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The Pineapple Express, the Alberta Clipper and the Colorado Low Meteorologists and journalists like to use these colourful names to brighten up their weather reports. But behind each is a major weather system that can have a big impact on our lives by HENRY HENGEVELD Pineapple Express. Alberta Clipper. Colorado Low. Texas Low. Nor'easter. Montreal Express. Some might be forgiven for guessing that these names refer to transportation companies. They are, in fact, the colourful terms that forecasters and the media often use to identify the different transportation systems that nature uses to bring winter storms to North America. What are they, and why do they occur? The answer lies partly in the basics of atmospheric dynamics. Across most of North America, two large-scale processes help determine how our weather changes from one day or one week to the next. The first is the Westerlies (so called because they come from the west) that dominate the broad movement of the atmosphere across the continent. This flow is strongest along the mid-latitude jet stream some 10 kilometers above the surface and weakest in the southern United States and in the high Arctic. The second factor is the development of low- and high-pressure centers that disrupt this flow in complex ways, much like boulders might affect the flow of a stream. Lows develop where various surface features and atmospheric processes interact to cause air in the upper atmosphere to expand, thus decreasing the weight of the atmosphere at that location and lowering the pressure at the surface. Winds circulate around lows in a counterclockwise direction. When they become intense, their spinning action can move large volumes of warm air northward to the east of the low centre, and cold air southward on its west side. High-pressure systems, on the other hand, occur where the total vertical atmospheric air mass increases. They rotate clockwise. When these opposing systems rub shoulders, they work together like giant egg-beaters -- or the meshed gears of Earth's weather machine -- sending warm air to heat up the frigid north and cold air to cool down the overheated equator. Westerlies drag these systems from west to east. Low-pressure centers, or cyclones, are of particular interest because they also cause most of our severe winter weather. Often these cyclones have distinguishing characteristics linked to their place of origin, hence the use of names that associate them with a specific region of North America. These winter cyclones frequently begin at locations far away from us, initially as a weak low-pressure centre in a location where the front that separates the cold Arctic air mass to the north from the warmer continental air to the south develops a kink. Two areas where this happens in central North America are in Colorado and in Alberta, both in the lee of the Rocky Mountains. Alternatively, such waves in the jet stream also occur when the difference between temperatures of air masses on the two sides of a weather front is very large. This often occurs in the Arctic and along the North American east coast. The Pineapple Express, a low system formed off Hawaii, is one such cyclone that can affect the west coast of America. Nor'easters usually form in the deep U.S. south and march their way up the east coast of North America, occasionally sideswiping Ontario. Arctic lows often bring extremely cold air sweeping across the Prairies. However, most of Ontario's major winter storms come from the cyclone systems that originate in the lee of the Rockies. The colder of these, often dubbed the Alberta Clipper, is a fast-moving system that can trek southeastward across the Prairies and into the upper U.S. Midwest and Ontario within a day or two. Because the Clipper, like the clipper ship, moves so quickly, it has little time to develop into an intense storm with heavy snow falls. However, intense cold Arctic temperatures usually follow in its wake. In areas where this cold air passes over large bodies of open water, like those found in the Great Lakes, during early winter, it can cause very heavy but localized snow falls directly downwind. It can also generate the howling winds, cold temperatures and poor visibility associated with a true blizzard. The other common Rocky Mountain cyclone system is called a Colorado Low. As the name suggests, its origin is in the region of Colorado, somewhat further south than the Alberta Clipper. It is therefore also warmer and moister, with much of its abundant water vapour hailing from the Gulf of Mexico. A typical Colorado Low tracks northeastward rather than southeastward, and moves much more slowly than the Clipper. The combination of slow movement, high moisture content and warmer temperatures often cause heavy falls of wet snow and freezing rain in its path, even the odd thunderstorm. By the time they reach Ontario, these storms have usually matured into a large wedge of warm, moist air surrounded on each side by cold air. From space, the clouds associated with these systems often take the shape of a large, wide comma. Along the leading edge of the comma, warm air is advancing overtop cold air to the north-east, accompanied by a layer of stratus cloud that produces relatively light precipitation. Along the west side of the comma, another weather front pushes cold air under the warm air trapped within the wedge of the comma. Here, the atmosphere can become highly unstable, causing cumulus clouds to develop and producing heavy precipitation, often in the form of sleet or freezing rain. Once the cold front has passed, temperatures usually plummet. Meteorological texts provide lots of information about how these cyclones are formed and how they develop, but few provide any reference to the mysterious names they seem to have attracted. Yet these names appear regularly in our media weather coverage, and even in our official weather forecasts. Some argue that these names likely come from meteorologists who, during long, dull nightshifts, seek to put some spice into their forecasts. Others suggest they originate with journalists responding to our desire to always blame bad weather on someone else. Not such a bad idea, really! BF Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada © copyright 2005AgMedia Inc.. top back