Gas molecules are always with us. The Earth has an atmosphere that surrounds us, bumps into us, pushes on us, and also keeps us alive. The wonderful thing about our atmosphere is that it isn’t static—it’s constantly shifting around and changing. The air is invisible to us, but if we could see it, we’d see huge masses of it heating up and cooling down, expanding and contracting, always moving. What our atmosphere is doing is dictated by the gas laws we’ve seen at work in this chapter, just like any other collection of gas molecules. Even though it’s not contained in a whale’s lungs or a steam engine, it’s still pushing. But since its surroundings are also air, that means that it’s constantly pushing itself around, readjusting to conditions. We can’t see the details, but we have a name for the consequences: weather.
The best place to watch a storm is a vast open plain. The day before, the air can be calm and the expanse of blue up above seems to go on forever. Invisible air molecules crowd together close to the ground and spread out farther up, always pushing, hassling, readjusting, and flowing. Air is shunted from regions of high pressure to regions of low pressure, responding to heating and cooling, always on the way to somewhere else. But the adjustments are slow and peaceful, and there’s no hint of the vast amounts of energy carried by the molecules.
The day of the storm dawns just as the one before it did, but the sky is clearer, so the ground is heating up more quickly. The air molecules take some of that energy and speed up. By early afternoon, a deep wall of cloud is approaching and expanding as it moves, until it stretches across the horizon. Energy is on the move. A pressure difference is pushing this slab of gaseous architecture across the plain. The drama comes because this giant structure is unstable. Although the air molecules are shoving hard on each other, they haven’t had time to rearrange themselves into a more balanced situation. Alongside that, vast amounts of energy are being shunted around, so the situation is constantly changing. Hot air warmed by the ground is pushing upward into the cloud, pummeling its way through and building towers that stretch high above the wall.
As the thundercloud arrives overhead, the expansive blue is replaced by a dark low lid on the landscape. On the ground, we are boxed in by the clash that is going on above. We can’t see the air molecules, but we can see the clouds churning and surging. And this is only a hint of the violence going on within them as air packets are buffeted and pummeled, because the imbalances of pressure are so strong that readjustment is a rapid and energetic process. As energy is exchanged by the air molecules, water droplets cool and grow and the first large raindrops start to fall. Strong winds stream past us, as the air molecules rush around even at ground level.
Big storm clouds remind us how much energy there is up there in the blue sky. We see hints of the bumping and shoving, and it looks extreme—but it’s only the merest hint of the real bumping and shoving happening at a molecular level above our heads. Air molecules may absorb energy from the Sun, lose energy to the ocean, gain energy from condensation as clouds form or lose energy by radiating it away to space, and they are constantly adjusting according to the ideal gas law. Our spinning planet with its rough and multicolored surface makes the adjustments more complicated, and so do clouds, tiny particulates, and the specific gases present. A weather forecast is really just a way of keeping track of the battles above our heads and picking out the ones that will affect us most down here on the ground. But the action right at the root of it all is the same as that used by an elephant, a rocket, and a steam engine. It’s all just the gas laws in action. The same bit of physics that makes popcorn pop also makes the weather work.
* We’ll get to the meaning of absolute temperature in chapter 6.
? This substitution is not the recommended way of doing science today.
? We don’t know how much of the air Otto’s vacuum pump removed. It won’t have been all the air, but it must have been a substantial proportion of it.
§ And also when we breathe. Every breath you take gets into your lungs because the atmosphere pushes it there.
? If you’ve ever wondered what Thomas the Tank Engine’s tank is all about, it’s all about the water. The water can be stored in a separate carriage with the coal (a tender) or it can be stored in a tank that sits around the engine. Thomas stores his water around the engine—that’s why he’s rectangular—and so he’s a tank engine.
# The Indian Airmail Society also experimented with rocket mail around the same time. They managed 270 flights, sending parcels as well as letters, but never established it as a long-term success. In the end, rocket mail was never going to be able to compete with the regular ground-based mail delivery systems on reliability and cost.
CHAPTER 2
What Goes Up Must Come Down
CURIOSITY RUNS IN my family. They’ll happily go and investigate anything new that comes along, they’re up for trying things out, and they do all this without making any fuss about it. So they weren’t all that surprised when I disappeared off to the kitchen during a family lunch on a suddenly urgent mission to find a bottle of lemonade and a handful of raisins. It was a beautiful summer day, and we were all sitting outside in my mother’s garden: my sister, aunt, Nana, and my parents. I found one of those 2-liter plastic bottles of cheap fizzy lemonade, took the label off, and then put the bottle in the middle of the table. This new madness was watched with quiet interest, but I had their attention, so I took the cap off and dropped the entire handful of raisins into the bottle. There was a whoosh of foam and then, when the bubbles cleared, we could see that the raisins were dancing. I had thought that this would only be entertaining for a minute or two, but Nana and my dad couldn’t stop staring at it. The bottle had been transformed into a raisin lava lamp. The raisins were rushing from the bottom of the bottle to the top and back again, twirling madly and bumping into each other on the way.