Page 244 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

208 part II The Water, Weather, and Climate Systems
Water has a leading role in the vast drama distinctive body of air is an air mass, and it initially re-
played out daily on Earth’s stage. It affects the
stability of air masses and their interactions and produces powerful and beautiful effects in the lower atmosphere. Air masses conflict; they move and shift, dominating first one region and then another, varying in strength and characteristics. Think of the weather as a play, Earth’s continents and oceans as the stage, air masses as actors of varying ability, and water as the lead.
Weather is the short-term, day-to-day condition of the atmosphere, contrasted with climate, which is the long-term average (over decades) of weather conditions and extremes in a region. Weather is both a “snapshot” of atmospheric conditions and a technical status report of the Earth–atmosphere heat-energy budget. Important elements that contribute to weather are temperature, air pressure, relative humidity, wind speed and direction, and seasonal factors, such as insolation receipt, related to day length and Sun angle.
Meteorology is the scientific study of the atmosphere. (Meteor means “heavenly” or “of the atmosphere.”) Meteo- rologists study the atmosphere’s physical characteristics and motions; related chemical, physical, and geologic pro- cesses; the complex linkages of atmospheric systems; and weather forecasting. Computers handle the volumes of data from ground instruments, aircraft, and satellites used to accurately forecast near-term weather and to study trends in long-term weather, climates, and climatic change.
Weather-related destruction has risen more than 500% over the past three decades as population has increased in areas prone to violent weather and as cli- mate change intensifies weather anomalies. In Canada, research and monitoring of violent weather are centred at Environment Canada, Meteorological Service, www .ec.gc.ca/meteo-weather/. Studies estimate that global annual weather-related damage losses could exceed $1 trillion by 2040.
In this chapter: We follow huge air masses across North America, observe powerful lifting mechanisms in the atmosphere, revisit the concepts of stable and unsta- ble conditions, and examine migrating cyclonic systems with attendant cold and warm fronts. We conclude with a portrait of violent and dramatic weather so often in the news in recent years.
Water, with its ability to absorb and release vast quantities of heat energy, drives this daily drama in the atmosphere. The spatial implications of these weather phenomena and their relationship to human activities strongly link meteorology and weather forecasting to the concerns of physical geography.
Air Masses
Each area of Earth’s surface imparts its temperature and moisture characteristics to overlying air. The effect of a location’s surface on the air creates a homogenous mix of temperature, humidity, and stability that may ex- tend through the lower half of the atmosphere. Such a
flects the characteristics of its source region. Examples include the “cold Arctic air mass” and “moist tropical air mass” often referred to in weather forecasts. The vari- ous masses of air over Earth’s surface interact to produce weather patterns.
Air Masses Affecting North America
We classify air masses according to the general moisture and temperature characteristics of their source regions: Moisture is designated m for maritime (wet) or c for con- tinental (dry). Temperature is directly related to latitude and is designated A for arctic, P for polar, T for tropical, E for equatorial, and AA for Antarctic. Figure 8.1 shows the principal air masses that affect North America in winter and summer.
Continental polar (cP) air masses form only in the Northern Hemisphere and are most developed in win- ter and cold-weather conditions. These cP air masses are major players in middle- and high-latitude weather, as their cold, dense air displaces moist, warm air in their path, lifting and cooling the warm air and causing its va- pour to condense. An area covered by cP air in winter ex- periences cold, stable air; clear skies; high pressure; and anticyclonic wind flow. The Southern Hemisphere lacks the necessary continental landmasses at high latitudes to create such a cP air mass.
Maritime polar (mP) air masses in the Northern Hemisphere sit over the northern oceans. Within them, cool, moist, unstable conditions prevail throughout the year. The Aleutian and Icelandic subpolar low-pressure cells reside within these mP air masses, especially in their well-developed winter pattern (see the January iso- baric pressure map in Figure 6.10a).
Two maritime tropical (mT) air masses—the mT Gulf/ Atlantic and the mT Pacific—influence North America. The humidity experienced in the North American East and Midwest is created by the mT Gulf/Atlantic air mass, which is particularly unstable and active from late spring to early fall. In contrast, the mT Pacific is stable to con- ditionally unstable and generally lower in moisture con- tent and available energy. As a result, the western United States, influenced by this weaker Pacific air mass, re- ceives lower average precipitation than the rest of the country. Please review Figure 6.11 and the discussion of subtropical high-pressure cells.
Air Mass Modification
The longer an air mass remains stationary over a region, the more definite its physical attributes become. As air masses migrate from source regions, their temperature and moisture characteristics slowly change to the characteris- tics of the land over which they pass. For example, an mT Gulf/Atlantic air mass may carry humidity to Chicago and on to Winnipeg, but it gradually loses its initial high hu- midity and warmth with each day’s passage northward.
Similarly, below-freezing temperatures occasion- ally reach into southern Texas and Florida, brought by