Unit 4: The Atmosphere and the Oceans Chapter 12: Meterology     Meteorology The prevailing westerlies are responsible for much of the movement of weather across the United States and Canada. This was first noted by Benjamin Franklin, who observed that weather in the American colonies moved from west to east. His studies also showed that weather conditions were much more changeable in the middle latitudes. Why? Driven by the wind, large-scale, low-pressure systems migrate across these latitudes. In this feature, you'll learn about one type of low-pressure system, a wave cyclone, and the technology used to track it. Wave Cyclones Wave cyclones are low-pressure systems caused by a combination of factors, including air masses, air motion resulting from temperature and pressure changes, the Coriolis effect, and fronts. A wave cyclone often begins along a stationary front. Some imbalance in temperature, pressure, or density, frequently referred to as a ripple or wave, causes the stationary front to split. One part moves south as a cold front and another part moves north as a warm front. This sets up a counterclockwise or cyclonic circulation. Steered by high-altitude winds, the resulting low-pressure system affects large portions of the middle latitudes. Weather conditions associated with the system vary, depending on moisture content and the stability of the air. In advance of the warm front, continuous precipitation is common, while behind the warm front, there may be light drizzle or rain. In advance of the cold front, there may be increasingly gusty winds and rain, with the strongest winds and heaviest precipitation-including thunderstorms-occurring as the cold front passes. Skies gradually clear behind the cold front. Usually, the cold front advances faster than the warm front; thus, the cold air eventually overtakes the warm air. This begins the formation of an occluded front. If the process continues, the wedge of warm air moving north will modify to the point at which it is no longer distinguishable as a warm front, and the low-pressure system will weaken. Meanwhile, the cold air has moved far from its source region and it, too, has become modified. Finally, the contrast between the air masses is no longer sufficient to keep the front moving, and it becomes stationary. The storm is over. Recall that wave cyclones are guided by the jet stream and prevailing westerlies-these winds occur high in the atmosphere. To track their movement, meteorologists often rely on upper-level weather technology, such as radiosondes and satellites. Radiosondes Radiosondes, or balloon-borne packages of sensors, are currently the most commonly used instrument to gather upper-level weather data. However, they do have one major disadvantage-they are expensive. Each balloon-borne package costs about $50. At more than 100 sites across the United States, two radiosondes are released each day. Thus, the annual cost of the balloons and radiosondes is more than $3.5 million. This does not include costs related to staff, ground stations, or computers. Currently, an average of one observation every 12 hours is taken by radiosonde, and there are roughly two observation sites per state. Given the high cost of radiosondes, data from weather satellites are often used to supplement radiosonde observations. Some weather satellites orbit Earth and take pictures of different parts of the planet. Then they transmit the data back to Earth. Other weather satellites are in geostationary orbits. In a geostationary orbit, a satellite orbits Earth at a speed that keeps it positioned over the same spot on Earth. This allows the satellite to continually observe and photograph one area so that a time-lapse film of the transmitted photos can be created. Meteorologists use these films to study changing cloud patterns and the weather systems that formed them. Such films, called satellite loops, are common features on television weather shows. LINK-UP: Find out more information about satellite loops. Activity As a group, monitor television weather reports for several days. Then, make a large poster of a daily weather map from your local newspaper. Include fronts, station models, and pressure systems on your map. Use the poster to give a weather report to your class, similar to the type of forecasts given by TV meteorologists. You may want to videotape your presentation.