The Global Energy Balance A. Ismail Jolboldin B. 20212001 PLANETARY EMISSION TEMPERATURE • Sun emits energy at a rate of Q = 3.87 × 1026 W • The flux of solar energy at the Earth, called the solar constant depends on the distance of the Earth from the Sun, r, and is given by the inverse square law, S0 = Q/4πr2. • The way radiation interacts with an atmosphere depends on the wavelength as well as the intensity of the radiative flux. • Sun emits radiation that is primarily in the visible part of the spectrum, energy flux decreasing toward longer (infrared, IR) and shorter (ultraviolet, UV) wavelengths. Why does the spectrum have this pattern? • Such behavior is characteristic of the radiation emitted by incandescent material, as can be observed for example in a coal fire. • The hottest parts of the fire are almost white and emit the most intense radiation, with a wavelength that is shorter than that coming from the warm parts of the fire, which glow red. • Incandescence is a natural phenomenon in which a hot object emits electromagnetic radiation, including visible light, due to its high temperature. This process occurs because as an object gets hotter, the thermal energy of its atoms and molecules increases. When this thermal energy reaches a certain level, it causes the electrons within the atoms to become excited and move to higher energy levels. • As these excited electrons return to their lower energy levels, they release energy in the form of photons, which are packets of electromagnetic radiation. The energy of these photons can fall within the visible light spectrum, and this is what we perceive as incandescent light. The specific color and intensity of the emitted light depend on the temperature of the object. Why does the spectrum have this pattern? • Experiment and theory show that the wavelength at which the intensity of radiation is maximum, and the flux of emitted radiation, depend only on the temperature of the source. Note that the hotter the radiating body, the more energy it emits at shorter wavelengths. Flux: refers to the flow or transfer of a quantity through a surface, a unit area, or a unit volum Blackbody: which is an idealized substance that absorbs all incident radiation and emits radiation across a continuous range of wavelengths or frequencies. Albedo The ratio of reflected to incident solar energy is called the albedo, -In equilibrium, the total terrestrial flux radiated to space must balance the solar radiation absorbed by the Earth- Earth intercepting the solar energy flux and radiating terrestrial energy. If at the location of the (mean) Earth orbit, the incoming solar energy flux is S0 = 1367 Wm−2, then, given that the cross-sectional area of the Earth intercepting the solar energy flux is πa2, where a is the radius of the Earth Albedo • If in total the spinning Earth radiates in all directions like a blackbody of uniform temperature Te (known as the emission temperature of the Earth) the Stefan-Boltzmann law gives: -> Earth Emission temp = 255K
