- The temperature of a liquid is a measurement of the average speed of the molecules. The molecules in a liquid are attracted to each other, but at a certain temperature they move so fast that they pull away from each other. That temperature is called the boiling point, at which the liquid turns into a gas as its molecules spread into the air. Individual molecules, however, can leave a liquid at any point, in a process called evaporation.
- The rate or temperature of individual molecules in a quantity of liquid varies greatly. Some move very quickly while others are very slow. A puddle of water has millions of molecules, and at any point a few move quickly enough to escape the pull of the other molecules and disperse into the air. When they do, these molecules are said to evaporate.
- When the fastest few molecules jump off the surface of the liquid, this lowers the average temperature. If a good breeze is blowing, it will pull some of the liquid molecules off, speeding up the rate of evaporation and cooling the liquid even faster. A low pressure area will also speed evaporation and cooling. Molecules naturally spread from areas of greater concentration to those of lesser concentration, so the liquid molecules will evaporate more quickly if the air has fewer molecules in it (i.e., the air is under low pressure).
- In a process called precipitation, molecules of water in the air can become part of a liquid. A passing water molecule, suspended in the air, can collide with the liquid and stick. When this happens, it adds energy to the liquid, warming it. Usually, evaporation outpaces precipitation, so there is usually some cooling of the liquid, but not always. The more humid the air, the more moisture it contains and the more precipitation it will cause. If the air is at 100 percent humidity, the liquid will not evaporate at all.
