Storm in a Teacup: The Physics of Everyday Life(7)

REACHING THE SOUTH Pole was a major landmark in human history. The great polar explorers—Amundsen, Scott, Shackleton, and others—are legendary figures, and the books about their achievements and failures are some of the greatest adventure stories of all time. And as if it wasn’t enough to deal with unimaginable cold, lack of food, fierce oceans, and clothing that wasn’t up to the task, the mighty ideal gas law was against them, quite literally.

The center of Antarctica is a high, dry plateau. It is covered in deep ice, but it hardly ever snows. The bright white surface reflects almost all of the feeble sunlight back into space, and temperatures can drop below ?112°F. It is quiet. At an atomic level, the atmosphere here is sluggish, because the air molecules have little energy (due to the cold) and are moving relatively slowly. Air from above descends on to the plateau, and the ice steals its heat. Cold air becomes colder. The pressure is fixed, so this air shrinks in volume and becomes more dense. The molecules are closer together, moving more slowly, unable to push outward hard enough to compete with the air around them pushing inward. As the land slopes away from the center of the continent toward the ocean, so this cold, dense air also slithers away from the center along the surface, unstoppable, like a slow waterfall of air. It is funneled through vast valleys, picking up speed as the funnels descend outward, always outward toward the ocean. These are the katabatic winds of Antarctica, and if you want to walk to the South Pole, they will be in your face all the way. It’s hard to think of a worse trick nature could have played on those polar explorers.

“Katabatic” is just a name for this sort of wind, and it’s found in many places, not all of them cold. As they descend, those sluggish molecules do warm up, just a little bit. And the consequences of that warming can be dramatic.

In 2007, I was living in San Diego and working at the Scripps Institution of Oceanography. As a northerner I was slightly suspicious of the eternal sunshine, but I got to swim in an Olympic-sized outdoor swimming pool every morning so I couldn’t really complain. And the sunsets were amazing. San Diego is on the coast with a clear view west across the Pacific Ocean, and the evening skyline was reliably stunning.

I really missed the seasons, though. It seemed as though time never moved on, almost like living in a dream. But then the Santa Ana winds came, and it went from sunny and warm and cheerful to sullenly hot and dry. The Santa Ana winds come every autumn, as air pours off the high deserts and flows over the coast of California out toward the ocean. These are also katabatic winds, just like the ones in Antarctica. But by the time they reach the ocean, the air is much hotter at the coast than it was on the high plateau. One memorable day, I was driving north up the I-5 freeway, toward one of the big valleys that funneled the hot air out to sea. There was a river of low cloud sitting in the valley. My boyfriend at the time was driving. “Can you smell smoke?” I asked. “Don’t be silly,” he said. But the next morning, I woke up in a weird world. There were huge wildfires to the north of San Diego, marching across the valleys, and there was ash in the air. A campfire had got out of control in the hot, dry conditions, and the winds were blowing the fire toward the coast. That river of cloud had been smoke. People went to work, and either were sent home or sat in huddles listening to the radio and wondering whether their houses were safe. We waited. The horizon was hazy because of ash clouds you could see from space, but the sunsets were spectacular. After three days, the smoke started to lift. People I knew had lost their houses to the flames. Everything had a layer of ash on it and health officials were advising against any outdoor exercise for a week.

Up on the high plateau, hot desert air had cooled, become more dense, and slithered downslope, just like the winds that faced Scott in Antarctica. But the wildfires started because that air wasn’t only dry, it was hot. Why would it get hotter as it came downhill? Where does the energy come from? The ideal gas law still applies—this was a fixed mass of air, and it was moving so quickly that there was no time for it to exchange energy with its surroundings. As that stream of dense air made its way downhill, the atmosphere that was already at the bottom of the hill pushed on it, because the pressure down there was higher. Pushing on something is a way of giving it energy. You can imagine individual air molecules hitting the wall of a balloon that is moving toward them. They’ll bounce off with more energy than they had to start with, because they’re bouncing off a moving surface. So the volume of the air in the Santa Ana winds decreased because it was squeezed inward by the surrounding atmosphere. That squeezing gave the traveling air molecules extra energy, and so the temperature of the wind increased. It’s called adiabatic heating. Every year, when the Santa Ana winds come, everyone in California is extra vigilant about open fires. After a few days of such hot, dry air stealing the moisture from the landscape, sparks can easily turn into wildfires. And the heat doesn’t just come from the California sun—it also comes from the extra energy given to the gas molecules as they are compressed by denser air closer to the ocean. Anything that changes the average speed of air molecules will change the temperature.

The same thing happens in reverse when you squirt whipped cream out of a can. The air that comes out in the cream has expanded suddenly and pushed on its surroundings, so it has given away energy and cooled down. The nozzle of the squirty cream canister feels cold to the touch for this reason—the gas that’s coming through it is giving away its energy as it reaches the free atmosphere. Less energy is left behind, so the can feels cool.