PPT09 - Variation in Solar Radiation

PPT09 - Variation in Solar Radiation - Factors controlling...

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Unformatted text preview: Factors controlling the Earths mean temperature Which of these factors influence the mean temperature of the Earths surface? A. The size and/or temperature of the Sun B. The radius of the Earth orbit C. The Earths cloud cover D. The size of Earth E. The concentration of water vapor in the Earths atmosphere Flux of solar radiation (W/m 2 ) emitted by the Sun F = T 4 (Stefan-Boltzmann's law) Total energy emitted by the Sun per unit time (W) = Energy emitted per surface area (W/m 2 ) x surface area (m 2 ) The size and temperature of the sun is important as they dictate the total amount of solar energy emitted by the sun per unit time The flux of solar radiation emitted by the Sun depends on its temperature and is calculated using Stefan-Boltzmanns equation With a temperature of 5785K, we find that each square meter of the Suns surface emits 63.5 106 joules of energy every second (or 63.5 106 Watts) A colder sun would emit less energy per unit surface area. The flux of solar radiation (W/m2) reaching Earth depends on the distance between Earth and the sun total amount of energy initially emitted by the suns surface is spread over a larger area as we move away from the Sun 30% of the incoming solar radiation is directly reflected back to outer space.. Earths albedo = fraction of incoming solar radiation that is reflected back to space In order to calculate the mean temperature of Earth, we must also know how much of it is actually absorbed by the Earth. This is because only solar radiations absorbed by Earth warm the planet It turns out that a fraction (30%) of the incoming solar radiations is directly reflected back to outer space and does not contribute to warming the planet. This fraction called the Earths albedo ~ 26% of incoming radiation reflected back to space by clouds.. A large contribution to the Earths albedo comes from the clouds, which reflect 26% of the incoming radiation Increasing cloud cover would increase the Earths albedo and lower the flux of solar radiation reaching the Earths surface A bigger planet has a larger cross section which intercepts a larger amount of solar energy.. .. but this solar energy is also spread over a larger surface area and the two effects cancel each other. (Connects to lecture 8 pg 13) Te = -18C; Ts = +15C Water vapor is an important contributor to greenhouse warming.. As such water vapor in the atmosphere is an important contributor to greenhouse warming which result from the trapping of IR radiations emitted by Earth surface and result in surface temperatures significantly higher than in the upper atmosphere In summary.. Earths mean temperature is controlled by: Solar radiations reaching Earth (Solar Constant) Size & temperature of the Sun Distance between Earth and Sun Solar radiations absorbed by Earth Albedo Atmospheric greenhouse gases concentration If either of these three factors changes in the course of Earths history, that will change...
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PPT09 - Variation in Solar Radiation - Factors controlling...

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