Air masses that influence weather conditions on the North American continent
What is an air mass? Examine the diagram of air masses that influence weather conditions on the North American continent, and discuss the predominant weather patterns produced and why the changes occur between summer and winter. (Use the following regions as examples: northwest; southern California; southwest interior; southeast; and northeast.)
A Idea that may help you sort out the information:
Construct a table with regions listed on the top; and summer and winter on the left. Fill in the air masses for each region in each season. Then looking at the properties of each air mass figure out the predominant weather producing systems affecting each region. For example, convectional thunderstorms in the southeast as the land heats in spring worsening through summer (also hurricanes in late summer). Remember the moist conditions of the air mass producing conditions with convectional lifting resulting in ……
Note: Use the PowerPoint to find the Figures
? Describe air masses that affect North America and relate their qualities to source regions.
1. How does a source region influence the type of air mass that forms over it? Give specific examples of each basic classification.
See Figure 8.2 for details. Earth?s surface imparts its varying characteristics to the air it touches. Such a distinctive body of air is called an air mass, and it initially reflects the characteristics of its source region. The longer an air mass remains stationary over a region, the more definite its physical attributes become. Within each air mass there is a homogeneity of temperature and humidity that sometimes extends through the lower half of the troposphere.
2. Of all the air masses, which are of greatest significance to the United States and Canada? What happens to them as they migrate to locations different from their source regions? Give an example of air-mass modification.
See Figures 8.2, 8.4, and 8.5. As air masses migrate from their source regions, their temperature and moisture characteristics are modified. For example, a mT Gulf/Atlantic air mass may carry humidity to Chicago and on to Winnipeg, but gradually will become more stable with each day?s passage northward. Similarly, temperatures below freezing occasionally reach into southern Texas and Florida, influenced by an invading winter cP air mass from the north. However, that air mass will have warmed from the ?50?C (?58?F) of its source region in central Canada. In winter, as a cP air mass moves southward over warmer land, it is modified, warming especially after it crosses the snowline. If it passes over the warmer Great Lakes, it will absorb heat and moisture and produce heavy lake-effect snowfall downwind as far east as New York, Pennsylvania, New England, and the Maritime Provinces of Canada.
? Identify types of atmospheric lifting mechanisms and describe four principal examples.
3. Explain why it is necessary for an air mass to rise if there is to be precipitation.
If air masses are to cool adiabatically (by expansion), and if they are to reach the dew-point temperature and saturate, condense, form clouds, and perhaps precipitate, then they must be lifted.
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.
The four principal lifting mechanisms causing adiabatic cooling are convergent lifting, convectional lifting, orographic lifting, and frontal lifting. Convergent lifting occurs along the ITCZ where air flows from areas of higher pressure towards areas of low pressure (pressure gradient force). The tradewinds and easterlies form this type of convergence, warming air masses that create high vertical cumulonimbus clouds and high amounts of precipitation.
Convectional lifting is stimulated by local surface heating, often associated with differential heating between land and water surfaces. Maritime tropical air masses migrate out from the ITCZ, transporting warm, moist unstable air to continental locations, causing precipitation.
Adiabatic lifting occurs in both convergent and convectional lifting as warm, moist air masses rise vertically in the atmosphere, and are cooled by the surrounding air. Orographic lifting occurs when air masses are forced over barriers, such as mountains. Such movement causes the vertical movement of air masses, which are cooled by the cold air at higher elevations.
And, frontal lifting takes place when air masses of differing temperature and humidity interact. Such collision is caused by the jet stream, affecting mostly midlatitude locations, as the jet streams pulls air masses from their source region into contact with other air masses. Adiabatic lifting occurs in frontal lifting when a cold air mass, or cold front, displaces warm air along the surface, or when a warm air mass, or warm front, slides over cooler air and is forced to rise vertically in the atmosphere.
5. Differentiate between the structure of a cold front and a warm front.
The leading edge of an advancing air mass is called its front. The leading edge of a cold air mass is a cold front, whereas the leading edge of a warm air mass is a warm front. A cold front is identified on weather maps by a line marked with triangular spikes pointing in the direction of frontal movement along an advancing cP air mass in winter. Warmer, moist air in advance of the cold front is lifted upward abruptly and is subjected to the same adiabatic rates and concepts of stability/instability that pertain to all lifting air parcels.
A warm front is denoted on weather maps by a line marked with semicircles facing the direction of frontal movement. The leading edge of an advancing warm air mass is unable to displace cooler, passive air, which is denser. Instead, the warm air tends to push the cooler, underlying air into a characteristic wedge shape, with the warmer air sliding up over the cooler air.
? Analyze the pattern of orographic precipitation and describe the link between this pattern and global topography.
6. When an air mass passes across a mountain range, many things happen to it. Describe each aspect of a mountain crossing by a moist air mass. What is the pattern of precipitation that results?
See Figures 8.8 and 8.9. The physical presence of a mountain acts as a topographic barrier to migrating air masses. Orographic lifting occurs when air is pushed upslope against a mountain and cools adiabatically. Stable air that is forced upward in this manner may produce stratiform clouds, whereas unstable or conditionally unstable air usually forms a line of cumulus and cumulonimbus clouds.
An orographic barrier enhances convectional activity and causes additional lifting during the passage of weather fronts and cyclonic systems, thereby extracting more moisture from passing air masses. The wetter intercepting slope is termed the windward slope, as opposed to the drier farside slope, known as the leeward slope, which is the location of a rain shadow. Moisture is condensed from the lifting air mass on the windward side of the mountain; on the leeward side the descending air mass is heated by compression, causing evaporation of any remaining water in the air. Thus, air can begin its ascent up a mountain warm and moist but finish its descent hot and dry.
The record precipitation for contiguous North America falls to Canada, where the focusing effects of local topography and orographic lifting on the west side of Vancouver Island produced an average annual amount at Henderson Lake, British Columbia of 650 cm (256 in.).
7. Explain how the distribution of precipitation in the state of Washington is influenced by the principles of orographic lifting.
Figure 8.9 illustrates the rain shadow effect created by orographic lifting as it affects four locations in Washington state. The illustration examines Quinalt, Sequim, Rainier Paradise, and Yakima, in order to illustrate the influence of mountain barriers upon the progression of air masses that migrate eastward across the state. Two of these locations, Quinalt and Rainier Paradise, are examples of the effects of orographic lifting on the windward side of mountain barriers. Quinalt, located on the western slope of the Olympic National Forest, receives 122.2 inches of precipitation each year, despite a relatively low elevation of 219 feet above sea level. Rainier Paradise, located on the Western slope of the Cascade Mountains, records 103.7 inches of rain per year at an elevation of 5550 feet. Sequim and Yakima are both located on the leeward slope of these mountain systems, where dry, descending air often desiccates these environments, and does not yield precipitation. Sequim, located in the Puget Trough at 180 feet above sea level, has an annual precipitation of 16.2 inches, while Yakima bears the majority of the rain shadow effect, receiving only 8.4 inches of precipitation a year, despite its elevation of 1061 feet.
? Describe the life cycle of a midlatitude cyclonic storm system, and relate this to its portrayal on weather maps.
8. Differentiate between a cold front and a warm front as types of frontal lifting and what you would experience with each one.
Compare Figure 8.10, illustrating a cold front, and Figure 8.11, showing a warm front. The captions for each describe the differences.
9. How does a midlatitude cyclone act as a catalyst for conflict between air masses?
Wave cyclones form a dominant type of weather pattern in the middle and higher latitudes of both hemispheres and act as a catalyst for air-mass conflict as they bring contrasting air masses into conflict. A migrating center of low pressure, with converging, ascending air, spiraling inwardly counterclockwise in the Northern Hemisphere (or converging clockwise in the Southern Hemisphere) draws surrounding air masses into conflict in the cyclonic circulation along fronts.
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?
A midlatitude cyclone, or extratropical cyclone, is born along the polar front, particularly in the region of the Icelandic and Aleutian subpolar low-pressure cells in the Northern Hemisphere. Strengthening and development of a wave cyclone is known as cyclogenesis. In addition to the polar front, certain other areas are associated with wave cyclone development and intensification: the eastern slope of the Rockies and other north-south mountain barriers, and the North American and Asian east coasts. As air moves downslope, the vertical axis of the air column extends, shrinking the system horizontally, intensifying wind speed. As the air travels downslope, it is deflected in a cyclonic flow, thus developing new cyclonic systems or intensifying existing ones.
11. Diagram a midlatitude cyclonic storm during its open stage. Label each of the components in your illustration, and add arrows to indicate wind patterns in the system.
See Figure 8.12. At first, diagram the storm in top and side view including a cross-section of the warm and cold fronts. Then diagram the storm during the occluded phase, including top and side diagrams.
? List the measurable elements that contribute to weather and describe the technology and methods employed.
12. What is your principal source of weather data, information, and forecasts? Where does your source obtain its data? Have you used the Internet and World Wide Web to obtain weather information? In what ways will you personally apply this knowledge in the future? What benefits do you see?
Personal analysis and response. The National Climate Data Center, of the National Environmental Satellite, Data, and Information Service, which is part of the National Oceanic and Atmospheric Administration, Department of Commerce, is located in Asheville, North Carolina 28801, and publishes a ?Daily Weather Map? series on a weekly basis. This features a detailed surface map, 500-millibar chart, highest and lowest temperatures, and precipitation areas and amounts for the week. From the same data center, you can obtain a poster entitled ?Explanation of the Daily Weather Map,? which is a guide showing all the standard symbols presently used. These poster-charts are available in single copies (free) or in lots of 50 at a very low price.
A subscription to Weatherwise (?The Magazine About the Weather?) is helpful, for it contains interesting articles, annual reviews of hurricanes and tornadoes, an annual weather photo contest, and historical information. It is published 6 times a year by the Helen Dwight Reid Foundation in association with the American Meteorological Society, Heldref Publications, Washington, DC, 1-800-365-9753. Other periodicals of interest are the Bulletin of the American Meteorological Society (monthly), Journal of Atmospheric Science, and Monthly Weather Review from the American Meteorological Society; Weather (monthly), from the Royal Meteorological Society; and NOAA (bimonthly), from the Office of Public Affairs NOAA. (See the many links on our Internet Home Page.)
? Analyze various forms of violent weather and the characteristics of each and review several examples of each from the text.
13. What constitutes a thunderstorm? What type of cloud is involved? What type of air masses would you expect in an area of thunderstorms in North America?
Tremendous energy is liberated by the condensation of large quantities of water vapor. This process is accompanied by violent updrafts and downdrafts. As a result, giant cumulonimbus clouds can create dramatic weather moments?squall lines of heavy precipitation, lightning, thunder, hail, blustery winds, and tornadoes. Thunderstorms may develop within an air mass, along a front (particularly a cold front), or where mountain slopes cause orographic lifting. Important here are the mT air masses of the Gulf and Atlantic source region.
14. Lightning and thunder are powerful phenomena in nature. Briefly describe how they develop.
Lightning refers to flashes of light caused by enormous electrical discharges?tens to hundreds of millions of volts?which briefly ignite the air to temperatures of 15,000? to 30,000?C (27,000? to 54,000?F). The violent expansion of this abruptly heated air sends shock waves through the atmosphere creating the sonic bangs known as thunder. The greater the distance a lightning stroke travels, the longer the thunder echoes. Lightning is created by a buildup of electrical energy between areas within a cumulonimbus cloud or between the cloud and the ground, with sufficient electrical potential to overcome the resistance of the atmosphere and leap from one surface to the other?it is like a giant spark.
A rule of thumb to use in determining the distance a lightning strike is from your location assumes that the flash arrived instantaneously at the speed of light, whereas the sound traveled at the speed of sound, some 3 seconds per km (1090 ft per sec, or 5 sec per mile). Simply begin counting at the moment of the flash to determine the elapsed time before you hear the thunder. Given the number of seconds elapsed, you will know the distance you are from the lightning in km, feet, or mile units given above speeds. Suffice it to say that if you experience no delay and witness a simultaneous flash and crack of thunder then you are in the wrong place at the wrong time!
Thunder is enhanced by greater moisture density within the cloud and by topography, which can act further to reverberate the sound waves.
15. Describe the formation process of a mesocyclone. How is this development associated with that of a tornado?
The updrafts associated with a cumulonimbus cloud appear on satellite images as pulsing bubbles of clouds. Because winds in the troposphere blow stronger above Earth?s surface than they do at the surface, a body of air pushes forward faster at altitude than at the surface, thus creating a rotation in the air along a horizontal axis that is parallel to the ground. When that rotating air encounters the strong updrafts associated with frontal activity, the axis of rotation is shifted to a vertical alignment, perpendicular to the ground. It is this spinning, cyclonic, rising column of mid-troposphere-level air that forms a mesocyclone.
A mesocyclone can range up to 10 km (6 mi) in diameter and rotate over thousands of feet vertically within the parent cloud. As a mesocyclone extends vertically and contracts horizontally, wind speeds accelerate in an inward vortex (much as ice skaters accelerate while spinning by pulling their arms in closer to their bodies). A well-developed mesocyclone most certainly will produce heavy rain, large hail, blustery winds, and lightning; some mature mesocyclones will generate tornado activity.
16. Evaluate the pattern of tornado activity in the United States. What generalizations can you make about the distribution and timing of tornadoes? Do you perceive a trend in tornado occurrences in the United States? Explain.
See Figure 8.20. Of the 50 states, 49 have experienced tornadoes, as have all the Canadian provinces. May is the peak month. A small number of tornadoes are reported in other countries each year, but North America receives the greatest share because its latitudinal position and shape permit conflicting and contrasting air masses to have access to each other. See the ?Recent Tornado Records? section in the chapter. The highest incidence of tornadoes in the United States occur along a four state corridor called ?Tornado Alley,? including Texas, Oklahoma, Kansas, and Nebraska, with another center in central Florida.
Generally, tornadoes occur in conjunction with a mesocyclone characterized by large cumulonimbus clouds, hail, intense winds, and lightning. Timing of tornadoes is still rather unpredictable, yet research using satellites, airplanes, and surface measurements is now enabling us to predict the occurrence of tornadoes. With the use of Doppler radar, which can detect the specific flow of moisture in mesocyclones, only 15% of American tornadoes strike without warning. Doppler radar enables forecasters to give tornado warnings 30 minutes to one hour in advance of an oncoming storm.
Even though this text is being written in central California, far from the ?Tornado Alley? of Oklahoma and Kansas, a tornado touched down just a block from this word processor on March 22, 1983 and several funnel clouds were sighted in February 2005. It was related to the incredible weather of the last intense El Ni?o phenomena?discussed in a focus study with Chapter 10. We were not at home and so no photos are included. It hit about 2 P.M., was moderate on the Fujita Scale, moved northeastward, hopping along and damaging about 30 homes and several businesses.
The long-term annual average number of tornadoes before 1990 was 787. Interestingly, after 1990 the average per year rose to over 1000. In 1998, the annual average reached 1270 tornadoes; the peak year was 2004, with 1820 tornadoes; 1156 tornadoes were reported in 2009.
Importantly, the number of EF-4 and EF-5 tornadoes is on the increase, reaching more than a dozen each season. Note on the graph in Figure 8.20 that the number of tornadoes nearly doubled in March and more than doubled in September, meaning that access to maritime tropical air masses is occurring earlier in spring and later in fall. This suggests climate change is forcing this trend.
17. What are the different classifications for tropical cyclones? List the various names used worldwide for hurricanes.
See Figure 8.22.
18. What factors contributed to the incredible damage cost of Hurricane Andrew? Why have such damage figures increased, whereas loss of life has decreased over the past 30 years?
The second greatest dollar loss from any natural disaster in history was caused by Hurricane Andrew as it swept across Florida and on to Louisiana during August 24?27, 1992. Sustained winds were 225 kmph, with gusts to 282+ kmph (140 mph, 175+ mph)?one of the few category five hurricanes this century (Saffir-Simpson scale). Studies recently completed by meteorologist Theodore Fujita estimated that winds in the eyewall reached 320 kmph (200 mph) in small vortices. Property damage exceeded $20 billion.
The tragedy from Andrew is that the storm destroyed or seriously damaged 70,000 homes and left 200,000 people homeless between Miami and the Florida Keys. By April 1993, 60,000 people were still homeless and reconstruction was progressing slowly. The storm is causing continued losses from reduced property assessments. Approximately 8% of the agricultural industry in Florida?s Dade County (Miami region) was destroyed outright, exceeding $1 billion in lost sales. About 25% of Louisiana?s sugar cane crop was lost. Many plants were killed by wind-driven saltwater that desiccated (dried) leaves that were not already stripped by the winds.
New buildings, apartments, and governmental offices are opening right next to still-visible rubble and bare foundation pads. Unfortunately, careful hazard planning to guide the settlement of these high-risk areas has never been policy. Due to improved technologies that enable us to predict and forecast hurricanes more easily, expansion of urban areas into hazard zones has increased, so that loss of life has been reduced, yet damage is even greater.
Source: Christopherson, R.W., 2012. Geosystems: An Introduction to Physical Geography 8th Ed. Prentice-Hall, New York.