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Weather Articles by Sander Schimmelpenninck - Clouds

Clouds by Sander Schimmelpenninck
ssvdo@cogeco.ca

Most clouds are just liquid or frozen fog aloft. They can produce rain, snow, and hail. In addition they give meteorologists an idea of weather in the upper atmosphere梥uggesting what to expect in the next few hours or beyond.

For knowledge of the upper air we now use radiosondes, first deployed en masse late in WWII, and now a plethora of satellite-mounted remote sensors. But before that clouds were the only game in town, and their description remains basic tradecraft for meteorologists.

If it moves, scientists describe it. (Some also revel in stationary stuff like Dead Sea scrolls and dinosaurs, but archaeology was never my strong suit.) Not surprisingly, then, a British naturalist, Luke Howard, first classified them in 1803. His method gained wide acceptance after it appeared in the 1896 International Cloud Atlas; it has changed very little since then.

Howard chose Latin for his method, because it was a lingua franca for scientists until French, German, and now English took over. His style benefits meteorology even today, because it keeps amateurs like me and other bothersome outsiders at bay. So it's nimbostratus not rain cloud.

Howard's system pigeonholes most clouds by altitude and shape.

The altitude ranges and their prefixes are: (nothing) = below 2 km above ground level (AGL), alto- = 2-5 km AGL, and cirro- = >5 km AGL. Aviators will recognize the intermediate limits as 6,500 and 16,500 ft.

The main shapes are stratus and cumulus.

The system for naming clouds resembles that for animals and plants: combinations of genera and species. In meteorology, the genera are levels and the species shapes. That gives combinations like cumulus (low heap-shaped clouds), altostratus (a mid-level, flat, featureless layer, and cirrocumulus (high-level, heap-shaped clouds).

Stratus comes from the Latin verb sterno, to put down a layer or a stratum. Hence street, of course. In meteorology it means a featureless, dreary sky without dark spots to suggest build-up. When more or less steady rain or drizzle falls from it, you call it nimbostratus. Higher forms are alto- and cirrostratus.

Cumulus is Latin for heap. This cloud type occurs at all three basic levels. A plain cumulus (or just `cu", an impolite word in French) is a medium-tall cloud. What's medium? I have never found an official definition of the implied limits for shallow and tall, but here are mine. Flat cumuli, called stratocu(mulus) are clearly wider than tall, say, a few hundred feet tall. Regular cu means a vertical extent between, say, 200 and 6,000 ft. Taller still and you have yourself a towering cumulus, a.k.a. TCU. At that point, airline pilots start asking permission to deviate, because tall clouds contain strong up- en downdrafts.

Stratus and nimbostratus usually mean little vertical air movement梛ust unremitting gloom.

Cu are often fair-weather clouds, but watch for vertical growth. TCUs bring rain or snow showers. When they sprout a gauzy anvil from their middle and upper levels, you have a cumulonimbus, or CB梐nd I don't mean the good-buddy kind of Convoy fame.

Mid-level clouds are altostratus and altocu. Watch out for altocumulus castellanus (ACC). In it you see turrets as on a medieval castle. They signal turbulence at mid-level and often presage thundershowers, sometimes half a day ahead.

At high levels, say, 17,000 feet in winter and 26,000 in summer, you have cirrus (Latin for hair curl), cirrocumulus, and cirrostratus. All tend to precede low-pressure areas and hence rain梐lthough cirrus patches can also form in large fair-weather systems. As the low draws near, the sky typically changes from cirrus to altostratus to nimbostratus梑ut the weather respects rule no more than the Greek poet Homer did his conjugations (every verb was an exception).

Aviation weather reports (METARs) include cloud heights and cover. Heights are shown in 100s of feet AGL and cover in the following code: FEW, SCT (scattered), BKN (broken), and OVC (overcast). In remarks of Canadian aviation weather reports (METARs) you will see abbreviations like CU(mulus), S(trato)C(umulus), A(lto)S(stratus), and so on. In those remarks the observer states cover in oktas (eighths of the whole sky).

One form of sky cover is `sky obscured.' By definition it means that all or part of the sky is invisible due to some phenomenon at ground level, e.g. rain, snow, or fog.

For cloud heights professionals use ceilometers. They're expensive; probably no amateur has one. But you can estimate cloud heights in two imperfect ways. First, if you live near an airport, you can determine the height of an airplane overhead, just barely visible in a cloud base. To do so, you must know the wingspan or length of the plane. Assume a Cessna 172, the most popular light single. Its wingspan is 20 ft, and a typical arm's length is 60 cm. Subtend the wingspan or length between your thumb and index at arm's length (e.g. 60 cm) and estimate the gap in cm. Divide the gap into your arm's length. If the gap is 1 cm, the plane's height is 60 times its wingspan. For a C-172 that gives 60x20 = 1,200 ft.

In addition, the height in feet of a cumulus base is usually about 400 times the dew point spread (the difference between the ambient temperature and the dew point in Celsius). That trick works only with cumuli.

Weather satellites like the American GOES-8 and ? sense the temperatures of cloud tops. Several websites show those data overlaid on maps of North America with false colouring to indicate temperature. In turn, temperature gives an idea of height梐ccording to a scale at the bottom of your computer screen. For instance, blue usually means something like ?0. That temperature range lies tens of thousands of feet above the earth. When you see a swath of blue, you're probably looking at cumulonimbus tops: watch out below.

Current US weather radars like the WSR-88-D also give top heights (in 100s of feet), but they see precipitation not clouds. Values above 250 can mean thunderstorms, and 650 certainly does. You've heard that airliners fly above the weather. Don't believe a word of it. Boeing 747s cruise around 33,000 feet.

Clouds form mainly by convection or advection. In weather, convection refers to the rising of air as a result of the actual temperature lapse rate: the rate of cooling with altitude. Advection means horizontal transport.

Understanding convection calls for a bit of thermodynamics. You probably know that a gas warms up when you compress it, and vice versa. If you didn't, inflate a bicycle tire with a hand pump. Feel the tube of the pump. It's warm, right? Air conditioners and heat pumps work on that principle. Similarly, a gas cools when it expands

Now imagine a parcel of air and lift it adiabatically, i.e. such that it does not exchange heat with the air around it. That parcel will cool at a rate of 3o Celsius per 1,000 ft until it gets so cold that the water vapour in it condenses. That is the dry adiabatic lapse rate. Once the water vapour in your air parcel has condensed, it cools more slowly with height, along the saturated adiabatic lapse rate (which is not a linear function).

Remember, the above assumes no exchange of heat between our hypothetical air parcel and the actual air aloft through which it may pass. Let our parcel have a temperature of 20 C at ground level. On the day of our experiment the temperature at 1,000 ft is only 15 C, not the 17 you expected. When our air parcel reaches 1,000 ft, it has cooled adiabatically to 17. It looks around and notices that it is still warmer than the air it meets there. So up it goes some more: warmer air is lighter than cooler air, so buoyancy causes further lifting. Eventually the air in the rising parcel condenses, and you have a cloud base.

If the actual temperature lapse rate is greater than the adiabatic one, the stage is set for convection and, often, the formation of low-level cumuli. When the difference is great, you will see towering cumulus and thunderstorms. Within the latter, hail has formed. It falls, gets caught in updrafts, and falls again梘rowing all the time. During its falls, it creates or strengthens downdrafts (rain can do so too). When this process gets out of hand, you can get a downburst, a phenomenon described by Theodore Fujita of Chicago University. One of those can squash big airplanes against the ground or flatten a stand of trees. Only the fastest military jets have a hope of outclimbing them. Airbuses and Boeings do not. So when you see lightning on final approach to Miami, pray that your captain is not bonused for on-time arrivals or that he values his wife and children.

Upper air layers can also be warmer than the adiabatic lapse rate would suggest. When the temperature rises with height, you have an inversion. Those often occur. In one scenario, nighttime cooling of the earth under a clear sky cools lowers the temperature of air near the ground, say within the bottom 700 metres of the atmosphere. Higher up the temperature may not vary much. The resulting condition is called a temperature inversion. It persists in the first few hours after sunrise and thus inhibits convection. Inversions can also occur in the middle of the day. You may then get some convective clouds, but the inversion above them keeps them pretty flat. It also prevents air pollution from dissipating into the upper atmosphere.

Various websites offer (pointers to) Skew-Ts (tephigrams in Canada): graphs of temperatures and dew points aloft; see the Ontario Weather Page. Their underlying grids show the dry and saturated adiabats. Those data come from radiosondes lofted at noon and midnight Co-ordinated Universal Time from hundreds of sites all over the world. They make interesting reading if you want to understand the three-dimensional nature of weather.

Finally, no article on this subject would be complete without reference to noctilucent clouds, clouds that shine in the night. Little is known about them. They are seen mostly on summer nights when the sun is 5 to 15 degrees below the horizon. They occur between 70 and 100 km above they earth. Rocket experiments have shown strong electric fields around them and suggested ice-covered extraterrestrial dust. Douglas Holdham, an Environment Canada staffer who kindly reviewed this text, writes: "They are a milky-white band across the sky in the dead of night with a thin touch of yellow along the edge near the horizon. As you watch them, they gradually fade. They are the most beautiful clouds I've ever seen!" Indeed the occasional beauty of weather is one reason I began to study it as a teenager in the 1940s.

This article is Copyrighted 2000: Sander Schimmelpenninck and used with his expressed written permission.  No unauthorized use allowed without prior written consent by him.

 

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