You've probably heard the weather geek speak: it's an inversion. What does THAT mean? It's when the air gets WARMER as you go up. Which is the opposite, or INVERSE (get it?) of the normal temperature profile of the atmosphere. Here's an example:
During the cool seasons, the low sun angle prevents the sun's rays from warming the ground enough to warm the air in the valleys. Even when a warm air mass moves in. The warming is realized aloft, but cooling overnight and the almost inevitable valley fog formation means cold air pools in the valleys. The atmosphere is quite comfortable with this arrangement, because cold air, being denser, has no motivation to try to displace the less dense warm air above it.
Think of a bottle of oil and vinegar salad dressing after is sits for a while. You get layers (in meteorology we call this stable stratification), with the lighter liquid above the heavier one. I can never remember which is which, oil over vinegar or vice-versa. But you get the picture. This is a stable situation and that's why atmospheric physics leave it alone, until something comes along to mix it up.
In and west of the Cascades, I've seen inversions so strong that some places in the valleys are stuck in the 40s with fog or low clouds while the Cascade foothills and Coast Range bask in 60-degree temperatures.
We should get a few cases similar to that Wednesday or Thursday this week.
KGW Chief Meteorologist