Page 274 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

238 part II The Water, Weather, and Climate Systems
orographic lifting (p. 211) rain shadow (p. 211) chinook wind (p. 212) cold front (p. 212)
warm front (p. 212) squall line (p. 214)
3. Explain why it is necessary for an air mass to be lifted if there is to be saturation, condensation, and precipitation.
4. What are the four principal lifting mechanisms that cause air masses to ascend, cool, condense, form clouds, and perhaps produce precipitation? Briefly describe each.
5. Differentiate between the structure of a cold front and that of a warm front.
■ Explain the formation of orographic precipitation, and review an example of orographic effects in North America.
The physical presence of a mountain acts as a topographic barrier to migrating air masses. Orographic lifting (oro- means “mountain”) occurs when air is forcibly lifted upslope as it is pushed against a mountain. It cools adi- abatically. An orographic barrier enhances convectional activity. The precipitation pattern of windward and lee- ward slopes is seen worldwide.
6. When an air mass passes over a mountain range, many things happen to it. Describe each aspect of a moist air mass crossing a mountain. What is the pat- tern of precipitation that results?
7. Explain how the distribution of precipitation in the province of British Columbia is influenced by the principles of orographic lifting.
■ Describe the life cycle of a midlatitude cyclonic storm system, and relate this to its portrayal on weather maps.
A midlatitude cyclone, or wave cyclone, is a vast low- pressure system that migrates across a continent, pulling air masses into conflict along fronts. These systems are guided by the jet streams of the upper troposphere along seasonally shifting storm tracks. Cyclogenesis, the birth of the low-pressure circulation, can occur off the west coast of North America, along the polar front, along the lee slopes of the Rockies, in the Gulf of Mexico, and along the East Coast. A midlatitude cyclone can be thought of as having a life cycle of birth, maturity, old age, and dissolution; or open, occluded, and dissolving stages. An occluded front is produced when a cold front over- takes a warm front in the maturing cyclone. Sometimes a stationary front develops between conflicting air masses, where airflow is parallel to the front on both sides.
midlatitude cyclone (p. 216) wave cyclone (p. 216)
storm track (p. 217) cyclogenesis (p. 217) occluded front (p. 217) stationary front (p. 217)
8. Differentiatebetweenfrontalliftingatanadvancing cold front and at an advancing warm front, and describe what you would experience with each one.
9. How does a midlatitude cyclone act as a catalyst for conflict between air masses? How do flows of warm, cold, and dry “conveyors” of air interact in such a system?
10. What is meant by cyclogenesis? In what areas does it occur and why? What is the role of upper-tropospheric circulation in the formation of a surface low?
11. Diagram a midlatitude cyclonic storm during its open stage. Label each of the components in your il- lustration, and add arrows to indicate wind patterns in the system.
■ List the measurable elements that contribute to mod- ern weather forecasting, and describe the technology and methods employed.
Synoptic analysis involves the collection of weather data at a specific time. Computer-based weather prediction and the development of weather-forecasting models rely on data such as barometric pressure, surface air tempera- tures, dew-point temperatures, wind speed and direction, clouds, sky conditions, and visibility. On weather maps, these elements appear as specific weather station symbols.
12. What is your principal source of weather data, in- formation, and forecasts? Where does your source obtain its data?
■ Identify various forms of violent weather by their characteristics, and review several examples of each.
The violent power of some weather phenomena poses a hazard to society. Severe ice storms involve freezing precipitation (freezing rain, ice glaze, and ice pellets), snow blizzards, and crippling ice coatings on roads, power lines, and crops. Thunderstorms are fueled by rapid upward move- ment of warm, moist air and are characterised by turbu- lence and wind shear. In strong thunderstorms known as supercells, a cyclonic updraft—a mesocyclone—may form within a cumulonimbus cloud, sometimes rising to the mid-troposphere. Thunderstorms produce lightning (elec- trical discharges in the atmosphere), thunder (sonic bangs produced by the rapid expansion of air after intense heating by lightning), and hail (ice pellets formed within cumulonim- bus clouds). Strong linear winds in excess of 26 m∙s−1, known as derechos, are associated with thunderstorms and bands of showers crossing a region. These straight-line winds can cause significant damage and crop losses.
A tornado is a violently rotating column of air in con- tact with the ground surface, usually visible as a dark gray funnel cloud pulsing from the bottom side of the parent cloud. A waterspout forms when a tornado circulation oc- curs over water.
Within tropical air masses, large low-pressure centres can form along easterly wave troughs. Under the right conditions, a tropical cyclone is produced. A tropical cyclone becomes a hurricane, typhoon, or cyclone when winds exceed 65 knots (119 km·h−1). As forecasting of weather-related hazards improves, loss of life decreases, although property damage continues to increase. Great damage occurs to occupied coastal lands when hurricanes make landfall and when winds drive ocean water inland in storm surges.
freezing precipitation (p. 221) mesocyclone (p. 222)