Construct in Figure 1 annual solar radiation curves for equatorial (0º) and midlatitude (45º N) locations. Do this by drawing a smooth curved line connecting the radiation values already plotted for each location (see map legend to identify plotted symbols).

1. The Figure 1 curves show average daily solar radiation at the top of the atmosphere varies the least over the period of a year at the [(polar)(midlatitude)(equatorial)] location. At that location, the daily period of daylight is 12 hours in length throughout the year.

2. The Figure 1 graph shows that there is a six-month period during which there is no sunlight received at the [(polar)(midlatitude)(equatorial)] location. At this location there is only one period of daylight per year, but it is six months long!

3. Comparison of the three annual radiation curves indicates that the annual range (the difference between the curve’s maximum and minimum) of solar radiation received daily [(decreases)(increases)] as latitude increases.

4. Based on how solar radiation received varies with latitude, it can be inferred that seasonal temperature contrasts would [(decrease)(increase)] as latitude increases.

5. Of the three latitudes for which radiation curves are drawn, the one location that annually experiences two maxima and two minima periods of incoming solar radiation at the top of the atmosphere is the [(polar)(midlatitude)(equatorial)] location.

6. The pattern of sunlight received at the equator over the course of a year indicates that tropical locations [(do)(do not)] experience warm and cold seasons as is characteristic of the higher latitudes.

The variations in the amount of solar radiation received at different latitudes over a period of a year arise because Earth’s axis of rotation is inclined 23.5º from a line normal (perpendicular) to the planet’s orbital plane. This inclination causes changes in the Sun’s path through the local sky. Figure 2 shows the Sun’s path through the local sky at the equator, 45ºN, and the North Pole on or near the dates of the solstices and equinoxes.

Figure 2. Path of Sun through the sky at (A) equator, (B) Northern Hemisphere midlatitudes, and (C) North Pole. A South Pole view would be similar to C but with its summer solstice (highest position) occurring on Dec 21.

Two major factors determine the amount of daily sunlight received at the top of the atmosphere at different latitudes. First, as depicted in Figure 2, is the path of the Sun through the local sky. Note in Figure 2(A) that the Sun’s path is always perpendicular to the horizon and always follows a 180º arc resulting in 12 hours of daylight every day of the year. In Figure 2(B), the Sun’s path is inclined to the horizon so the length of the arc above the horizon changes throughout the year. The longer the length of the Sun’s path above the horizon, the greater the length of daylight. In Figure 2(C) the Sun’s daily path from the spring equinox onward is continuously above the horizon, gradually spiraling upward until the first day of summer. It then gradually spirals downward until the Sun sets on the first day of fall. From the first day of spring to the first day of fall, there are 24 hours of daylight every day.

Second, the maximum altitude the Sun attains above the location’s horizon impacts the amount of energy intercepted by Earth. The greater the Sun’s altitude, the greater the intensity of solar energy arriving on a horizontal surface located at the top of the atmosphere.

7. Figure 2’s depiction of the local sky at the equator shows that the variation in average daily solar radiation over the year, as reported in Figure 1, must be due to changes in the daily [(path of the Sun across the sky)(period of sunlight)].

8. In Figure 1, it can be seen there is a period during the year more than 2 months long when both the midlatitude and polar locations receive more solar radiation on a daily basis than the equator ever does on a daily basis. It can be inferred from Figure 2 that the major factor(s) that make(s) this happen at the North Pole is(are) the [(maximum solar altitude)(daily length of daylight)(both of these)] at that latitude.

9. The same NASA data set from which Figure 1 is drawn shows that the average daily insolation averaged over a year in kWh/m2/day is 10.02 at the equtor, 7.34 at 45º N, and 4.13 at the North Pole. This shows that the midlatitude location receives about 73% as much solar energy as does the equator and the North Pole receives [(23%)(32%)(41%)] as much solar energy as does the equator over the period of a year. This non-uniform receipt of energy sets the stage for Earth’s weather and climate.

10. Seasonal weather contrasts at the middle and higher latitudes can be inferred by comparing insolation values during the months centering on the summer and winter solstices. As seen from the midlatitude curve in Figure 1, the June/July average top of the atmosphere insolation is about 11.4 kWh/m2/day while the December/January average is about 3.1 kWh/m2/day. This indicates that during the June/July period the midlatitude location receives about [(0.3 times)(the same amount as)(3.7 times)] the amount of top-of-the atmosphere solar energy received during December/January.

Fundamental to Earth’s interception of solar radiation so essential to weather, climate, and life itself is Earth’s rotation and its orbital relationship to the Sun. As described earlier and shown in Figure 3, the inclination of Earth’s axis to the plane of its orbit about the Sun brings about the changes in the path of the Sun through the local sky. Because Earth’s rotational axis remains in the same orientation relative to the stars (i.e., the North Pole steadily points to the North Star), it therefore changes its position relative to the Sun’s rays as it travels its orbit. Twice a year (on the vernal and autumnal equinoxes) the axis is aligned perpendicular to the Sun’s rays. On the winter and summer solstices, it is most inclined to the Sun’s rays. On the Northern Hemisphere’s summer solstice, the North Pole is at its most tipped position towards the Sun; on the winter solstice it is in its most tipped position away from the Sun.

Figure 3. Earth’s orbital relationship to the Sun on the solstices and equinoxes.

Examine the three visible satellite images in Figures 4, 5, and 6 These are actual images which were obtained on or near the first days of the Northern Hemisphere’s fall, winter and summer seasons. Next, examine the small drawing to the right of each Earth image. The drawing shows the relative positions of Earth, the satellite, and rays of sunlight at the time each image was recorded. (In the small drawing, the view is from above Earth’s Northern Hemisphere.) If you were located on the satellite, you would have seen the same view of Earth as shown in each accompanying satellite image.

Figure 4. Visible satellite image for 23 September.

Figure 5. Visible satellite image for 21 December.

Figure 6. Visible satellite image for 21 June.

The Earth images in Figures 4, 5, and 6 were acquired when sunset was occurring at the point on the equator directly below the viewing satellite (in the center of the Earth’s disk). Sunset was occurring along the dashed line passing through the sub-satellite point. The arrows to the left in each image represent incoming rays of sunlight at different latitudes. When answering the following questions, ignore the effects of Earth’s atmosphere on the Sun’s rays.

11. Look at Figure 4, the 23 September image. Note that the Earth’s axis is perpendicular to the Sun’s rays, so the sunset line and Earth’s axis line up together in the perspective shown. Also note that each latitude line, including the equator, is half in sunlight and half in darkness. Because the Earth rotates once in 24 hours, the period of daylight is [(0)(6)(12)(18)(24)] hours everywhere except right at the poles.

12. Now look at Figure 5, the 21 December satellite image. On the Northern Hemisphere’s winter solstice, the Earth’s North Pole reaches its maximum tilt away from the Sun for the year. Consequently, poleward from the Arctic circle, the daily period of daylight is [(0)(6)(12)(18)(24)] hours.

13. Examine the Northern Hemisphere latitude lines in the 21 December satellite image and compare how much of each line is in sunlight with the amount that is in darkness. The comparison shows that at all latitudes in the Northern Hemisphere of the rotating Earth, the daily period of daylight is [(greater than)(equal to)(less than)] the daily period of darkness.

14. Poleward from the Antarctic Circle on 21 December, the daily period of daylight is [(0)(6)(12)(18)(24)] hours. This is the time of the South Pole’s maximum tilt towards the Sun for the year.

15. Now look at Figure 6, the 21 June satellite image. This shows that on the Northern Hemisphere’s summer solstice, Earth’s North Pole attains it maximum tilt towards the Sun for the year. Consequently, poleward from the Arctic Circle, the period of daylight is [(0)(6)(12)(18)(24)] hours.

16. Poleward from the Antarctic Circle on 21 June, the period of daylight is [(0)(6)(12)(18)(24)] hours.

Along with these variations in the length of daylight at various latitudes as shown in the satellite views, the intensity of incoming sunlight varies with the angle of incidence of the Sun’s rays striking Earth’s surface. (You could add a latitude line on the satellite views at your location along with a solar ray to visualize the angle of the incoming solar rays.) Thus, the solar energy received at a location over the course of the year depends on the varying solar altitude (angle of the Sun above the horizon) and the period of daylight at that location.