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

Chapter 2Where We’re Headed: Solar Energy,

Seasons and the Atmosphere

Our Solar System

Figure 2.1

Earth’s Orbit

• Perihelion – closest to sun January 3– 91,500,000 mi

• Aphelion – farthest at July 4– 94,500,000 mi

Earth’s Energy Budget

Figure 2.8

The Electromagnetic Spectrum

• Sun radiates shortwave & longwave energy

• Earth radiates longwave energy

Solar and Terrestrial Energy

Figure 2.7

Solar radiation peaks in the visible light spectrum

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

Figure 2.9

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

Revolution and Rotation

Figure 2.11

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 http://esminfo.prenhall.com/science/geoanimations/animations/01_EarthSun_E2.html

– North pole points toward the North Star (Polaris)– Thus, the subsolar point (declination) will vary in location

between the Tropic lines.

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.

Axial Tilt and Parallelism

Figure 2.12

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– Subsolar point Equator

Annual March of the Seasons: Know Well!

Figure 2.13

Seasonal Observations

Figure 2.15