I wrung out my top and shorts as well as I could, used a towel to dry them as much as I could (which got them to “damp”), and then hung them up and went to get dinner. There they stayed until six the next morning, when it was time to get up and start work. But when I picked up my shorts, they weren’t just damp—they were wetter than the night before. And not only were they wet, they were also very cold, because the temperature had dropped overnight. Yuck! But I didn’t have any duplicates, so I had to put them on, and walk along the beach trying to look soulful in the sunrise without visibly shivering.
In a gas, the molecules generally aren’t attracting each other, which is why they can spread out to fill whatever container you put them into. In a liquid, things are a bit different. The game of bumper cars is still going on, but the molecules are much closer together—so close that they’re touching almost all the time. In air at room temperature, the average distance between any pair of gas molecules is about ten times the length of a single molecule. But in a liquid, the molecules are right next to each other. They’re still jiggling about as they bounce into the molecules next to them, and they can move past each other quite easily, but they’re moving more slowly than the gas molecules. Because they’re slower and closer to each other, molecules in a liquid feel the attraction of other molecules near them. That’s why liquids form droplets. Temperature is all about the amount of movement energy that the molecules have. In a cool liquid droplet, the molecules aren’t moving much and so they stick together. If you heat the droplet up, the average speed of all the molecules will increase, and some will happen to end up with quite a bit more energy than average.
For a molecule to escape from the liquid, it needs enough energy to escape the attraction of the others. This is evaporation, and it happens at the moment a molecule has acquired just enough energy to escape from a liquid and float off by itself to join a gas. My wet clothes were full of liquid water, molecules sluggishly moving around each other but without the energy to escape.
For three days in that monsoon, I tried everything I could to dry my clothes off. Drying clothes generally means putting them in a situation that will give the liquid water molecules enough energy to escape, so that they just drift off elsewhere. During bursts of hot sunshine, the liquid water absorbed the sun’s energy and the water molecules would slowly escape. But when it was cloudy, I was fighting a losing battle. The problem was that there was just too much water in the air. The air blowing toward the beach from the ocean was full of it. As the sun shone on the hot ocean, it warmed the surface layer. The water molecules in the ocean are also playing bumper cars, and the hotter the water gets, the faster they move, on average. As the ocean surface heated up, more molecules happened to end up with enough speed to escape. These molecules drifted up into the atmosphere to become gas instead of liquid. So the warm, moist air that arrived at the beach was already full of escaped water molecules. They were now playing bumper cars with the other molecules in the air.
When I got rained on, the warmth from my body was heating up my clothes, giving some of the water molecules I was carrying around enough energy to escape into the air. That was making the clothes drier. But the kicker was that there were so many water molecules in the air that they were bumping into my clothes and sticking. When that happened, they would just sink into the liquid mob, making my clothes wetter. The reason my clothes never dried was that the numbers of water molecules evaporating from them into the air was exactly balanced by the number of water molecules that were condensing on to them from the air. This is what 100 percent humidity means: that every molecule that evaporates is replaced by another one condensing. If the humidity is lower than 100 percent, more molecules will leave the liquid than arrive. The bigger that difference is, the faster things dry.
At night, it got worse. As the air cooled, all the molecules slowed down. So even more of them slowed down enough to stick to my top and shorts, and the clothes got even wetter. The point at which more molecules are condensing than evaporating is called the dew point, and the liquid drops that form are dew. Occasional molecules will still have enough energy to leave the liquid and join a gas. But their numbers are insignificant compared to the molecules that are coming the other way. If I had been able to heat up the clothes, I would have increased the number of molecules evaporating, perhaps enough to tip the balance back so that the clothes got drier. As it was, I was stuck with the wet, and so was the rest of India.
The point is that there’s always an exchange going on. That statistical way of looking at a sea of molecules is important because the molecules aren’t all doing the same thing. At exactly the same time, in exactly the same place, some molecules will be evaporating and some will be condensing. What we see just depends on the balance between those two possibilities.
There are times when it’s really helpful that each molecule in a mob is behaving differently. For example, when sweat evaporates, it’s only the molecules with the most energy that escape. The consequence is that the average speed of the ones left behind decreases. That’s why sweating cools you down; the escaping molecules take lots of energy away with them.