Page 98 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

62 part I The energy–Atmosphere system
13. What is the solar constant? Why is it important to know?
14. Study the graph for Winnipeg at 50° N latitude on Figure 2.9. How do the trends of daily insolation throughout the year compare with those at the North and South Poles?
15. If Earth were flat and oriented at right angles to incoming solar radiation (insolation), what would be the latitudinal distribution of solar energy at the top of the atmosphere?
■ Define solar altitude, solar declination, and daylength, and describe the annual variability of each—Earth’s seasonality.
The angle between the Sun and the horizon is the Sun’s altitude. The Sun’s declination is the latitude of the subsolar point. Declination annually migrates through 47° of latitude, moving between the Tropic of Cancer at about 23.5° N (June) and the Tropic of Capricorn at about 23.5° S latitude (December). Seasonality means an an- nual pattern of change in the Sun’s altitude and chang- ing daylength, or duration of exposure. Daylength is the interval between sunrise, the moment when the disk of the Sun first appears above the horizon in the east, and sunset, that moment when it totally disappears below the horizon in the west.
Earth’s distinct seasons are produced by interactions of revolution (annual orbit about the Sun) and rotation (Earth’s turning on its axis). As Earth rotates, the boundary that di- vides daylight and darkness is the circle of illumination. Other reasons for seasons include axial tilt (at about 23.5° from a perpendicular to the plane of the ecliptic), axial parallelism (the parallel alignment of the axis throughout the year), and sphericity.
Earth rotates about its axis, an imaginary line extend- ing through the planet from the geographic North Pole to the South Pole. In the Solar System, an imaginary plane touching all points of Earth’s orbit is the plane of the eclip- tic. The Tropic of Cancer parallel marks the farthest north the subsolar point migrates during the year, about 23.5° N latitude. The Tropic of Capricorn parallel marks the far- thest south the subsolar point migrates during the year, about 23.5° S latitude. Throughout the march of the seasons, Earth experiences the December solstice, March equinox, June solstice, and September equinox (illus- trated in Geosystems in Action). At the moment of the December solstice, the area above the Arctic Circle at about 66.5° N latitude is in darkness for the entire day. At the June solstice, the area from the Antarctic Circle to the South Pole (66.5°–90° S latitude) experiences a 24-hour period of darkness.
altitude (p. 52) declination (p. 52)
16.
17.
18. 19.
20. 21. 22.
23.
daylength (p. 52)
sunrise (p. 52)
sunset (p. 52)
revolution (p. 53)
rotation (p. 53)
axis (p. 53)
circle of illumination (p. 53) axial tilt (p. 54)
plane of the ecliptic (p. 54) axial parallelism (p. 54) Tropic of Cancer (p. 55) Tropic of Capricorn (p. 55) December solstice (p. 55) Arctic Circle (p. 55) March equinox (p. 55) June solstice (p. 55) Antarctic Circle (p. 55) September equinox (p. 55)
Assess the 12-month Gregorian calendar, with its months of different lengths, and leap years, and its relation to the annual seasonal rhythms—the march of the seasons. What do you find?
The concept of seasonality refers to what specific phenomena? How do these two aspects of seasonality change during a year at 0° latitude? At 45°? At 90°? Differentiate between the Sun’s altitude and its dec- lination at Earth’s surface.
For the latitude at which you live, how does day- length vary during the year? How does the Sun’s altitude vary? Does your local newspaper publish a weather calendar containing such information?
List the five physical factors that operate together to produce seasons.
Describe Earth’s revolution and rotation and differ- entiate between them. DefineEarth’spresentaxialtilt—whatisthe
angle? Does the axial tilt change as Earth orbits
about the Sun?
Describe seasonal conditions at each of the four key seasonal anniversary dates during the year. What are the solstices and equinoxes, and what is the Sun’s declination at these times?
Answer for Critical Thinking 2.2: Hypothetically, if Earth were tilted on its side, with its axis parallel to the plane of the ecliptic, we would experience a maximum variation in seasons worldwide. In contrast, if Earth’s axis were perpendicular to the plane of its orbit—that is, with no tilt—we would experience no seasonal changes, with something like a perpetual spring or fall season, and all latitudes would experience 12-hour days and nights.
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