Typically when one thinks about a high pressure “ridge” moving into the forecast, you think to head outside and enjoy the sun. That’s because High Pressure promotes sinking air which limits the form of cloud cover. With that said, why have we been fogging up and not seeing much sun in the valley lately?
If you’re a city bug, I’m sure you noticed all the fog formation we have had over the last week. That morning fog typically burns off, or as meteorologist like to say, “mix out.” But there have been several days where that fog doesn’t go anywhere. Meanwhile the coast and the Cascades haven’t seen a cloudy day since the last week of November. There are a couple of reasons for this which I will get into.
The first reason is from what I stated earlier. High pressure promotes “sinking air.” Cold air is more dense, or heavier, than warmer air which is why it’s been so bloody cold in the valley. All that cold air is “sinking” due to the higher pressure and it has no where else to go except the bottom of the valley. It’s sliding down the mountains of the Coast Range and Cascades and pooling up right here in the Eugene, Corvallis, Roseburg, and others along the way. What we need is a little bit of wind to dry it up or a storm to blow it out of here. But that hasn’t happened.
Another reason for the fog is due to the season. During the winter season, we have a very low sun angle. That sun doesn’t get much time to warm us up since daylight hours are so limited. And since it is at such a low angle, we don’t get the direct rays (seen in summer months) that could warm us up to create air movement. These reasons combined are why valley areas tend to see clouds during a “high pressure” time and other locations see sun.
I’d like to take a moment to make sure you all know that these situations make forecasting extremely difficult. It’s hard to calculate how much wind you think will be in the area during this weather trend. The wind is the perfect mechanism needed to dry out that fog and if you get a little bit of it, you can have it burn off by the afternoon. Sometimes models indicate a 5-10 mph wind coming in (which would limit fog growth) but then that wind never comes. So when a winter high pressure ridge builds in I say, “Alright Mother Nature lets see who can out-forecast who.”
Mother Nature definitely has her moments of success. I’m just saying…
