Lately I feel like I'm living in a primitive culture in the Amazon. Every day I wake up and face innumerable life-threatening dangers, forage for food, struggle to survive. That's because I'm living in Utah in the winter. And I have very little money, but that's a different matter. But despite Utah's ferocious landscape, it is my home and I love it. I love the magnificent snowy mountains, despite the daily avalanche warnings that kept me away from physics conferences. I love the view of the mountain from my backyard, even if it means taking a look for cougar and coyote tracks before we let the dog out. I do love the snow, the snow shoeing, the snow throwing, even the snow shoveling, despite the inevitable snow driving. And I love the not-quite-small-town-not-quite-big-city feel of Salt Lake City, even if it means occasionally dealing with the worst air in the country.
How does a city with less than 200,000 residents achieve such poor air conditions, you ask? Well, allow me to blog about the physics of SLC. The city lies at the northern end of the Salt Lake Valley, a five hundred mile squared oval that was once part of Lake Bonneville (circa 17,000 years ago). The lake covered over 19,000 square miles of what is now known as The Great Basin (that would have made the lake as large as Lake Michigan). Apparently geological evidence says the lake may have drained and reformed as many as 28 times in three million years, which totally astounds me. The last time the lake left, its remnants drained into five different lakes in the area, including the rotting corpse we call the Great Salt Lake. Maybe that's harsh...I call it a rotting corpse not out of disdain, but of smell: the only thing that can survive in the lake are brine shrimp, which make the whole thing smell like...well, shrimp discharge.
Geology, biology...where's the physics?! Back on track - the Salt Lake Valley has two yearly events in January or February that you can count on like sunrise: a Christmas concert by one of the Osmonds and an inversion. The physics is in the latter: a temperature inversion is a reversal of the normal temperature balance of the atmosphere. In meteorological folk lore, I bet it's known as the witching hour.(I will not actually put money on that - but only because I don't have any.)
As sunlight passes through the atmosphere (losing most of its UV rays and other harmfuls at the outer layers) it mostly just passes right through the air above us and hits the ground. I always assumed, based on experience with ovens and what-not, that the air closer to the sun might get warmed up more. That's not to say planets closer to the sun aren't toastier, but within the earth's atmosphere the air is a relatively bad conductor of heat. So the sunlight mostly passes through it and hits the ground. Amazingly enough, the balance of heat in the atmosphere has as much to do with direct sunlight as it does with the heat reflected off the ground. Basic physics principles tell us that hot air will rise above cold air, and it does. The warm ground heats up the air immediately above it, and that warmer air slowly rises. Colder air sinks down to the ground where it starts to warm up as well. As the warm air rises high into the atmosphere, it expands and cools down. This creates a somewhat counter-physics-intuition situation: the cold air rests above the warm air. Now, no laws are being broken because the air higher up is of lower density. Thus, for most of the year the valley is warmer than the mountain tops.
BUT! Such is not always the case. In winter, the ground cools off during the long nights and the air immediately above it cools down as well. The layer of air near the ground is cold and dense, and it can't rise. Without this lower heating mechanism, the air higher up is actually warmer than the air near the ground, and still lighter. Long nights, the low angle of the sun in the sky and overcast days add to this, as does a lack of circulation in the boxed-in Salt Lake Valley. In the polar regions, the air is almost always inverted during the winter because the sun hardly ever comes up. In a valley, without a big storm or wind front, there's nothing to stir up the cold air near the ground and push it out. Over days, the cold air continues to build up near the ground and the warmer air higher up puts a cap on the valley, preventing the cold air from escaping.
Alright, cold air doesn't equal bad air, just unpleasantness. But as residents pump smog from cars and smoke from fires into the air, the cap of warm air prevents the particulates from rising out of the valley. They get stuck under the cap, and for once we get to see exactly how much crap we're pumping into the atmosphere. That bad air makes for bad breathing conditions and it further prevents sunlight from getting through to the ground. This traps the poor residents of SLC in a bubble of cold and gloom, while the mountain peaks are often warm and sunny. You can quite literally drive up the mountain during an inversion and pass through the top-most layer of smog, quickly emerging into a bright, blue skied day. It's like the transition from "She's So Heavy" to "Here Comes the Sun" on Abbey Road.
But don't cry for us, Salt Lake City. The inversion slips away once an inevitable storm front sweeps through and starts circulating the air system. The cold air in the incoming clouds and the moisture they carry flush the valley, as do heavy winds, and put the natural atmospheric conditions back in place. Still, the longest inversion I remember in Utah lasted for nearly a month, dropped the average temperature in the valley to a few degrees above zero, and turned it into a dismal, gray little town.
Don'tcha hate it when physics gotta be like that?