| Section E | E index | 231-239 of 498 terms |
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energy balance—The balance between the net warming or cooling of a volume and all possible sources and sinks of energy. The main sources and sinks of energy typically include the net fluxes of sensible heat, latent heat, and radiant energy. Conservation of energy requires that the energy received by a surface must equal that lost from the surface plus that stored. For water and land surfaces, the main source of energy is net radiation, which equals the sum of short and long waveband radiation downward minus radiation reflected or emitted upward. This energy is normally transferred into the soil (soil heat flux), into the air (sensible heat flux), or into latent heat flux (evapotranspiration or ET). Small amounts of the incoming energy can change the heat content of water or crops at the surface or are converted to other forms of energy (e.g., photosynthesis). Energy balance is often used to estimate evapotranspiration by 1) measuring net radiation, soil heat flux, and sensible heat flux; 2) entering those values into an energy balance equation; and 3) solving for the latent heat flux (ET). Under hot, dry, windy (advection) conditions, heat from the air in addition to net radiation is sometimes available at an underlying cool surface. Advection can potentially increase evaporation rates to higher than the energy available from net radiation alone. See also surface energy balance.
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energy conversion—(Also energy transformation, energy transfer.) A process in which energy changes from one form to another. Energy is conserved for a system that does not interact with its surroundings, and the total energy of such a system may often be expressed as the sum of energies of different kinds:  Thus if E1 decreases in any process, E2, etc., must increase correspondingly for E to remain constant, and we may say that energy of type 1 has been converted into energies of type 2, 3, etc.
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energy equation—1. Thermodynamic energy equation; same as the first law of thermodynamics. 2. Mechanical energy equation (or kinetic energy equation): an expression for the rate of change of kinetic energy, which is obtained by scalar multiplication of the three-dimensional vector equation of motion by the vector velocity u; it may be written in the form  where φ = gz is the geopotential energy, is density, p is pressure, F is the vector frictional force per unit volume, and ∇ is the del operator. 3. Total energy equation: An expression relating all forms of energy obtained by combining the thermodynamic energy equation with the mechanical energy equation. When integrated over a fixed volume of the atmosphere, this equation takes the form  where dV is the volume element, ds is the element of the surface of the volume, and Vn is the inwardly directed velocity normal to the surface of the volume. This equation expresses the fact that the combined internal, kinetic, and potential energy in a given volume can vary only as a result of 1) the transport of these forms of energy across the boundaries of the volume; 2) the work done by pressure forces on the boundary; 3) the addition or removal of heat; and 4) the dissipational effect of friction.
Gill, A. E., 1982: Atmosphere–Ocean Dynamics, Academic Press, 76–82.
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energy grade line—The line above a datum used for expressing the total energy of a flow.
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energy level—Any of the possible discrete energies of an atom, molecule, or nucleus. According to Newtonian mechanics, the energy of a body is a continuous variable, but according to the quantum theory, measured energies of bound states are discrete, some said to be allowed, others forbidden. We are not aware of this discreteness for macroscopic objects because of the extreme (relative) smallness of the separation between their energy levels. During transitions from one energy level to another, quanta of radiant energy are emitted or absorbed, their frequency depending on the difference between energy levels. Emissions of this type are responsible for the aurora borealis, for example.
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. It is assumed that the total energy
is finite.
