Chapter 2
Where 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 mostly shortwave &
some longwave energy
• Earth radiates longwave energy
Solar and Terrestrial Energy
Solar
radiation
peaks in the
visible light
spectrum
Figure 2.7
Distribution of Insolation
• Tropics receive more concentrated insolation due
to the Earth’s curvature. (see next slide)
• Annually, tropics receive 2.5x more insolation 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. (long days)
•
*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.
paste into browser one of following:
http://dli.taftcollege.edu/streams/geography/Animations/Eart
hSun.html
http://esminfo.prenhall.com/science/geoanimations/animatio
ns/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
Figure 2.13
Seasonal Observations
Figure 2.15