Stacey asks:
How Does Evaporation Work?

Put simply, evaporation is the process of converting molecules from a liquid state (water, for example) into a gaseous state (water vapor). You see, all states of matter---solids, liquids, and gases---are made up of molecules. The molecules in solids are stable for the most part, while the molecules in liquids move about more freely, and the molecules in gases are the most active. So what’s all this got to do with evaporation, you ask?

The most important factor in all of this is temperature. Use your imagination for a minute and think about those cold winter mornings. You know, the ones that freeze your nostril hairs into prickly little icicles. If you’re like me, your activity slows when it’s that cold. We don’t feel like doing anything but huddling our arms to our bodies, right? But as the air warms up, we become more and more energetic. Well, the very same thing happens to molecules. As the temperature of molecules increases, they become more energetic and active.

Take an ice cube, for example. When you add heat to this solid, what happens? It begins to melt. That’s because as the molecules warm up, they begin to move fast enough to escape into a liquid form. If you were to then take all that melted water and heat it in a pan, the molecules would get even hotter and faster until they began to escape into the surrounding air as a gas. That’s evaporation!

Of course, evaporation in nature occurs below the boiling point and thus at a much slower rate. (In fact, different liquids evaporate at different rates.) As the sun warms a body of water, evaporation occurs and the resulting water vapor is transferred into the atmosphere. This process creates a cooling effect because only the warmer molecules depart, leaving the cooler ones behind. That’s why the human body sweats. As our perspiration evaporates, our skin is cooled.

About 80% of all evaporation on Earth is from the oceans, while the rest comes from inland water (lakes, ponds, rivers, etc.) and plants. Plants? Yes, they also contribute to the process through transpiration. Transpiration occurs when plants draw water up from the ground through their roots, stems, and finally to their leaves. Water then evaporates from the leaves back into the atmosphere. The combination of evaporation and transpiration is called evapotranspiration.

So what happens to all of this water vapor that passes into the atmosphere? As winds move it around the globe (changing humidity levels throughout the world), the vapors form water droplets around dust particles. These water droplets create clouds and fog, finally falling back to Earth as precipitation (rain, ice, snow, etc.). Some of the precipitation flows into the ground and some runs off into streams, lakes, and oceans. Then evaporation begins the cycling process all over again. This is known as the water cycle, a process that is absolutely essential to life on our planet.