BOISE, Idaho (CBS2) — We are quickly approaching that time of the year when air stagnation becomes an issue for the Treasure Valley. The same process that traps smoke and haze in the valley triggers the formation of morning clouds and fog. Here’s why:
Low sun angle – you’ve probably noticed shorter days and longer shadows as we move toward the winter season. The low sun angle creates less direct sunlight. This in turn means we have less solar radiation warming the ground. So there is less heat available to warm the air. This is the main reason why our temperatures naturally get colder.
Cold air sinks – have you ever opened a freezer only to see the cold air sink to the ground? That’s because cold air is dense. It actually sinks to the lowest spot it can find. So when cold, mountain air sinks into the valley it’s like water going to the bottom of a bowl. Once that happens, the cold air can remain in place until a significant front moves in to stir the pot.
High pressure – air associated with high pressure is very stable. Once a high pressure ridge settles into an area, the air becomes stagnant. The cold air is not stirred. The pollution has no where to go.
This is what we refer to as “an inversion”. This is when the atmosphere becomes “inverted”. In most scenarios, the higher up you go, the colder it gets. But in an inversion, the opposite can happen. The colder air is stuck to the surface and it can actually get warmer in the mountains.
The solution? There is none. This is what happens in every high desert or mountain valley. The only think we can hope for is a big storm that mixes things up. The only problem is that behind most storms you’ll find high pressure. Some of the coldest scenarios occur after the storm has moved through. Once the clear, cold, dense air of high pressure moves in, temperatures get very cold and stay that way. This process happens over and over until the sun angle climbs high enough in the sky to mix things up.
If you get tired of this, relief is found by going to the mountains. Thank goodness we have plenty of those.
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