Normally on Green Mountain you get a wonderful view of Denver.  Often the air is so clear that we can see the terminals of DIA across the Platte valley.  
Not yesterday.
Yesterday, we saw a thick grey smudge across the skyline – pollution held in place by an air inversion.  We all know what pollution is, and we often hear about an air inversion.  But what is an air inversion, really?
Typically, air temperature gets colder the higher up you go, because the thicker atmosphere traps the heat at lower levels.  The warm air eventually rises, mixing the air.
Once in a while, though, the air becomes very stable because the ground is cold too, such as when it is covered with snow.  When this happens, there is no mixing because no warm air is rising.  Then the cold, dense air on the nearby slopes can slip down underneath the cap of relatively warm air above.  Now, the usual cold-on-top-warm-on-bottom layers are reversed in an air inversion.
Air inversions are most common in the winter, when the air is coldest and most dense and snow insulates the ground.  Land features such as mountains, hills and valleys can trap the colder air, and funnel it into low lying areas.  This means that the slopes of hills or mountains are often  warmer than the frigid bottom lands.
Air inversions are common in the South Platte Valley that holds Denver,  but very cold temperatures and deep valleys means they occur more often and for longer in the mountains. North, Middle and South Park, the San Luis, Eagle and many other mountain valleys often have excellent conditions for air inversions.  Fraser has earned the nickname “Icebox of the Nation” because of them, while Maybelle, in the Yampa River Valley, holds the dubious distinction of having the coldest temperature on record in Colorado of -61 o F on February 1, 1985.
To add to the problems, the inverted air can act like a lid on a pot, keeping the cold air trapped inside the valley.  If the cold lower air is humid, we have fog, if it becomes polluted, we have a high pollution day.