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Ask the Meteorologist – Fog

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ASK THE METEOROLOGIST

“What causes ground fog?”
Riley Valle
Rogue River

Fog is actually more difficult to forecast than a cold front or a warm front. Fog forms when the air temperature drops to the dewpoint, which is the temperature at which condensation can form. It is easier to form fog with a higher dewpoint which often comes after a storm rolls through dropping rain which moistens the earth. With a high dewpoint fog can form quicker at night and can often last longer through the day. Weather models are terrible at forecasting fog and low clouds, so it really is up to the forecaster.

Fog happens more often in the winter months when there is a low sun angle, when you have longer days with more intense sun the atmosphere mixes more easily which prevents the temperatures to drop to the dewpoint. When fog does form and there is a thick low-cloud layer it is tougher for the sun to penetrate and burn off the fog. This goes for an inversion as well, the stronger the temperatures inversion (warmer air above cooler air) the cool, moist air gets trapped near the surface and tends to last longer. High pressure’s also allow fog to form easily because there are light to calm winds and sinking air; when air sinks overnight it can pool in valley locations and become colder which makes it easier to reach the dewpoint.