Who among us hasn’t spent some time gazing at the clouds? Perhaps we have lain in a grassy field or lawn and looked for shapes in the puffy white blobs that floated lazily across the blue expanse above. Or we watched the sky catch fire at the setting (or rising) of the day. For some, maybe the only relevance of clouds is whether they will produce rain (or hail, or snow, or a tornado). Regardless of the specific nature of our relationships with clouds, we have them.
For me, I am most fascinated by the shapes and colors clouds can assume. The absolute best cloud formation I’ve seen was here in the Adirondacks. I was driving back from Ray Brook and there in the sky was a herd of banthas* – must’ve been a hundred of them. Each cloud was the same shape, and as they slowly changed, they changed in unison. It was pretty amazing.
Clouds, at least here on Earth, are made from condensed water vapor.** It doesn’t sound very exciting, does it? Warm air absorbs water vapor (this is why winter air is dry), and warm air rises. As the warm, moist air rises, it cools. As it cools, the water condenses into droplets, or ice crystals. If enough of these droplets are close enough together, they form a visible mass we call a cloud.
Why are clouds white? And why are they not always white? This has to do with how light bounces on, around, off, water particles. Take your average cloud – it’s large, it’s deep, and it is highly reflective of all wavelengths of light within the visible spectrum. In other words, it reflects all light we can see, and thus it looks white (the color white is made up of all the colors). As the sunlight penetrates further into the cloud, it is scattered more and more, leaving less to be reflected. This is why the bottoms of clouds are often darker, even grey. Think rain clouds. These are very dense – lots of condensed water vapor.
We’ve all see clouds that are red, orange and pink – glorious shades that show up when the sun is low on the horizon. These colors, however, are not IN the clouds, though. These colors appear as reflections from the sun. A great explanation I found for this is that it is the same as if you shone a red flashlight onto a sheet – the sheet reflects the red light, it doesn’t turn red itself.
But some clouds look bluish, or greenish, or even yellowish. These are all structural. For example, the blueish-grey clouds are caused from light scattering within the cloud. Blues and greens are short wavelength colors and are very easily scattered by the water droplets (reds and oranges are long wavelengths, and they are reflected, see paragraph above).
If you see a green cloud, it is that color because the sunlight is being scattered by ice instead of water droplets. This can be a clue to weather prognosticators as to what kind of weather we can expect (hail, snow, tornadoes). Yellow clouds are apparently quite rare, and their color tends to come from pollutants in the atmosphere, like smoke.
Then there are iridescent clouds. These are very uncommon. Iridescent clouds usually sport pastel colors, looking much like mother-of-pearl. Sometimes, however, their colors can be quite intense. Iridescent clouds are formed when the light shines through thin clouds (often the edges of clouds) made from nearly uniform droplets. Each ray of light strikes one droplet and all the droplets participate in cumulative diffraction, the end result of which is a cloud that shimmers with all the visible colors.*** I’ve only seen this once, and that was because I was wearing polarized sunglasses at the time – dark glasses can help make these events visible. It was amazing.
Cloud gazing isn’t something that should be left to children or the idle. Everyone should take the time to watch the clouds. Not only can it be a relaxing activity (can an activity be relaxing?), but it can also be informative. Just think, our ancestors knew their clouds and had a weather sense that most of us have lost today, traded in for the ease of technology. Sometimes I think our ancestors had the better plan.
* For those who don’t get this reference, banthas are the creatures from “Star Wars” that the Sand People and Tuskan Raiders rode. They are imaginary, obviously, but even so, that’s exactly what the clouds looked like.
** Clouds can form on any moon or planet that has an atmosphere, but this doesn’t mean they are made from water vapor. Venus’s clouds are made of sulfuric acid. On Mars, they are made of ice. If you go to Jupiter and Saturn, be prepared for ammonia clouds, and if you travel to Uranus or Neptune, you’ll find the clouds are made from methane gas. Even outer space has clouds made of space debris – these are often called nebulae.