Clearing out the fog in the valley can be a tough task


Fog can be a forecast buster when it comes to sunshine and daytime temperatures

PORTLAND, Ore. (KOIN) – With surface high pressure building in the coming days, we are in store for some sunny afternoons.

However, it also allows for a clear sky at night and fog to form in the valley. How do we push that fog out earlier some days but keep it around until the afternoon others?

Fog can be a forecast buster when it comes to sunshine and daytime temperatures. If that fog hangs around until 2 or 3 p.m., it will inhibit the temperature by 5 to 10 degrees. As you know we live in a valley that can essentially resemble a bathtub. Where we live at the bottom of that tub with what you could imagine as some walls around the outside.

Fog can take much longer to dissipate in a valley because there are locations that are protected from the wind. Fog can also be pretty thick in a valley as it can form down at the surface because the cold dense air sinks to the valley floor and we have water sources with moisture near the bottom of the valley (think all the rivers). An example in the image below of the Willamette Valley.

Example of Valley Fog

What does it take to help mix out that fog? Well, it starts with warming the air, which gradually takes time because we are counting on the sun for that. As the sun chips away and aids in the warming of the surface, it helps attack the fog and will eventually lead to the moisture evaporating. Increasing the sunshine aloft and cranking the heat under the fog should create a rising motion in particles that will eventually mix with drier air aloft. Once that drier air is added to the equation, it will start to attack that fog in the process of evaporation.

The wind is also a component that will obviously allow the mixing of drier air and the decrease of stagnant conditions. What generally gets the wind going? That answer again is the sun. It is that heat generated that will tend to increase the wind. That is why in the morning, coming off of the night sky, we will see this development of fog until we get multiple hours of sunshine above that foggy layer.

The question to answer now is why do we clear out some days by 9 a.m. but refrain from clearing others until the early afternoon? A big portion of that depends on the wind and also how thick the fog was actually allowed to draw. If a thin layer of fog forms at 7 a.m. as the sun is already on its way, that fog will lift much quicker. If that fog developed earlier in the night and that layer is much thicker, it’s going to take more work to get that mixed out. That isn’t always the case, but it is in consideration while forecasting.

Additionally, while forecasting, we look at when the wind is going to start picking up and what direction is it coming from. Generally, if we have wind coming down the mountains it will result in the action of warming and tend to be drier. This will help clear up that fog quicker. When we see the wind that is gentle from the north, it doesn’t tend to do that much and it takes a little more action. Furthermore, around Portland, if it is running from the north, the Columbia River is right there.

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