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Terminology of Radiation Balance

Updated on January 17, 2014

Radiation Budget: The temperature of the Earth’s surface, atmosphere and oceans and the rate of change of those temperatures depends on the detailed balance of energy fluxes; commonly referred to as the ‘Radiation Budget/Balance’,

Energy balance: The difference between the total incoming and total outgoing energy. If this balance is positive, warming occurs; if it is negative, cooling occurs.

Radiation: Radiation is the propagation and emission of energy in the form of electromagnetic waves. The types of radiation include, light, heat and sound.

Short-wave Radiation: Shortwave radiation (SW) is the radiant energy with wavelengths in the visible (VIS), near-ultraviolet (UV), and near-infrared (NIR) spectra. In broad sense we may defined it to all radiation with a wavelength between 0.1μm and 5.0μm.

Long-wave Radiation: Longwave radiation is the infrared energy emitted by the earth and atmosphere at wavelengths between about 5 and 25 micrometers.

Wavelength: Forms of electromagnetic radiation like radio waves, light waves or infrared (heat) waves make characteristic patterns as they travel through space. Each wave has a certain shape and length. The distance between peaks (high points) is called wavelength.

Solar Radiation: Solar radiation is the electromagnetic radiation emitted by the Sun. Almost all known physical and biological cycles in the Earth system are driven by the solar radiation reaching the Earth.

Solar Spectrum: The distribution of solar radiation as a function of the wavelength is called the solar spectrum, which consists of a continuous emission with some superimposed line structures.

Solar Constant: The solar constant is the amount of solar radiation received outside the Earth’s atmosphere on a surface normal to the incident radiation per unit time and per unit area at the Earth’s mean distance from the Sun.

Solar Insolation: The solar insolation is the actual amount of solar radiation incident upon a unit horizontal surface over a specified period of time for a given locality.

Direct Solar Radiation: The radiation coming directly from the Sun received at the Earth’s surface is called direct solar radiation.

Diffuse Solar Radiation: The amount of scattered radiation coming from all other directions is called diffuse solar radiation.

Terrestrial Radiation: Long-wave electromagnetic radiation originating from Earth and its atmosphere. It is the radiation emitted by naturally radioactive materials on Earth including uranium, thorium, and radon.

Net Radiation: Difference in intensity between all incoming energy and all outgoing energy carried by both shortwave and longwave radiation.

Flux: Rate of energy transfer by electromagnetic radiation.

Radiant Flux Density: The radiant flux density is the amount of radiant flux (or power) crossing a unit of area, and is measured in W m-2. Radiant flux density is also known as intensity or irradiance.

Blackbody Radiation: the electromagnetic radiation that would be radiated from an ideal black body; the distribution of energy in the radiated spectrum of a black body depends only on temperature and is determined by Planck's radiation law.

Emissivity: The emissivity of a surface is defined as the ratio of the radiation emitted by the surface to the radiation emitted by a blackbody at the same temperature. Thus,

0≤ ε≤1

Absorptibity: Absorptivity is the fraction of irradiation absorbed by the surface.

Reflectivity: Reflectivity is the fraction of irradiation reflected by the surface.

Transmissivity: Transmissivity is the fraction of irradiation transmitted through the surface.

Radiosity: total radiation energy streaming from a surface, per unit area per unit time. It is the summation of the reflected and the emitted radiation.

Diffuse surface: Diffuse surface is a surface which its properties are independent of direction.

Gray surface: Gray surface is a surface which its properties are independent from wavelength.

Sensible Heat Flux: The flux of heat from the Earth’s surface to the atmosphere that is not associated with phase changes of water; a component of the surface energy budget.

Latent Heat Flux: The flux of heat from the Earth’s surface to the atmosphere that is associated with evaporation or condensation of water vapour at the surface; a component of the surface energy budget.

Soil Heat Flux: The amount of heat flowing into a cross-sectional area of soil per unit time.

The Principle of Conservation of Energy: A principle stating that the total energy of an isolated system remains constant regardless of changes within the system.

Heat Capacity: The ratio of the heat energy absorbed by a substance to the substance's increase in temperature.

Specific Heat: The amount of heat, measured in calories, required to raise the temperature of one gram of a substance by one Celsius degree.

Evaporation: The process by which a liquid becomes a gas.

Evapotranspiration: The combined process of evaporation from the Earth’s surface and transpiration from vegetation.

Condensation: The process by which a gas or vapor changes to a liquid.

Ideal Surface: An ideal solid surface is one that is flat, rigid, perfectly smooth, chemically homogeneous, and has zero contact angle hysteresis.

Albedo: Some of this incoming flux is reflected straight back to space by the atmosphere, the clouds and the Earth’s surface. The fraction of the radiation that is reflected is called the albedo of the Earth or planetary albedo (α). In present-day conditions, it has a value of about 0.3.

Surface Temperature: Surface Temperature is the temperature of the atmosphere which represents the average kinetic energy of the molecular motion in a small region and is defined in terms of a standard or calibrated, thermometer in thermal equilibrium with the air.

Air temperature: The temperature describes the kinetic energy, or energy of motion, of the gases that make up air. As gas molecules move more quickly, air temperature increases.

Bowen Ratio: The ratio of sensible to latent heat fluxes from the Earth’s surface up into the atmosphere.

Horizontal surface: a flat surface at right angles to a plumb line.

Absorption: Absorption is a process by which incident radiant energy is passed to the molecular structure of a substance.

Scattering: Is an atmospheric process where small particles and gas molecules diffuse part of the incoming solar radiation in random directions without any alteration to the wavelength of the electromagnetic energy.

Solar thermal: Solar thermal is the conversion of solar radiation into thermal energy (heat). Thermal energy carried by air, water, or other fluid is commonly used directly, for space heating, or to generate electricity using steam and turbines.

Blackbody: A blackbody is defined as a perfect em itter and absorber of radiation. At a specified temperature and wavelength, no surface can emit more energy than a blackbody.

Irradiance: Irradiance is the power of electromagnetic radiation per unit area (radiative flux) incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter (W/m2).


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