GO IN DEPTH

We Really Do Know Clouds After All

Asperatus cloud formation. Asperatus is one of the new additions to the latest volume of the International Cloud Atlas issued today. Photo Credit: Ave Maria Mõistlik, Licensed under CC BY-SA 3.0, via Wikimedia Commons.

Rows and flows of angel hair 
And ice cream castles in the air
And feather canyons everywhere
I've looked at clouds that way

But now they only block the sun
They rain and snow on everyone
So many things I would have done 
But clouds got in my way

I've looked at clouds from both sides now
From up and down, and still somehow
It's clouds' illusions I recall
I really don't know clouds at all

Contrary to Joni Mitchell's "Both Sides, Now" lyrics, we do know clouds. Even if we don't yet understand what happens when cloud droplets and aerosols interact, we know clouds better than ever before thanks to the brand new edition of the International Cloud Atlas, published today by the World Meteorological Organization. And while looking for shapes in the clouds overhead may be a fun pastime, it no longer has to be the only one spectators engage in, with the Cloud Atlas accessible online and available free to the general public for the first time in its 121-year history.

The Cloud Atlas is the definitive guide to cloud identification and classification. A cloud's size, shape, structure, texture, color, and luminance (how light interacts with it) all help meteorologists organize clouds into categories. Clouds are classified similarly to plants and animals: genus, species, and variety. Just like an animal can have only one genus and one species, the same is true for clouds. There are 10 genera—including cirrus, cumulus, and stratus—and 15 species, including volutus (also known as the roll cloud), a new addition in this latest version of the Cloud Atlas. There are nine varieties of clouds, which refer to their transparency and how their features are arranged. Finally, clouds also sometimes have supplemental features or accessory clouds attached to them, such as the funnel of a tornado (tuba) or an asperitas. Asperitas is a cloud formation that resembles rough ocean waves, appearing in varying shades of gray. After heavy lobbying by the citizen science group, the Cloud Appreciation Society, it is the first new term approved for use in the Cloud Atlas since 1953.

Additionally, there are also clouds that only occur in very limited, specific circumstances: upper atmospheric clouds and other, special clouds, such as flammagenitus clouds that begin from a forest fire or volcano eruption and homogenitus clouds that indicate they were caused by human activity, such as airplane contrails or power plant tower plumes.

As well as offering information about clouds, the Cloud Atlas also shares data and photos of other weather phenomena, including hail, rainbows, and lightning. Additionally, the site offers tips for observing clouds (both from the ground and from airplanes), a flowchart for cloud genus identification, a photo/video gallery, and the ability to compare two photos of clouds against each other.

The first International Cloud Atlas was published in 1896 and included paintings when color photographs, still an expensive process at the time, were unavailable.The latest edition of the Cloud Atlas was published today just in time for World Meteorological Day, the theme of which this year is "Understanding Clouds."

If you want to learn more about clouds, check out these Science NetLinks lessons: Sky 1: Objects in the Sky, Measuring Cloud Coverage, The Water CycleRobert Gardner: Teaching Scientific Inquiry, and Air Masses. The Science Update podcasts Asian Brown Cloud, Traveling Dust, and Raindrop Fossils and the tool The Water Cycle at Work also contain related information.

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