“Frost in low lying areas.” It’s not an uncommon forecast here from mid-August on, though thankfully it’s  not in the immediate forecast. But why low-lying areas? Doesn’t it get colder as you go higher? Don’t some mountains have snow on top all year round?

Well, yes – but we’re talking about two different processes. A great deal depends on whether the air is heated or cooled from below, where it is in contact with the ground.

Cold air is denser than warm air at the same pressure. If cold air and warm air are side by side at the same pressure, the cold air will flow under the warm air, just as water flows under oil. This kind of structure, with cold air under warm air, is called an inversion. It is very stable, which means very resistant to vertical mixing. This means that any pollutants put in the air cannot mix very far vertically, so inversions are closely associated with air pollution.

At night (effectively all winter here in Alaska) the ground radiates more energy to space than it gets from the sun, and actually cools the overlying air. This cooler air tends to run downhill if there is any slope to the ground, and pools in the lowest parts of the landscape. If it’s very close to freezing, the lowest areas may wind up with frost.

If the sun is up and heating the ground, however, a different process takes over. The warm ground will heat the air. We now have warm air lying under cooler air, much like oil lying under water. The warm air will rise. At it rises, it moves into lower pressure and expands. The energy for the expansion has to come from the heat stored in the air, so the air cools as it rises.

We know exactly how fast dry air cools as it rises: 10 ° Celsius for each kilometer of rise, or 5.5°F for each 1000 feet. This is called the adiabatic lapse rate. With enough heating at the bottom the air will rise, cooling at the adiabatic rate as it does. So in the daytime the air normally cools with height, and mountaintops are cooler than valley bottoms.

Inversions can also form when warm air rides over colder air. This often happens when high-altitude air is sinking and warming, which is common in anticyclones. Thus anticyclones are known for air pollution.

In most latitudes, solar heating of the air will eventually break the inversions caused by nighttime cooling. Not in Alaska in winter! Here solar heating is very slight during the winter, and it is normal for the air to be coldest near the ground. Often the warmest air is a mile or more up. This means we are very prone to air pollution in the winter – astonishingly so, for such a small population. It also means the hilltops are very popular places to live – they’re normally warmer than the valley bottoms. But at this time of year, the main effect of inversions is those pesky frosts in low-lying areas.