The National Weather Service of San Diego issued a dense fog advisory on Jan. 3, which was the second time this month and we’re only three days into the new year. Meteorologists say these dense fog conditions form under certain parameters, such as when there is a neutral pressure gradient, or neither strong onshore, nor offshore flow.
Plus, when we have high pressure building aloft, that can also make the marine layer shallower. Especially during the overnight and morning hours, temperature differences between land and sea, often around 10 degrees, can also aid in the formation of fog.
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Inversions are also common, which is when temperature increases with height and can almost act as a cap to trap fog. As the sun comes up in the morning, the land temperature often warms up to or above the ocean water temperature, which can provide mixing in the lower atmosphere and help clear out or mix out dense fog.
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It’s not always as easy at the coast, which is because onshore flow can keep cold air coming in and there won’t be as much heating from the sun.
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