Solar Radiation and the Seasons

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Solar Radiation and the Seasons

• 2) The causes of the Earth seasons

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A few considerations about the Sun

• The Sun surface is extremely hot 5800K
• The electromagnetic energy is not depleted as it
  propagates towards the earth
• However as it moves away from the sun it is
  distributed over a greater area, which reduces
  its intensity (that is, the intensity of the
  solar radiation is less over Earth than it is
  over Mercury, for instance)

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From Stefan-Boltzman Law Es T4 (w/m2)
E6.4x107 Watts per
m2
In other words, to know the total solar emission
we need to calculate E for the entire sun!!!
The total solar emission E x Sun area E x
4pR2sun Where Rsun 6.955x108m Total solar
emission 3.865 x 1026W
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Imagine a sphere completely surrounding the Sun
whose radius is equal the mean distance Earth-Sun
1.5x1011m
As the distance from the Sun increases the
intensity of the radiation decreases in the
proportion of the distance squared (the inverse
square law) The incoming radiation
SOLAR CONSTANT
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FOR THE SAKE OF COMPARISON
The solar constant for Mars (2.25x1011 m from
the Sun) 608 W/m2 For Earth 1367 W/m2
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Causes of Earths Season
Earths orbit around the Sun is not perfectly
circular but is an ellipse. Earth is nearest to
the Sun (perihelion) on about January 4 and
farthest away (aphelion) on July 4. These
different represent only about 3 in terms of
distance and 7 in terms of changes in the
intensity of radiation . THESE DIFFERENCES DO
NOT CAUSE SEASONS ON EARTH

• interesting sites with movies
• http//astro.unl.edu/naap/motion1/animations/seaso
  ns_ecliptic.swf
• http//www.youtube.com/watch?vDuiQvPLWziQ

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So, what is the big deal about the solar
constant?

• Wellsolar variability has been considered, a
  priori, to be an external forcing factor for
  climate, although it remains a controversial
  mechanism of climate change, across all time
  scales.
• At times of high sunspot number, the value of the
  solar constant increases. Normally a change is
  less than 1 over the cycle.
• However, there have been periods in the past
  history of our planet when changes in the solar
  activity have been related to the decrease of
  temperature of our planet (such as the little ice
  age)

If you want to learn more about this issue and
global climate change go to http//www.ace.mmu
.ac.uk/resources/gcc/2-5-3.html
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So, what causes seasons!!!!Lets think of an
extreme example
Around July 22 the entire SH would be dark 24
hours - that would be summer in the NH (24 light
90o north of the equator)
If Earth was tilted 90 deg with respect to the
plane of its orbit (Ecliptic)
Around Dec 22 90o North of the Equator would be
darkness for 24 hours - 24h or incoming solar
radiation in the SH (24h light south of the
equator)
During the Equinox March 21, incoming solar
radiation would be equal in the entire planet
(12h light 12h dark)
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Imagine you living in Southern CA of this planet.
..
End of Winter
Mid summer
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How would be the elevation of the Sun in Southern
CA during the NH Summer solstice?
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12 hours later
30
30
Equator
NP
30
60
90
30
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And how would likely be the sunset in CA during
summer
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In summary

• The tilting of the axis of our planet is
  responsible for seasons
• Larger tilting larger differences in seasons
  for higher latitudes

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Our actual planet tilt of the axis 23.5o
As of today, only north of 90.0-23.5 66.5o is
completely dark (24h) by December 21-22 On the
other hand, south of -66.5 is 24h with sun light
Equinox March 22-23, incoming solar radiation
is equal in both hemispheres (12h light 12h
dark)
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Solar declination the latitudinal position of
the subsolar point can be identified as the
latitude at which the noontime Sun appears
overhead

Equator
-
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Equinox
Which guy would see the sun at the Zenith (top
of his head) during the equinox at noon?
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NH summer solstice
Which guy would see the sun at the Zenith (top
of his head) during the NH summer solstice at
noon?
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This is to have an idea how is the sunset north
of 66.5 during NH summer (or south of -66.5
during SH summer)
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NH winter solstice
Which guy would see the sun at the Zenith (top
of his head) during the SH summer solstice?
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In conclusion The tilt of the Earth rotation
axis influences the energy received on Earth by
its effect on length of day and night

• So, in June all latitudes poleward of 66.5N
  (arctic circle) receives 24 hours of continuous
  daylight.
• Moving toward the equator the period of daylight
  decreases until reaching the equator where the
  daylight period is 12 hours.
• In December, poleward of 66.5N is 24 hours of
  continuous darkness

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Solar Elevation Angle
It is certainly noticeable the warming when you
go outside and the sun is very high in the sky or
when it is low in the sky
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This is because of the beam spreading
The same beam is spread in a larger area the
greater the spreading, the less intense radiation
is
Incoming radiation is received at 90o angle
More obliquely same radiation distributed to a
larger area less energy/area
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What would be the solar elevation angle at noon
during the Equinox ?
30o (from Zenith)
Latitude 30N
West
North
South
30
Angle60o
90 latitude solar declination 90 -30 0
60o
East
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What would be the solar elevation angle at noon
during the SH solstice?
53.5o (from Zenith)
Latitude 30N
West
North
South
30 23.5
Elevation Angle36.5o
90 latitude solar declination 90 30 -23.5
36.5o
East
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Third way in which the tilt of the axis
influences heating is in determining the amount
of atmosphere that sunlight must penetrate before
reaching surface
The greater the thickness of the atmosphere the
more the beam is weakened by reflecting back the
light, sometimes absorption by particles in the
air
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Summary What cause seasons?
June Solstice Equinoxes December solstice
Solar declination23.5o Every latitude in the NH receives more energy than the corresponding latitude in the SH (Cause less beam spreading and depletion) Every place north of the Arctic Circle (south of the Antarctic circle ) receives 24 hours daylight (night) The equator receives 12h of daylight For the NH as a whole, sunlight travels through less atmosphere than it does in the SH Solar declination0o Every place on Earth receives 12 hours of daylight Both hemispheres receive equal amounts of insolation Solar declination-23.5o Every latitude in the SH receives more energy than the corresponding latitude in the NH Every place South of the Antarctic Circle (north of the Arctic circle ) receives 24 hours daylight (night) The equator receives 12h of daylight For the SH as a whole, sunlight travels through less atmosphere than it does in the NH
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Quizzes for fun
C
B
A
Ecliptic plane
Rotation axis

1. In which situation you would experience the most
   severe seasonal variability?
2. What would be the length of daylight on December
   21 at the North Pole in Situation A?
3. What would be the length of daylight in
   California on your birthday in situation A?
4. What would be the length of the daylight at the
   Equator on your birthday in situation C?

C
12h
12h
12h
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Quizzes for fun
B
Ecliptic plane
Rotation axis

1. If the solar declination were 10o, where would we
   find the Arctic and Antarctic Circles? Would this
   cause a change in the dates of solstices,
   equinoxes, perihelion and aphelion ?
2. What should be de tilt of the Earth for the Sun
   to be overhead in Santa Barbara- CA during the NH
   summer solstice?

(90o-10o80o S or N)
(tilt should be equal to the latitude of SB
34.41o)
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Explanation for the 12 h daylight at the equator
with different tilts
Earth Axis
Earth Axis
Earth Axis
SUN
12h Light
12h Light
12h Dark
12h Light
12h Dark
12h Dark
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If you have curiosity to find more about
astronomical theory for climate change you can
check at (that are not in the text book)
http//www.ncdc.noaa.gov/paleo/milankovitch.html