Saturday, July 25, 2020


"I've looked at clouds from both sides now"
Joni Mitchell

Lenticular clouds in Centennial, Colorado.

People have looked for images in clouds for at least as long as humanity has been recording it's activities.

I lived most of my life in the Southeast United States leaving only for a construction job in Montana, four trips to or through Denver (and the states I passed through on the way), and two seasons on a laybarge in the Gulf of Mexico. But I've seen some weather.

Every area has its distinctive weather. The reason we could predict our weather with any certainty in Alabama was that it was coming from where I live now, Colorado, and we had plenty of time to see it coming.

As each area has its characteristic weather, each has its characteristic clouds.

The massive high-topped thunderheads with anvil shaped tops are rare here. We get a lot more hail; they get a lot more tornadoes. They also get hurricanes.

We get these weird lenticular clouds like the ones in the picture above.

As air flows across the Rockies and out over the plains, it can set off standing waves, like those that form when a person blows across the top of a soda bottle. At the bottom of the waves, the air is warm and can hold moisture as invisible vapor, but when the air is carried to the top of the waves, it cools off and releases the water as cloud droplets. A person from the east might think "rain coming," but these are actually stable weather clouds, like the big fluffy cumulus clouds of the Southern summer.

Those Southern cumulus clouds could build up into far from stable cumulonimbus clouds. I had one drop a tornado into a farmer's field next to where I was driving once. I didn't usually speed. I did that time. Clouds can cause you to get excited.

I was driving near Tuskegee, Alabama on I 75 and there was this huge thunderhead over me. The bottom was flat and it looked like I could reach up and touch it. It was electric blue. There was so much water in the cloud that it piped the color of the sky above it right down to the bottom. That's the kind of cloud that can drop a tornado down right on top of you without any warning.

The funny thing is that I saw so much storm activity before I became a storm watcher about five years before my retirement and never saw sign of another tornado, not even during the disastrous outbreak of 2011.

One of the weirdest cloud formations I have ever seen were the hole-punch clouds one summer in Selma. An even sheet of clouds had big circular holes in it, all the way up to clear blue. We didn't know what caused them back then. It turns out that planes flying through clouds can cause droplets to coalesce, like when you put a drop of detergent in a bowl of greasy water.

The most beautiful display of weather I have seen was a very darkly overcast afternoon on a laybarge. There was a clear band on the horizon where the sun was setting blood red. It cast a red glow over the bottom of the clouds and there was a brilliant triple rainbow.

I've been chased up Mount Carbon three times by thunderstorms. Walking east from Morrison on the Bear Creek Trail carries you through a broad, treeless area of Bear Creek Lakes Park. With no cover behind me and no cover and a lung busting switchback before me, I heard a rumble at my back and turned to see massive black clouds boiling up over Mount Falcon. There was green-ness in them, indicating hail, which I didn't want on me, so I walked fast.

All three times I reached the shelter at Mountain View before the storm hit.

What causes cloud colors. Well, I talked about sunrise and sunset clouds in the last blog. Why are clouds white and, of all colors, dark gray? Is water gray?

We talked about Rayleigh and Mei scattering last time. Mei scattering is caused by aerosols, particles that aren't molecules but they're still small enough to stay suspended in air. And remember that Mei scattering doesn't pick and choose specific wavelengths of light like Rayleigh scattering does, so it's not surprising when clouds are white, but gray?

A few years ago, I built a Joly Photometer. It's basically two chunks of paraffin separated by aluminum foil. Here it is.

If you put a standard light source on one side and a light source of unknown brightness on the other, then move the unknown source nearer or further away until both sides of the Photometer look the same, you can calculate from the difference in distances of the light sources from the photometer how bright the unknown source is.

But notice that the two halves, which are from the same slab of wax and, therefore, the same color, look different. The apparent color is from the white light illuminating the wax. The top slab is lit by more white light ..therefore, whiter.

Clouds are the same way. Less light makes its way down through the cloud so places with less light look less white and our eyes are rigged to emphasize contrasts so, next to the bright white of the tops of the cloud, the bottom sometimes looks positively black.

Notice that the gray parts of clouds are usually a cool gray until sunset. The blue of the sky also comes through misty clouds.

If you ever visit the Great Smoky Mountains, you will see that they, indeed, do look smokey. That area has always funneled aerosols in, hardwood tree pollen, smoke, and more recently, abundant industrial pollutants and they disperse white light efficiently. Add in the sky blue and there's smoky mountains.

Green clouds are a place where light has to pass through so much dispersion that all the blue is gone. Notice that it's a sorta dirty brown, meaning that light at the red end of the spectrum is there, too . Clouds with that much water and ice in them will often mean hail. I run from those! Mother Nature doesn't want me there.

Hail forms in clouds with strong updrafts. High up in cumulonimbus clouds, water freezes into tiny ice crystals until they are too heavy to remain suspended in the air and then they start to fall, but they get caught in the updraft and are blown back up. They pick up more moisture and freeze another layer. These little balls of ice will ride winds up and down until they're no longer little balls of ice. When they finally fall to the ground, they can be destructively big balls of ice!

Cloud watching is a great pastime. You can get a good idea of what to expect of the weather in your area and, occasionally, you'll see something rare and spectacular!

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