.

The second type of fog is visible in sheets over the flat areas surrounding the lake. This fog occurs because, on clear nights, the ground and the layer of air immediately above it can cool below the dew point simply by radiating infrared light out toward space. The atmosphere becomes saturated with water vapour as it cools and a cloud forms but only extends upward for a few metres at the most. This is usually called radiation fog. Both of these fogs require relatively still air to be created. Moving air will mix the colder super saturated pockets away.

In Victoria at this time of year one can sometimes find extremely dense clouds of radiation fog standing above the local soccer fields in the early morning. The soccer fields are typically wet (they are watered) and surrounded by trees and houses which act to trap the cooling air in a blob* over the field. As the sun rises its radiation warms the ground. This has the dual effect of raising the dew point in the air immediately above the surface and causing breezes to begin to mix the air. The fog soon disappears. Sometimes people say that the sun has "burned it off". The steam fog disappears when the air temperature rises above that of the water body or when a strong enough breeze develops and mixes away the supersaturated air too quickly to see much fog.

*Blob is not really a meteorological term.

UVic / SEOS / Climate Group / About Front Page Picture / Morning fogs in Victoria, BC Last updated: Tuesday, 04-May-2010 11:15:33 PDT
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