Posts Tagged ‘cirrus’

Just for fun!

The Latin students among you will readily recognize what sort of ideas are expressed by all these syllables, which are very clear and definite.

Stratocumulus perlucidus below Cirrus intortus

Upper level cirrus in various tangles with a large sheet of cumulus that still allows the light to come around and through.

Stratocumulus means a sheet of lumpy clouds. Stratus is sheet; cumulus is lumpy or humpy. Perlucidus means the light goes around it — the sheet does not cover the sky. In fact, convection in the center of the sheet makes it high there while the edges are not growing.

Above the cumulus — in the upper section of the photo, but also thousands of feet higher in the sky, the cirrus, the wispy clouds, are tangled in various ways. Having words for it makes it easier to remember what you saw and how it was laid out. Figuring what weather is implied is a different exercise, but The Weather Identification Handbook gives some basic clues.

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In the post about the size of a raindrop, we began a consideration of the magnitudes of the objects studied in meteorology. Today we continue, but we are going the opposite direction — not down to the objects smaller than a raindrop, but upwards in size, through some of the cloud forms.
Just to get oriented, we begin once more with [0] — the consideration of things that are one meter or so in length, width, or height. We might have said: anything from 1/3 or a meter to 3 meters or so. A child, a young scientist, standing in a puddle with an umbrella wondering whether he will be the lucky one to find a weather balloon would be a display of some weather objects in this size range.
When I was in college, I used to ride a bike to Mass in the morning in Florida, and there would be miniature fogs about 4 feet off the ground, and just a few feet wide. I’ve never seen that sort of fog body since.
Tendrils of sea smoke can be in this size range, possibly smaller.

[1] Ten meters is the next size range: about the size of a house or a mature maple tree.

  • Let us consider that right above our young meteorologist, the smallest of the fair weather clouds are barely beginning to form. They form, float away from their sources, and quickly die.
  • But on a different kind of day, only the curled edges of a cloud would be in the size range of his house, though they seem smaller because of their distance.
  • Before the sky is entirely caught into gray sheets, a few small gray shreds still float by at the size of small establishments such as houses, stores, and small yards. Because of the immense distance of the sun, its rays arrive on earth so close to parallel that as long as there is some sun, our meteorologist can verify the sizes of small clouds by the sizes of their shadows.
  • Little drifts of fog form in the evening near creeks or low places in a field.
  • Widths of tornadoes are generally in this size range.
  • Typical waterspouts are here.

[2] But as weather systems build, 100 meter items, the size of football fields, quickly turn up.

  • Small clouds are readily incorporated into larger ones.
  • If it’s evening, gathering fogs are likely to appear in small valleys and fields, or to wind along creeks for great distances.
  • In the tree-covered mountains of New England, roads may be marked after a summer rain because the mist rises above their hot pavements.
  • This is the usual tornado diameter.
  • Destructive waterspouts can be in this size range, though their sizes very quite a bit. Here’s a list of historic and destructive waterspouts.
    • All of us have seen the little whirlwinds of snow that sculpt our drifts all winter. But snonados can get a little larger. Here’s an interesting view from You tube, just by way of reminding us that the physics of the wind is the same everywhere. It’s what heat adds to it that makes the most intense forms of weather. Never much heat in a snowdrift.

    [3] Here we are at 1000 meters, one kilometer.

    • Low, fair-weather cloud heights might be as little as a kilometer.
    • As the sky fills, many cloud-bases will be one or a few kilometers above the ground.
    • A cumulus congestus is likely to be a kilometer wide and high.
    • Larger clouds come in many shapes and at many heights. It’s hard to judge the width of a cloud, but you can learn to do it. When you drive along, notice how far you have to drive to actually pass a cloud. Bring your cloud viewers in the car and watch the odometer — don’t do this when you’re the driver, of course. In time, you’ll get used to what you are seeing.
    • In the tree-covered mountains of New England, roads may be marked after a summer rain because the mist rises above their hot pavements and hovers above the treetops.
    • The pannus clouds that form on the front end of a storm are only as deep as a field or two but may roil along in lines that are a few kilometers long.

    [4] What features of weather are 6 miles tall, wide, or long? This is about 10 kilometers?

  • Cumulonimbus clouds with their anvil tops can be 3 miles or more in height, (not counting rom the ground, but from the base of the cloud) so they belong to this order of size. (order of magnitude, right?).
  • Since they already begin a mile or two up in the atmosphere thunderheads may reach to the top of the troposphere, that 6-10 mile veneer of atmosphere which covers the earth and houses most of our weather.
  • An average storm cell consists of a cumulonimbus cloud and a few of its associates moving in. When an entire front moves in, the storm cells are spaced a few miles apart because they are pulling in the air from some distance around.
  • Of course there are many other such objects that might have been listed. If I left out your favorite, drop me a line.  We’ll extend the size range next time. Meantime, ask yourself, what is the largest object you can think of which makes a contribution to weather on Earth? And what is the smallest?

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    Humdity on high

    Convection trail in late August 2009

    Condensation trail in late August 2009


    Everyone sees vapor trails from time to time. Some are long and knotted; some are short, disappearing in less than half a minute; some are swept across the sky by upper winds and persist for hours and hours. I’ve always wondered whether the length of the contrail could be used as an indicator of humidity — at least humidity on high, which is related to what’s below, even if it’s not the same. Anyway, the ability of a condensation trail to hold its own must be related to how much other water is out there.

    The way you’d measure the trail would be to hold out your arm and hold up your hand as we do for astronomy; then see how many fingers it takes to cover the trail — this is a way of measuring how many degrees of sky it covers. One finger is about 1 degree. Three is about five degrees. The fist is ten, the fist with the thumb outstretched is 15°, and the measure from thumbtip to little fingertip with the hand outstretched is 22°. Graph this against the humidity in the weather report and send me the results.

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    Cirro stratus

    It’s hard to be enthusiastic about photographing high sheets, even high sheets of angel hair.  They’re probably 5 miles up, maybe six, and they’re made of ice. It implies that warm, humid air is moving in.  


    Cirro stratus with sundog

    Cirro stratus with sundog



    On the other hand, this is a perfect sky for certain types of light display. Mike got a sundog for me at 7:30 in the morning. I was on the way to Mass without my camera, and called him from the car. You can see the sun through the trees at exactly the same altitude as its protecting sundog. Sundogs mean there are ice crystals in the air, and sometimes you can hardly even see the clouds that make them. If you look closely, you can see that there is much more than a spot of light; there is a whole circumsun, a whole arc around the sun. 

    Actually, as I write, it’s nearly noon, and there is still a halo around the sun. It’s about 15° out, and it’ll be there as long as the clouds are cirrus and let the sun come through a little. Hold up two hands to cover the sun and take a look.

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