Chapter 3, Part I
Seasonal Temperatures
Differences Summer & Winter
•
•
•
•
Warmer in summer
Days are longer in summer
Sun is more vertical (overhead) in summer
Why?
Earth’s Orbit
Note that we are actually closer to the sun in the winter,
but 147 million km is approximately 152 million km.
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What causes seasons?
• Amount of solar energy received at surface.
• This is determined by
– the angle of the sun in the sky
– the length of a day.
• Both of these are due to the 23.5o tilt of the
earth as it goes around the sun.
Orbit and Tilt
North. Hemisphere Day Length
Latitude
0
10
30
50
70
90
Vernal Summer Autumn
Equinox Solstice Equinox
Mar. 20 June 21 Sept. 22
12 hr
12 hr
12 hr
12 hr
12.6 hr
12 hr
12 hr
13.9 hr
12 hr
12 hr
16.3 hr
12 hr
12 hr 2 months 12 hr
12 hr 6 months 12 hr
Winter
Solstice
Dec. 21
12 hr
11.4 hr
10.1 hr
7.7 hr
0 hr
0 hr
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Insolation & Tilt – Part 1
• Insolation = incoming solar radiation.
More vertical implies a brighter spot.
Insolation and Tilt – Part 2
Radiation in the far northern
lattitudes must pass through
more atmosphere to reach the
earth’s surface.
Insolation at Winter Solstice
• Because of both the day length and the
angle of the sun in the sky, at winter solstice
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Energy Balance
Incoming
Radiation
Outgoing
Radiation
Local Seasonal Variations
Summary
• Seasonal variations are due to the 23.5o tilt
of the earth.
• The tilt leads to changes in both day length
and the angle of the sun in the sky.
• A more vertical sun in the sky has more
incoming solar radiation.
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