ENERGY is typically defined as the ability to do the work. According to the First Law of Thermodynamics, energy is always conserved, so it cannot be destroyed nor created, it only can be transformed from one form to another. Examples of different energy forms include thermal, radiant, potential, and kinetic energy. In particular, thermal energy or heat is energy transported across the system boundary by temperature difference. The energy transfer in the Earth's atmosphere defines the energy cycle of the planet. The fundamental mechanisms for energy transport are radiation, convection, conduction, and evaporation/condensation.
Radiation is energy transfer due to the oscillations of electromagnetic fields or photons that have properties of both particles and waves. The properties of radiation are defined by its wavelength (X), which represents a distance between two successive wave crests. Atmospheric science is usually concerned with wavelengths that fall into ultraviolet, visible, and infrared parts of the electromagnetic spectrum. All objects radiate energy, as long as their temperature is higher than absolute zero. The Stefan-Boltzmann law defines the total emitted energy in terms of a power law based on the object's surface temperature. Furthermore,
Plank's law defines the inverse proportion between the surface temperature and the peak wavelength, which is characterized by maximum energy output. Typical wavelengths for the Sun's radiation are in the range of 0.15- 3.0|im, which is called short-wave or solar radiation, while Earth emits radiation in the wavelength range of 3.0-100|im, which is called longwave radiation. The incoming solar radiation first encounters the atmosphere, which reflects, absorbs, and transmits this energy to the Earth's surface. The composition of the atmosphere plays an important role in reflectivity, absorptivity, and transmissivity. The atmosphere also emits energy to the surrounding environment, which is characterized as atmospheric emissivity.
Convection, also know as advection, represents heat transfer via bulk, or microscopic, motion of fluids, which can be liquids or gases. There is no convection in solids. For convection in the vicinity of solid surfaces, a boundary layer forms at the interface to reduce the velocities of the unobstructed fluid motion to the velocity of the solid surface, which is typically zero. Also, a temperature boundary layer forms in the vicinity of solid surfaces that have temperatures different than the bulk fluid. The connective transport due to the different densities of fluid is called natural convection. An example of natural or free convection would be vertical movement of the air masses heated at the warm Earth's surface, which receives energy via solar radiation. When free convection is the dominant motion for the air mass, the atmosphere is called unstable. A stable atmosphere has forced convection to take over the fluid motion. This forced motion is mechanically-induced by terrain roughness or pressure differences, and the atmosphere has a predominantly horizontal motion. In nature, the most typical mode of convection is mixed convection, which combines natural and forced convective heat transfer regimes.
Conduction is associated with random molecular motion. It represents an effective heat-transfer mechanism for solids, because of short molecular distances, while it is less effective for fluids and gases. For conduction, there is no bulk motion of molecule characteristics as for convection. Fourier's Law describes this process by relating the total heat transfer to temperature difference and the distance or length of the system under consideration. Conduction is important for heat transfer beneath the Earth's surface, but for atmospheric science conduction is negligible, except at the very thin laminar sub-layer in direct contact with the Earth's surface.
The evaporation/condensation process is a heat-transfer mechanism that involves a phase change of the matter in the system. Evaporation occurs when energy is added to the system during the heat transfer process, while condensation is characterized by energy release to the surrounding environment. The total energy necessary for the phase change from fully-saturated liquid to fully saturated vapor is called latent heat, and represents material property dependent on the temperature and pressure of the system. For example, the latent heat would be different for the same matter, such as water, at different elevations in the Earth's atmosphere. Furthermore, the water cycle is defined by the water evaporation at the ocean surface and condensation that forms clouds and rain. Another important contributor to the water cycle is plant life that releases water vapor to the atmosphere by evapotranspiration, because it combines evaporation and transpiration.
sEE ALso: Condensation; Radiation, Absorption; Radiation, Infrared; Radiation, Long Wave; Radiation, Short Wave; Radiative Feedbacks.
bibliography. Rodney Cotterill, Biophysics: An Introduction (Wiley, 2002); Uri Haber-Schaim, et al., Forces, Motion, and Energy (Science Curriculum Inc., 2002); Philip Nelson, Biological Physics: Energy, Information, Life (W. H. Freeman, 2003).
Jelena Srebric Pennsylvania State University
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