Q&A: Water in the Atmosphere

Question: I’m a junior HS student and have to do an assignment about water in the air. But I have no idea where to start. Any chance you could help me out? — WJ, Winnipeg, Manitoba

Answer: Your question isn’t very specific, so I’ll try to answer it in a general sense. That should give you some leads, and you can take it from there. If you’re writing a paper on this topic, be sure to check out the links in my answer for additional useful material. For example, see this page on the US Geological Survey (USGS) website.

First, just to put things in perspective, very little of Earth’s total water supply is in the atmosphere at any time, whether in a vapor, liquid, or solid state. It’s estimated that Earth has 1.4 billion cubic kilometers (336 million cubic miles) of water in all forms, fresh and saltwater included. On average, only 0.25% of that is in the atmosphere.

If you took all that atmospheric water and spread it evenly around the surface of the Earth, it would make a “puddle” only 2.5 cm (about an inch) deep. By comparison, if you did the same with all the oceans, lakes, and (melted) ice, that layer would be around 2.8 km (1.7 miles) deep.

Water in the atmosphere can take many forms, as shown in the above graphic. In its vapor state (which is most of what’s in the air) it takes the form of single water molecules represented by the chemical formula H2O. In its liquid state, when the droplets are very small, they make up the clouds we see in the sky. See my June 24 post “What Clouds Are.” When those droplets get larger (and heavier) they will fall as precipitation.

Interestingly, falling raindrops are not the classic “teardrop” shape. Once they get large enough for air drag to overcome surface tension, they assume the oblate shape shown in the graphic. If they grow even larger, things get more complex.

In the form of ice crystals, the variety is essentially limitless. The only common ground is that all these shapes are based on the tendency of solid water to assemble in hexagonal crystals (like the classic snowflake). This is a consequence of the asymmetric structure of H2O, which makes it a polar molecule. The hydrogen atoms acquire a slight positive charge, and the oxygen atom a slight negative charge. This is because oxygen has twice the electron affinity of hydrogen.

Despite the small amount of water in the atmosphere, it has a huge impact on climate, visibility, geology, and weather circulation patterns. And, of course, it’s a critical component of the water cycle. But I’d be remiss if I didn’t mention that water in the atmosphere is responsible for countless beautiful visual phenomena: rainbows, sun pillars, parhelia, coronae, and others I’ve posted about in the past. The Sky Lights archive contains many fine examples.

Next Week in Sky Lights ⇒ How Altimeters Measure Altitude

Light and Color at Sunset
Q&A: How Altimeters Measure Altitude