The reason ice crystals fall from a parent cloud of water drops was discussed last October in a posting called fallstreaks<\/a>.\u00a0In the picture shown at that time, the crystals fell, more or less, straight down because the wind through which the crystals fell did not change much with height. Under circumstances when they don’t fall straight down, they often take a parabolic path, a shape that looks like a hook, thus, the name,\u00a0uncinus. Why does cirrus so often assume a parabolic shape? Curiously, the reason for the parabolic shape of uncinus is quite easy to account for using simple calculus, but is quite difficult to explain using simple English. This is just one of many situations where natural language falls short. Natural language often presents a view\u00a0through a glass, darkly, under conditions that mathematics shows things face to face. Despite language limitations,\u00a0here is an inevitably flawed attempt.<\/p>\n The ice crystals are all falling at roughly the same terminal velocity, but they fall though a wind that changes with height at a constant rate. Such a wind variation is common. One might expect that the streaks would take the same shape as this linear wind profile and so appear as a sloping straight line. This does not happen.<\/p>\n The problem is that each crystal falls through a succession of different winds and so its path is the cumulative effect of an acceleration from each level above it. The result is a streak with a parabolic shape. Of course, to the mathematically inclined, this is trivial: the integral of a linear function is a quadratic. (Another example of this behaviour is the parabolic path of a ballistic projectile, where the constant acceleration is not applied by a wind changing uniformly with height, but by gravity).<\/p>\n Often, the wind in the atmosphere changes at a constant rate with height. When ice crystals fall through such an atmosphere, the streaks are parabolic in shape. They look like a hooked lock of hair: cirrus uncinus. Uncinus is Latin for hooked and when applied to cirrus—itself Latin for a lock of hair—it nicely characterizes the cloud, below: formally, it is\u00a0cirrus uncinus. A familiar name for these clouds is fallstreaks, for they are streaks of ice … Continue reading ![\"\"](\"https:\/\/blog.kootenay-lake.ca\/wp-content\/uploads\/2014\/02\/crystalpath4.gif\")
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