A. Earth-Sun Relationships
(1). Radiation and the Electromagnetic Spectrum
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solar energy/radiation is part of the electromagnetic spectrum of radiant
energy
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which travels at the speed of light
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it exhibits both electrical and magnetic properties
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all objects emit electromagnetic energy (EMR)
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the EMS is made up of many different forms of electromagnetic energy: radio
waves, microwaves, gamma waves, etc.
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EMR travels in waves and does not need molecules to propogate them - thus,
they can move through a vacuum (i.e. space)
they can be characterized by their wavelength - the distance
measured along a wave from one crest (trough) to the adjacent crest (trough)
Electromagnetic Spectum
Electromagnetic Spectum
(2) . Earth's Orbit
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earth has an elliptical orbit about the sun
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average distance to the sun is ~ 150 million km
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plane on which the earth orbits about the sun is called the "plane
of the ecliptic"
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"perihelion" - Earth is closest to the Sun - January 3
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"aphelion" - Earth is farthest from the Sun - July 4
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only 3.4% difference in solar energy receipt
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earth's orbit changes over a 100,000 yr. cycle, by ~ 18 million km - may
contribute to glacial and interglacial periods
Earth's Orbit
(3). Direct Versus Indirect Rays from the Sun
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direct rays provide more heating (more concentrated energy) than
do indirect rays
why ? ..... (1) Energy is spread over a larger surface area; and
(2) incoming solar radiation (insolation) must pass through more atmosphere
Direct vs. Indirect Insolation
Rays Must travel Through More Atmosphere
(4). Reasons for the Seasons
Terms and Important Dates and Geographic Locations:
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Seasonality - a reference to the
seasonal
variations in the Sun's rays above the horizon, & the changes
in daylength throughout the year
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Insolation - incoming solar radiation
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Sun's Altitude - the angular distance between the horizon
and the Sun
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Declination - the latitude that receives direct (at a 90
degree angle) rays from the sun (migrates between 23.50N &
23.50S)
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Summer Solstice (June 20-21) - @Tropic of Cancer (23.5 N)
- Sun's rays directly overhead
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Winter Solstice (Dec 21-22) - @ Tropic of Capricorn (23.5
S) - Sun's rays directly overhead
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Vernal Equinox ( March 20-21) - Sun's rays directly overhead
at the equator
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Autumnal Equinox (Sept. 22-23) - Sun's rays directly overhead
at the equator
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Daylength varies throughout the year depending on latitude
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except at the equator and on the two equinoxes, all latitudes experience
uneven daylength throughout the course of the year
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@ equator: 12 hrs of daylight & 12 hrs night
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@ 40 N/S: ~6 hours difference in daylight
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@ 50 N/S: ~8 hours difference in daylight
Changes in Daylength
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Altitude (sun angle above the horizon) changes as well.
What causes the changes in daylength and altitude?
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They occur because Earth's orientation to the Sun continually changes
as it travels along its orbit.
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because of the Earth's curved surface, the angle at which the Earth receives
radiant energy from the sun varies with latitude
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the lower latitudes receive rays that are more direct (closer to the perpendicular)
than do the higher latitudes
Summary of the Physical Factors Responsible for Seasonality:
(1) Earth's revolution & rotation; (2) Earth's tilt on it's axis (23.50
from the vertical); and (3) its sphericity.
Earth-Sun Relationships
(5). Net Radiation At the Top of the Atmosphere
incoming - outgoing = net radiation
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have positive values of net radiation at lower latitudes
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negative values toward the poles
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poleward of ~ 36 degrees N & S lat, values of net radiation are negative
- thus, a net loss in energy !!
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thus, we have an overall imbalance in radiant energy from the equator
to poles!!
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this leads to atmospheric & oceanic circulations !!
Energy Budget by Latitude