Cirrus (from the Latin meaning a curl, a tuft, a filament) clouds are wispy, white high clouds that often precede a change in the weather. A fairly common feature is a drift of thinning cloud that fades to nothing. These are called mare’s tails, which is appropriate because they do look like the curl of a horse’s tail.

Mare’s tail, or drifting, falling ice crystals

These clouds just happened to be in the direction of the sun, so I had to place a light post over the sun to get a picture.

Cirrus clouds are composed of ice crystals, and those wispy tufts of the mare’s tail are ice crystals falling out of the cloud and sublimating as they drop into drier air. They are technically virga, which is precipitation that falls from a cloud and evaporates before it reaches the ground. I usually think of virga as liquid precipitation that evaporates before hitting the ground. If you watch the television weather, you might see a radar image showing rain over you, but when you look outside there’s no rain. That means there’s virga up there somewhere above you.

Cirrus clouds over the mountain

The mare’s tails can tell you the direction of the wind. It often also tells you that there is wind shear at altitude. If the mare’s tail is moving away from the cloud, then it must be falling into air that’s moving differently from where the cloud is. Wind shear can refer to a change in the velocity of the wind with altitude, or with the direction of wind with altitude. If there is any wind, there will be some wind shear close to the ground, because the surface of the Earth slows the wind. That’s why windmills work better when they are raised as high as possible. For large windmills, like some I have seen in the western US, there can be significant differences in wind speed from where the blades come closest to the ground to their highest point.

The physical forces at work also ensure that there will be some change in direction of the wind with altitude. Most of us have seen cloud layers that move across the sky in one direction while clouds that are higher move in a different direction. I like it best when it’s night and there’s a moon involved.

We were socked in from Christmas Eve through sometime Christmas afternoon. I went outside to walk Zeke about 10 pm and could barely see our neighbor’s outdoor Christmas tree. Late Tuesday morning we went for a longer walk down Fouche Gap Road into Texas Valley. It was very foggy.

Do you see anything down there?

I estimated the visibility to be about 100 yards. Everything was evenly lighted; there appeared to be no shadows at all, anywhere. The sky was lighter directly overhead, but it was pretty light in all directions. That means to me that the fog was not very thick overhead, so more of the sunlight made it to the ground. Think about all of those photons from the sun, flying down into the clouds, bouncing around from water droplet to water droplet. As it turns out, a good fraction bounce into the same direction they were going, and a good fraction bounce backwards. Some of them go into other directions. Eventually, if the fog is thick enough, they get pretty well confused, going pretty much in all directions, and eventually finding their way out in almost every direction, up, down, sideways. If the fog is too thick, the ones that get too deep get lost and never find their way out. That’s why most clouds look white from pretty much any direction you look at them, but thick clouds look dark from below. From the top they are as white as any other cloud.

The fog got thinner as we walked down the mountain, until at the bottom, there was almost no fog.

Pretty clear down here

Down in the valley the visibility was pretty good. The shiny metal roof of a house is visible across the valley on the side of Rocky Mountain, if you know just where to look, and you hold you mouth just right. It was probably close to a mile away.  Looking back up, the top of Lavender Mountain was hidden. So up on the top of Lavender Mountain we had fog, but from down in the valley, we had clouds.