Page 253 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 8 Weather 217
The intense high-speed winds of the jet streams guide cyclonic systems, with their attendant air masses, across the continent (review jet streams in Figures 6.13 and 6.14) along storm tracks that shift in latitude with the Sun and the seasons. Typical storm tracks cross- ing North America are farther northward in summer and farther southward in winter. As the storm tracks begin to shift northward in the spring, cP and mT air masses are in their clearest conflict. This is the time of strongest frontal activity, featuring thunderstorms and tornadoes.
Life Cycle of a Midlatitude Cyclone
Geosystems in Action, Figure GIA 8, shows the birth, ma- turity, and death of a typical midlatitude cyclone in several stages, along with an idealized weather map. On average, a midlatitude cyclonic system takes 3–10 days to progress through this life cycle from the area where it develops to the area where it finally dissolves. However, every day’s weather map departs from this model in some manner.
Cyclogenesis The first stage of a midlatitude cyclone is cyclogenesis, the atmospheric process in which low- pressure wave cyclones develop and strengthen. This process usually begins along the polar front, where cold and warm air masses converge and are drawn into conflict, creating potentially unstable conditions. For a wave cyclone to form along the polar front, a compensating area of divergence aloft must match a surface point of air convergence. Even a slight distur- bance along the polar front, perhaps a small change in the path of the jet stream, can initiate the converging, ascending flow of air and thus a surface low-pressure system in Figure GIA 8.1, Stage 1.
In addition to the polar front, certain other areas are associated with wave cyclone development and intensifi- cation: the eastern slope of the Rockies and other north– south mountain barriers, the Gulf Coast, and the east coasts of North America and Asia.
Open Stage In the open stage, to the east of the de- veloping low-pressure centre of a Northern Hemisphere midlatitude cyclone, warm air begins to move north- ward along an advancing front, while cold air advances southward to the west of the centre. See this organiza- tion in Figure GIA 8.1, Stage 2, and around the centre of low pressure, located over Winnipeg, in the GIA map. As the midlatitude cyclone matures, the counterclock- wise flow draws the cold air mass from the north and west and the warm air mass from the south. In the cross section, you can see the profiles of both a cold front and a warm front and each air mass segment.
Occluded Stage Next is the occluded stage (in Fig- ure GIA 8.1, Stage 3). Remember the relation between air
temperature and the density of an air mass. The colder cP air mass is denser than the warmer mT air mass. This cooler, more unified air mass, acting like a bulldozer blade, moves faster than the warm front. Cold fronts can travel at an average 40 km·h−1, whereas warm fronts av- erage roughly half that, 16–24 km · h−1. Thus, a cold front often overtakes the cyclonic warm front and wedges be- neath it, producing an occluded front (occlude means “to close”).
When there is a stalemate between cooler and warmer air masses such that airflow on either side is almost parallel to the front, although in opposite di- rections, a stationary front results. Some gentle lifting might produce light to moderate precipitation. Even- tually, the stationary front will begin to move, as one of the air masses assumes dominance, evolving into a warm or a cold front.
Dissolving Stage Finally, the dissolving stage of the midlatitude cyclone occurs when its lifting mechanism is completely cut off from the warm air mass, which was its source of energy and moisture. Remnants of the cyclonic system then dissipate in the atmosphere, perhaps after passage across the country in Figure GIA 8.1, Stage 4.
Although the actual patterns of cyclonic passage over North America are widely varied in shape and du- ration, you can apply this general model of the stages of a midlatitude cyclone, along with your understanding of warm and cold fronts, to reading the daily weather map. Examples of the standard weather symbols used on maps are shown below the map in Figure GIA 8.1.
Weather Maps and Forecasting
Synoptic analysis is the evaluation of weather data col- lected at a specific time. Building a database of wind, pressure, temperature, and moisture conditions is key to numerical, or computer-based, weather prediction and the development of weather-forecasting models. Development of numerical models is a great challenge because the atmosphere operates as a nonlinear system, tending toward chaotic behaviour. Slight variations in input data or slight changes in the model’s basic as- sumptions can produce widely varying forecasts. The accuracy of forecasts continues to improve with techno- logical advancements in instruments and software and with our increasing knowledge of the atmospheric inter- actions that produce weather.
Weather data necessary for the preparation of a syn- optic map and forecast include the following:
• Barometric pressure (sea level and altimeter setting) • Pressure tendency (steady, rising, falling)
• Surface air temperature
• Dew-point temperature
• Wind speed, direction, and character (gusts, squalls) • Type and movement of clouds
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