A. The Electromagnetic Spectrum

• "insolation" (in-sol-ation) - incoming solar radiation
• solar energy/radiation is part of the electromagnetic spectrum of radiant energy
• which travels at the speed of light
• it exhibits both electrical and magnetic properties
• all objects emit electromagnetic energy (EMR)
• the EMS is made up of many different forms of electromagnetic energy: radio waves, microwaves, gamma waves, etc.
• 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 (or trough) to the adjacent crest (or trough)

• earth has an elliptical orbit about the sun
• average distance to the sun is ~150 million km
• plane on which the earth orbits about the sun is called the "plane of the ecliptic"
• "perihelion" - Earth is closest to the Sun - January 3
• "aphelion" - Earth is farthest from the Sun - July 4
• the distance to sun accounts for only a 3.4% difference in solar energy receipt
• earth’s orbit changes over a 100,000 yr. cycle, by ~18 million km - may contribute to glacial and interglacial periods

• "seasonality" - a reference to the seasonal variations in the Sun’s rays above the horizon, & the changes in daylength throughout the year
• Sun’s Altitude - the angular distance between the horizon and the Sun
• Declination - the latitude that receives direct (at a 90 degree angle) rays from the sun (migrates between 23.5⁰N & 23.5⁰S)
• Summer Solstice (June 20-21) - @Tropic of Cancer (23.5 N) - Sun’s rays directly overhead
• Winter Solstice (Dec 21-22) -

• @ Tropic of Capricorn (23.5 S) - Sun’s rays directly overhead

• Vernal Equinox ( March 20-21) - Sun’s rays directly overhead at the equator
• Autumnal Equinox (Sept. 22-23) - Sun’s rays directly overhead at the equator
• daylength: varies throughout the year depending on latitude
• @ equator: 12 hrs of daylight & 12 hrs night
• @ 40 N/S: ~6 hours difference in daylight
• @ 50 N/S: ~8 hours difference in daylight
• except for the two equinoxes, away from all latitudes (except the equator) experience uneven day-length throughout the course of the year
• because of the Earth’s curved surface, the angle at which the Earth receives radiant energy from the sun varies with latitude
• the lower latitudes receive rays that are more direct (closer to the perpendicular) than do the higher latitudes
• direct rays provide more heating (more concentrated energy) than do indirect rays

(1) Earth’s revolution & rotation; (2) Earth’s tilt on it’s axis (23.5⁰ from the vertical); and (3) its sphericity
 
 

D. Net Radiation at the Top of the Atmosphere:

(shortwave - longwave) = net radiation
 

• positive values at lower latitudes
• negative values toward the poles
• poleward of ~ 36 degrees N & S lat, values are neg - thus, a net loss in energy !!
• thus, we have an overall imbalance in radiant energy from the equator to poles!!
• This leads to atmospheric & oceanic circulations !!