subsolar point= zenith= point directly beneath sun (where sun's rays are perpendicular to the Earth's surface). This point migrates between the Tropic of Capricorn and the Tropic of Cancer.
declination= angular distance between equator and subsolar point.
Similar to figure 5.8 in text - shows contours of energy received at different latitudes and times of the year. Note that poles get little energy; most energy is at the equator. Why is there more energy received in the southern summer than the northern summer? Because the distance to the sun is different at those two times. Remember Earth is closer to the sun during the northern hemisphere winter.
More on the sun's energy
Most of the sun's energy is in the form of light. Visible light is part of the electromagnetic spectrum. Electromagnetic energy is transmitted through waves made of an electrical field interacting with a magnetic field. This wave has a wavelength and an amplitude. The sun's energy is strongest in the blue visible light wavelength.
breakdown of wavelengths in sun's energy: 47% visible light, 45% infrared, 8% short-wave radiation (ultraviolet, x-rays, gamma rays). Temperature directly influences what wavelengths the sun emits energy in. The surface temperature of the sun is about 6000 Kelvins whereas the temperature of the Earth's surface is about 280 Kelvins. So you'd expect the radiation from Earth to be much lower energy, longer wavelength radiation.
The atmosphere reduces the amount of incoming radiation, especially in certain wavelengths. When you compare the amount of incoming energy measured at the thermopause, or the top of Earth's atmosphere, you find it is greater at every wavelength. The average insolation at the thermopause is about 1372 W/m2 (Watts/meter2)=about 2 calories/min./cm2=2 Langleys/min. And to think that only 2 billionths of the Sun's energy is intercepted by Earth!
aside: how else does the Earth emit energy besides electromagnetic radiation?
composition: N2 78%, O2 20.9%, and Ar 0.93%, plus trace gases. The main gases are also called the constant gases and total 99.83% of the atmosphere. Note that nitrogen and argon are inert while oxygen is not and is important for life. The trace gases include:
An aerosol is a suspended particle of liquid or solid in air.
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