1. Chapter 2 Where We’re Headed: Solar Energy, Seasons and the Atmosphere

2. Our Solar System
Figure 2.1

3. Earth’s Orbit Perihelion – closest to sun January 3
91,500,000 mi
Aphelion – farthest at July 4
94,500,000 mi

4. Earth’s Energy Budget
Figure 2.8

5. The Electromagnetic Spectrum
Sun radiates shortwave & longwave energy
Earth radiates longwave energy

6. Solar and Terrestrial Energy
Solar radiation peaks in the visible light spectrum
Figure 2.7

7. Distribution of Insolation
Tropics receive more concentrated insolation due to the Earth’s curvature. (see next slide)
Annually, tropics receive 2.5x more ins. than poles*, though total hours of daylight per year are the same.
However, in June, more radiation falls in a day on the top of the atmosphere at latitudes like Vancouver, BC than above a similar sized tropical area.
*per unit area, top of atmosphere

8. Figure 2.9

9. Seasonality Caused by changes in intensity & duration of insolation
Key related concepts
Solar altitude – angle above horizon (affects intensity at ground level)
Daylength (duration)
Declination – location of the subsolar point* in degrees of latitude (in tropics)
*latitude where sun is directly overhead at solar noon

10. Revolution and Rotation
Figure 2.11

11. Reasons for Seasons: 
Primary explanation: variations in the duration and intensity of solar radiation received drive the annual march of the seasons. What causes these variations?
Tilt of Earth’s axis
Axis is tilted 23.5° from plane of ecliptic
Axial parallelism
Axis maintains alignment during orbit around the Sun animation earth sun relations
North pole points toward the North Star (Polaris)
Thus, the subsolar point (declination) will vary in location between the Tropic lines.

12. Reasons for seasons (cont)
From March 21 to Sept. 22, declination (subsolar pt.) is in northern Hemisphere. Day length is longer the farther north one is on the Earth. For all places north of Tropic of Cancer, solar altitude is highest June 21.
From Sept. 22 to March 21, declination is in southern Hemisphere. Day length is longer the farther south one is on the Earth. For all places south of Tropic of Capricorn, solar altitude is highest Dec. 21.
From Sept 22 to March 21, declination is in the ___ Hem. Day length is longer the farther south one goes.

13. Axial Tilt and Parallelism
Figure 2.12

14. Annual March of the Seasons (know perfectly!!)
Winter solstice ~December 21
Subsolar point Tropic of Capricorn
Spring equinox ~March 21
Subsolar point Equator
Summer solstice ~June 21
Subsolar point Tropic of Cancer
Fall equinox ~September 22

15. Annual March of the Seasons: Know Well!
Figure 2.13

16. Seasonal Observations
Figure 2.15