NOTE: terms in definitions that appear in bold italics are also defined in this glossary.

absolute zero

The theoretical temperature at which molecular motion vanishes and a body would have no heat energy; the zero point of the Kelvin and Rankine temperature scales. Absolute zero may be interpreted as the temperature at which the volume of a perfect gas vanishes or, more generally, as the temperature of the cold source that would render a Carnot cycle 100 percent efficient. The value of absolute zero is not estimated to be -273.15° Celsius, -459.67° Fahrenheit, 0° Kelvin, and 0° Rankine.

absorption

The process by which radiant energy is absorbed and converted into other forms of energy. A substance that absorbs energy may also be a medium of refraction, diffraction, or scattering; these processes, however, involve no energy retention or transformation and are to be clearly differentiated from absorption.

advection

1. The process of transport of an atmospheric property solely by the mass motion of the atmosphere; also, the rate of change of the value of the advected property at a given point.

2. Regarding the general distinction (in meteorology) between advection and convection, the former describes the predominantly horizontal, large-scale motions of the atmosphere whereas convection describes the predominantly vertical, locally induced motions.

3. To transport or carry. In air quality, the rate at which particulate matter is transported.

aerosols

Particles, other than water or ice, suspended in the atmosphere ranging in radius from one-hundredth to one-ten-millionth of a centimeter -- or 10² to 10^-3 microns (µ). Aerosols are important as nuclei for the condensation of water droplets and ice crystals, and as participants in various atmospheric chemical reactions. Perhaps most significantly, they absorb solar radiation, then emit and scatter it, influencing the radiation budget of the Earth-atmosphere system, which in turn influences the climate on Earth's surface. Aerosols from volcanic eruptions can lead to a cooling at the surface, which may delay greenhouse warming for a few years following a major eruption.

anomalous

Not usual or regular; abnormal. Difficult to explain or classify.

Antarctic circumpolar current

A current that flows eastward completely around Antarctica. It is caused by the west wind surface drift.

anticyclone

An atmospheric high-pressure closed circulation with clockwise rotation in the Northern Hemisphere, counterclockwise in the Southern Hemisphere, and undefined at the Equator.

anticyclone gyre

A closed circulation system, ranging in size from a few hundred meters to thousands of kilometers; associated with the concentrated center of highest pressure in an anticyclone.

atmospheric window

The spectral region between 8.5 and 11.0 microns where the atmosphere is essentially transparent to longwave radiation.

attenuation

1. Reduction in intensity.

2. The decrease in the magnitude of current, voltage, or power of a signal in transmission between points. Attenuation may be expressed in decibels, and can be caused by interferences such as rain, clouds, or radio frequency signals.

augmentation

The apparent increase in the semidiameter of a celestial body as its altitude increases.

AVHRR (Advanced Very High Resolution Radiometer)

A five-channel scanning instrument that quantitatively measures electromagnetic radiation, flown on NOAA environmental satellites. AVHRR remotely determines cloud cover and surface temperature. Visible and infrared detectors observe vegetation, clouds, lakes, shorelines, snow, and ice.

blackbody

1. An ideal emitter that radiates energy at the maximum possible rate per unit area at each wavelength for any given temperature. A blackbody also absorbs all the radiant energy in the near visible spectrum incident upon it. No actual substance behaves as a true blackbody, although platinum black and other soots rather closely approximate this ideal. However, one does speak of a blackbody with respect to a particular wavelength interval. This concept is fundamental to all the radiation laws, and is to be compared with the similarly idealized concepts of the whitebody and the graybody. In accordance with Kirchhoff's law, a blackbody not only absorbs all wavelengths but emits at all wavelengths and does so with maximum possible intensity for any given temperature.

2. A laboratory device that simulates the characteristics of a blackbody. See blackbody radiator.

blackbody radiation

1. The electromagnetic radiation emitted by an ideal blackbody; it is the theoretical maximum amount of radiant energy of all wavelengths that can be emitted by a body at a given temperature. The spectral distribution of blackbody radiation is described by Planck law and the related radiation laws. If a tiny opening is made into an otherwise completely enclosed space (hohlraum), the radiation passing out through this hole when the walls of the enclosure have come to thermal equilibrium at some temperature will closely approximate ideal blackbody radiation for that temperature.

2. Any physical body absorbs and emits electromagnetic radiation when its temperature is above absolute zero. Planck's law determines the radiant flux of a body at a specific wavelength. In atmospheric chemistry, the calculation involving Earth's blackbody radiation shows that Earth's surface temperature would be below the freezing point of water if it did not have an atmosphere that absorbed some of the outgoing radiation.

blackbody radiator

A hypothetical, ideal radiator that totally absorbs and reemits all energy incident upon it. Actual objects only approach this ideal.

boundary current system

The series of currents that form the eastern and western segments of the oceanic gyres in the three tropical oceans, carrying water poleward in the stronger western boundary currents and toward the equator in the weaker eastern currents.

brightness temperature

A measure of the intensity of radiation thermally emitted by an object, given in units of temperature because there is a proportional correlation between the intensity of the radiation emitted and physical temperature of the radiating body.

brine

A general term for sea water. Sea water that is unusually high in salt content because of evaporation or freezing. Any water with a high content of dissolved salts.

calving

The breaking away of a mass of ice from a glacier, an ice shelf, or an iceberg.

circulation

The water current flow within a large area.

cirrus clouds

A type of cloud composed of ice crystals and shaped in the form of hair-like filaments. It is formed at an altitude of approximately 9700 m.

condensation

Change of a substance to a denser form such as gas to a liquid. The opposite of evaporation.

conduction

The transfer of energy within and through a conductor by means of internal particle or molecular activity and without any net external motion. Conduction is to be distinguished from convection (of heat) and radiation (of all electromagnetic energy).

Coriolis force

An apparent force arising from the fact that Earth turns on its axis. It is an apparent force that makes sense only because Earth is a noninertial frame of reference. Earth's spinning creates a constant centrifugal acceleration in which objects appear to curve because Earth is spherical, with different points on the surface spinning at different speeds. If, instead of being a spherical, rotating planet, Earth were flat, there would be no Coriolis force because all points would spin at the same speed.

current

A horizontal movement of water in a well-defined, established pattern, as in a river or stream. The movement of a definite body of air in a certain direction.

decay

In ocean wave studies, the loss of energy from wind generated ocean waves after they have ceased to be acted on by the wind; this process is accompanied by an increase in length and a decrease in height of the wave.

divergence

A horizontal flow of water outward from a common center or zone, often associated with upwelling.

downwelling

The process of accumulation and sinking of warm surface waters along a coastline. A change of air flow of the atmosphere can result in the sinking or downwelling of warm surface water. The resulting reduced nutrient supply near the surface affects the ocean productivity and meteorological conditions of the coastal regions in the downwelling area.

Ekman convergence

A zone of convergence of warm surface water caused by Ekman transport, creating a marked depression of the ocean's thermocline in the affected area.

Ekman layer

One of the main layers of the troposphere, it is the layer of transition between the surface boundary layer, where shearing stress is constant, and the free atmosphere, where the atmosphere is treated as an ideal fluid in approximate geostrophic balance. Also called spiral layer. Together with the surface boundary layer, it makes up the planetary boundary layer.

Ekman spiral

A theoretical representation that a wind blowing steadily over an ocean of unlimited depth and extent and uniform viscosity would cause, in the Northern Hemisphere, the immediate surface water to drift at an angle of 45° to the right of the wind direction, and the water beneath to drift further to the right, and with slower and slower speeds, as one goes to greater depths.

Ekman theory

See Ekman spiral.

Ekman transport

The net mass displacement of water from one place to another, caused by wind blowing steadily over the surface; the net mass transport is 90° to tie right (in the Northern Hemisphere) of the wind's direction.

El Niño events

An irregular variation of ocean current that from January to March flows off the west coast of South America carrying warm, low salinity, nutrient poor water to the south. It does not usually extend farther than a few degrees south of the equator, but occasionally it does penetrate beyond 12° S displacing the relatively cold Peru Current. The effects of this phenomenon are generally short-lived, and fishing is only slightly disrupted. Occasionally (in 1891, 1925, 1941, 1957-58, 1965, 1972-73, 1976, and 1982-83), the effects are major and prolonged. Under these conditions, sea surface temperatures rise along the coast of Peru and in the equatorial eastern Pacific Ocean and may remain high for more than a year, having disastrous effects on marine life and fishing. Excessive rainfall and flooding occur in the normally dry coastal area of western tropical South America during these events. Some oceanographers and meteorologists consider only the major, prolonged events as El Niño phenomena rather than the annually occurring weaker and short-lived ones. The name was originally applied to the latter events because of their occurrence at Christmas time.

El Niño Southern Oscillation (ENSO)

Interacting parts of a single global system of climate fluctuations, ENSO is the most prominent known source of interannual variability in weather and climate around the world, though not all areas are affected. The Southern Oscillation (SO) is a global scale seesaw in atmospheric pressure between Indonesia and North Australia and the southeast Pacific. In major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SO pattern. Many of the countries most affected by ENSO events are developing countries that depend on their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events can have a global impact. While ENSO is a natural part of Earth's climate system, whether its intensity or frequency may change as a result of global warming is an important concern. See also El Niño events and Southern Oscillation.

electromagnetic radiation

Energy propagated through space or through material media in the form of an advancing disturbance in electric and magnetic fields existing in space or in the media. The term radiation alone is used commonly for this type of energy, although it actually has a broader meaning. Also called electromagnetic energy or simply radiation. See electromagnetic spectrum.

electromagnetic spectrum

The ordered array of known electromagnetic radiations, extending from the shortest cosmic rays, through gamma rays, X-rays, ultraviolet radiation, visible radiation, infrared radiation, and including microwave and all other wavelengths of radio energy. See absorption spectrum. The division of this continuum of wavelengths (or frequencies) into a number of named subportions is rather arbitrary and, with one or two exceptions, the boundaries of the several subportions are only vaguely defined. Nevertheless, to each of the commonly identified subportions there correspond characteristic types of physical systems capable of emitting radiation of those wavelengths. Thus gamma rays are emitted from the nuclei of atoms as they undergo any of several types of nuclear rearrangements; visible light is emitted, for the most part, by atoms whose planetary electrons are undergoing transitions to lower energy states; infrared radiations are associated with characteristic molecular vibrations and rotations; and radio waves, broadly speaking, are emitted by virtue of the accelerations of free electrons (the moving electrons in a radio antenna wire).

emission spectrum

The array of wavelengths and relative intensities of electromagnetic radiation emitted by a given radiator. Each radiating substance has a unique, characteristic emission spectrum, just as every medium of transmission has its individual absorption spectrum.

emissivity

A property of a material, measured as the emittance of a specimen of the material that is thick enough to be completely opaque and has an optically smooth surface.

evaporation

The phase change from liquid water to water vapor.

exponent

The part of a floating-point number specifying the power of ten by which the mantissa should be multiplied. In the common notation, e.g., 3.1E8, the exponent is 8.

exponentially

Expressible or approximately expressible by an exponential function; especially characterized by or being an extremely rapid increase (as in size or extent); an exponential growth rate.

free atmosphere

That portion of Earth's atmosphere above the planetary boundary layer in which the effect of Earth's surface friction on the air motion is negligible, and in which the air is usually treated (dynamically) as an ideal fluid. The base of the free atmosphere is usually taken as the geostrophic wind level. Also called free air.

freezing point

The temperature at which a substance in liquid form freezes, equal to the temperature at which its solid form melts; this represents equilibrium between the liquid and solid phases.

friction

A force that opposes the relative motion of two material surfaces that are in contact with one another; the direction of the force on each body is opposite to the direction of its motion relative to the other body.

geopotential height

The height of a given point in the atmosphere in units proportional to the potential energy of unit mass (geopotential) at this height, relative to sea level. In the CGS system, the relation between the geopotential height H and the geometric height Z is where g is the acceleration of gravity, so that the two heights are numerically interchangeable for most meteorological purposes. Also, 1 geopotential meter is equal to 0.98 dynamic meter. At present, by convention of the World Meteorological Organization, the geopotential height unit is used for all aerological reports.

geostrophic balance

The balance of the horizontal pressure gradient and the acceleration provided by the spinning of Earth; i.e., the Coriolis force. The horizontal pressure gradient arises because of differential solar heating that occurs at different latitudes. The Coriolis force arises because of Earth's (hence the atmosphere's) spinning on its axis. See pressure gradient force and Coriolis force.

geostrophic wind

The horizontal wind velocity for which the Coriolis force exactly balances the horizontal pressure gradient force.

glacial ice

Any ice that is or once was part of a glacier.

glacier

A multiyear surplus accumulation of snowfall in excess of snow melt on land and resulting in a mass of ice at least 0.1 km² in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of fresh water on Earth, and second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia.

Greenwich meridian

The meridian through Greenwich, England, that serves as the reference for Greenwich Time; in contrast with local meridians, it is accepted almost universally as the prime meridian, or the origin of measurement of longitude.

Gulf Stream

A warm, swift ocean current that flows along the coast of the eastern United States and makes Ireland, Great Britain, and the Scandinavian countries warmer than they would be otherwise.

gyre

A closed circulatory system, but larger than a whirlpool or eddy.

harmonic

1. An integral multiple or submultiple of a given frequency; a sinusoidal component of a periodic wave.

2. A signal having a frequency that is a harmonic (sense 1) of the fundamental frequency.

heat budget

A listing of all the sources of heat transfers for some thermodynamic system, to account for the total heat transfers into or out of the system.

heat capacity

The ratio of the amount of heat transfer applied to a body to the change in temperature produced by this heat, usually expressed as the heat energy required to raise the temperature of a specific substance by 1°C at constant pressure and volume.

heat energy

Energy associated with the random motion of atoms and molecules in a substance; in general, this energy can be measured by temperature. Heat energy exists as either sensible heat or latent heat.

heat flux

The flow of heat across a surface of unit area in a unit amount of time; commonly expressed in units of cal/cm²-sec or W/(M²-A).

iceberg

A large floating block of freshwater ice that has broken off the edge of a glacier and been carried out to sea; about 90% of its mass lies under the water.

incoming solar radiation

All the constituents making up the Sun's emission, including electrons, protons, neutrinos, and rare and heavy atomic nuclei.

infrared radiation

Electromagnetic radiation lying in the wavelength interval from 0.7 µ to 1000 µ. Its lower limit is bounded by visible radiation, and its upper limit by microwave radiation. Most energy emitted by Earth and its atmosphere is at infrared wavelength. Infrared radiation is generated almost entirely by large-scale intramolecular processes. The triatomic gases, such as water vapor, carbon dioxide, and ozone, absorb infrared radiation and play important roles in propagating infrared radiation in the atmosphere. Abbreviated IR; also called "longwave radiation."

interannual

Pertaining to changes that take place over two or more years.

isotherm

A line connecting points having equal or constant temperature.

latent heat

1. Energy transferred from Earth's surface to the atmosphere through the evaporation and condensation processes.

2. If a change of state occurs from gas to liquid or liquid to solid, internal energy in the form of heat is released. If a change of state occurs from solid to liquid or liquid to gas, heat is required. Different compounds absorb and release different amounts of latent heat. Water takes nearly 600 kilocalories for each kilogram of water condensed.

longwave radiation

See infrared radiation.

mantissa

A fixed point number composed of the most significant digits of a given floating point number. The positive decimal part of a common logarithm.

meridian

A north-south reference line, particularly a great circle through a planet's geographical poles. A terrestrial meridian is a line on Earth's surface that connects points having the same astronomical longitude -- also called astronomical meridian. A geodetic meridian is a line connecting points of equal geodetic longitude. Geodetic and sometimes astronomical meridians are also called geographic meridians. Geodetic meridians are shown on charts. The prime meridian passes through longitude 0°. A fictitious meridian is one of a series of great circles or lines used in place of a meridian for certain purposes. A transverse or inverse meridian is a great circle perpendicular to a transverse equator. An oblique meridian is a great circle perpendicular to an oblique equator. Any meridian used as a reference for reckoning time is called a time meridian. The meridian through any particular place or observer, serving as the reference for local time, is called local meridian, in contrast with the Greenwich meridian, the reference for Greenwich time. A celestial sphere, through the celestial poles and the zenith.

mesopause

1. The upper limit and the coldest portion of the mesosphere. The transition zone between the mesosphere and the thermosphere.

2. The top of the mesosphere around an altitude of 85 km where temperatures reach their lowest in the entire atmosphere.

mesosphere

The atmospheric shell in which temperature generally decreases with heights that extend from the stratopause at about 50-55 km to the mesopause at about 80-85 km. Compare to stratosphere and troposphere.

Mie scattering

1. Developed by Gustav Mie in 1908, this is a complete mathematical-physical theory of the scattering of electromagnetic radiation by spherical particles. In contrast to Rayleigh scattering, the Mie theory embraces all possible ratios of diameter to wavelength. See size parameter. The Mie theory is very important in meteorological optics, where diameter-wavelength ratios of the order of unity and larger are characteristic of many problems regarding haze and cloud scattering. Scattering of radar energy by raindrops constitutes another significant application of the Mie theory. Compare to Rayleigh Scattering.

2. Processes by which particles of similar size and electrical characteristics separate or disperse different wavelengths (colors) of light. Since the Sun's visible spectrum contains a mixture of red, orange, yellow, green, blue, indigo, and violet colors, these wavelengths are differentially scattered by particles as they travel through the atmosphere.

monsoon

A seasonal wind caused primarily by a greater annual variation of temperature over large land area than over neighboring ocean surfaces causing an excess of pressure over the continents in the winter and a deficit in the summer; monsoons are strongest in Asia, but also occur on the coasts of tropical regions.

NOAA

National Oceanographic and Atmospheric Administration.

pixel

Term derived from "picture element." The smallest resolved area of information on an image display, representing an element from a data array.

Planck's law

This is a derived formula, from the German physicist Max Planck that portrays the amount of radiation emitted by a blackbody as theoretically determined by its temperature. It is an equation that produces a curve, termed Planck's blackbody radiation curve that illustrates that the warmer a body is, the greater is its blackbody emission at each wavelength and the shorter is the wavelength of which emissions peak.

planetary boundary layer

1. The transition region between the turbulent surface layer and the normally nonturbulent free atmosphere. This region is about 1 km thick and is characterized by a well-developed mixing generated by frictional drag as the air masses move over Earth's surface. This layer contains approximately 10% of the mass of the atmosphere. Also called the "atmospheric boundary layer" or "frictional layer."

2. That layer of the atmosphere from a planet's surface to the geostrophic wind level including, therefore, the surface boundary layer and the Ekman layer. Above this layer lies the free atmosphere. Also called friction layer, atmospheric boundary layer.

Polar Orbiting Meteorological Satellite

Operated by NOAA, they are designated "NOAA satellites." Included in this group are the current series of TIROS-N satellites, the third-generation polar-orbiting environmental spacecraft operated by NOAA.

pressure gradient force

The force that causes acceleration from higher to lower pressure in a fluid such as the air. It arises because of a pressure gradient or change between higher and lower pressure. As an analogy, consider the pressure gradient that exists at the nozzle of a vacuum cleaner when it is turned on. A motor inside creates a region of relatively low pressure contrasted with the region of higher pressure outside. The dust and air molecules outside the vacuum, being at a higher pressure, feel a force pulling toward the low pressure inside the vacuum. This is a pressure gradient force. In the atmosphere, regions of high and low pressure form ultimately as a result of unequal heating. In moving from a high pressure region to a low pressure region, the motion is said to be along the pressure gradient. The pressure gradient force itself points in the direction of the lower pressure. In the vacuum cleaner example, this is evidenced by the fact that air is sucked into the lower pressure inside. The magnitude of the pressure gradient force is proportional to the magnitude of the pressure gradient itself. The lower the pressure inside the vacuum cleaner, the more powerful the vacuum will be. In the atmosphere, pressure gradient is related to a quantity called geopotential height. The pressure gradient force is proportional to the geopotential height gradient.

Rayleigh-Jeans law

A relation bet ween the differential intensity (dE) of blackbody radiation within a narrow range of wavelengths; it is equal to 2ckT^-4d, where c is the speed of light, k is the Boltzmann constant, T is the absolute temperature, and d is the wavelength; the law is only experimentally valid for long wavelengths and leads to the ultraviolet catastrophe for short wavelengths.

Rayleigh scattering

1. Any scattering process produced by spherical particles whose radii are smaller than about one-tenth the wavelength of the scattered radiation. Compare Mie scattering. See also size parameter. In Rayleigh scattering, the scattering coefficient varies inversely with the fourth power of the wavelength, a relation known as the Rayleigh law. The angular intensity polarization relationships for Rayleigh scattering are conveniently simple. For particles not larger than the Rayleigh limit, there is complete symmetry of scattering about a plane normal to the direction of the incident radiation, so that the forward scatter equals the backward scatter.

2. The scattering of light by a body with a particle diameter (Dp) less than 0.03 micrometers is termed Rayleigh Scattering. The wavelength of light scattered is dependent on the Dp, and the amount of light scattered is dependent on the number of particles present per unit volume. Shorter visible wavelengths, such as blue, are scattered by smaller particles than are the longer wavelengths like red.

salinity

The weight ratio between dissolved salts and water in seawater. The amount of dissolved salts in any solution.

Scanning Multichannel Microwave Radiometer (SMMR)

An imaging system consisting of lenses, moving mirrors, and solid-state image sensors used to obtain observations of Earth and its atmosphere. Scanning radiometers, which are the sole imaging systems on all current operational weather satellites, have far better long-term performance than the vidicon TV camera tubes used with earlier spacecraft.

scattering

The process by which small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions. In scattering, no energy transformation results, only a change in the spatial distribution of the radiation. Also called scatter. Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about 1/10, Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described by the Mie theory; at a ratio of the order of 10, the laws of geometric optics begin to apply.

sea ice

Ice formed by the freezing of seawater, as distinguished from glacier ice or other land ice. Generally, any ice floating in the sea.

seasonal cycle

The annual cyclical pattern in any atmospheric variable, whether temperature or trace gas concentration, caused by the seasons. Also called an annual cycle. The seasonal cycle or seasonal variability is one type of variability. Other types of variability include short-term (day-to-day or week-to-week), interannual (year-to-year), or long-term (decade-to-decade or longer). Thus, the amount of ozone in a particular location has a short-term variability, a seasonal variability, an interannual variability, and a long-term variability.

semienclosed basin

A body of seawater that does have some access to the open ocean. The Gulf of Mexico is an example of a semienclosed basin.

sensible heat

Heat energy that can be felt or measured directly with a thermometer.

shortwave radiation

In meteorology, a term used loosely to distinguish radiation in the visible and near-visible portions of the electromagnetic spectrum (roughly 0.4 to 1.0 micron in wavelength) from long-wave (infrared) radiation.

size parameter

The ratio of circumference of the sphere (of an aerosol particle) to the wavelength of incident radiation. For Rayleigh scattering, the particle radii are smaller than about one-tenth the wavelength of the scattered radiation. For Mie scattering the particle radii are on the order of the size of the wavelength of the incident radiation. See both Rayleigh scattering and Mie scattering.

solstice

The moment that the Sun reaches its greatest extent in declination, north or south.

Southern Oscillation

A large-scale atmospheric and hydrospheric fluctuation centered in the equatorial Pacific Ocean. It exhibits a nearly annual pressure anomaly, alternatively high over the Indian Ocean and high over the South Pacific. Its period is slightly variable, averaging 2.33 years. The variation in pressure is accompanied by variations in wind strengths, ocean currents, sea-surface temperatures, and precipitation in the surrounding areas. The Southern Oscillation is coupled directly with the El Niño phenomenon, giving rise to the El Niño-Southern Oscillation (ENSO) phenomenon. See El Niño events and El Niño Southern Oscillation (ENSO).

Special Sensor Microwave Imager (SSM/I)

The SSM/I is flown aboard Defense Meteorological Satellite Program (DMSP) satellites F8, F10, F11, F12, and F13. SSM/I instruments are microwave radiometers that sense emitted microwave radiation. The SSM/I is a 7-channel, 4-frequency, orthogonally polarized, passive microwave radiometric system. The instrument measures combined atmosphere and surface radiances at 19.3, 22.2, 37.0, and 85.5 GHz.

Stefan-Boltzmann law

One of the radiation laws that states that the amount of energy radiated per unit time from a unit surface area of an ideal blackbody is proportional to the fourth power of the absolute temperature of the blackbody. This law was established experimentally by Stefan and was given theoretical support by thermodynamic reasoning of Boltzmann. This law may be deduced by integrating Planck's law over the entire frequency spectrum.

stratification

The arrangement of water masses in a lake or other body of water into two or more horizontal layers having different characteristics.

stratiform

Consisting of parallel bands, layers, or sheets. Used to describe clouds of extensive horizontal rather than vertical development.

stratocumulus

A principal cloud type predominantly stratiform, in the form of a gray or whitish layer of patch, which nearly always has dark parts.

stratopause

The boundary between the stratosphere and the mesosphere. It occurs at an atmosphere height of approximately 50 km; however this depends on latitude. The atmosphere is characterized by a decrease in pressure with respect to increased altitude. More important, regions within the atmosphere, like the troposphere, stratosphere, and mesosphere, are distinguishable because of distinct temperature gradients with relatively well-defined starting and ending points. The stratopause is the highest portion of the stratosphere, with a temperature of approximately 0°C; the stratopause can also be described as the warmest region between the mesosphere and the stratosphere.

stratosphere

The thermal atmospheric region of the atmosphere between the troposphere and the mesosphere. The lower boundary of the stratospheric region is marked by the tropopause and begins at approximately 13 km; however, this altitude of the troposphere depends on latitude. The upper limit of the stratosphere is marked by the stratopause at approximately 50 km. The stratosphere is characterized by relatively stable temperatures (between -80°C and -50°C) in the lower regions, and begins warming near 20 km, reaching its maximum temperature of approximately 0°C at the stratopause. Stratospheric chemistry is of particular interest to scientists because ozone, the principal substance that shields Earth from incoming solar ultraviolet radiation, is found in the stratosphere. It should also be noted that wind currents in the stratosphere are primarily horizontal in nature.

stratus clouds

A principal cloud genus characterized by a gray later having a relatively uniform base; often occurring in the form of ragged patches or fragments (stratus fractus) and usually composed of fairly widely dispersed water droplets; similar to stratocumulus, but lower and lacking the latter's uniform relief.

Sun synchronous orbit

An orbit timed for a satellite to proceed over any given point on the landscape at roughly the same local sun time, providing repeatable sun illumination conditions during specific seasons.

surface boundary layer

The thin layer of air adjacent to Earth's surface, extending to the Ekman layer. Thus, it is the lowest layer of the troposphere. Within this layer the wind distribution is determined largely by the vertical temperature gradient and the nature and contours of the underlying surface; shearing stresses are approximate constant. Also called surface layer, friction layer, atmospheric boundary layer, ground layer. Together with the Ekman layer it forms the planetary boundary layer.

thermal emission

The phenomenon by which electrons or ions are emitted from a heated object.

thermal infrared

Infrared radiation with wavelengths ranging from 3 to 1000 micrometers.

thermocline

A temperature gradient as in a layer of seawater, in which the temperature decrease with depth is greater than that of the overlying and underlying water.

tidal current

Currents associated with the cyclical rise and fall of the water level as described in the definition of tide.

tide

The cyclical rise and fall of the water level in the oceans and other large bodies of water that is caused by the interaction of the rotating Earth with various forces, principally the gravitational attraction of the Moon and the Sun.

transmission

The complete passage of radiation through a medium.

tropics

Either of two parallels of latitude 23°27' north and south of the equator, being the Tropic of Cancer and the Tropic of Capricorn; also the region bounded by these latitudes.

tropopause

The boundary between the troposphere and stratosphere, usually characterized by an abrupt change of lapse rate. The change is in the direction of increased atmospheric stability from regions below to regions above the tropopause. Its height varies from 15 to 20 km in the tropics to about 10 km in polar regions. In polar regions in winter it is often difficult or impossible to determine just where the tropopause lies, since under some conditions there is no abrupt change in lapse rate at any height.

troposphere

That portion of the atmosphere from Earth's surface to the stratosphere; that is, the lowest 10 to 20 km of the atmosphere. The troposphere is characterized by decreasing temperature with height, appreciable vertical wind motion, appreciable water vapor content, and weather. Compare to stratosphere. Dynamically, the troposphere can be divided into the following layers: planetary boundary layer, which includes both the surface boundary layer and the Ekman layer, and the free atmosphere. See free atmosphere, Ekman layer, planetary boundary layer, and surface boundary layer.

ultraviolet catastrophe

The prediction of the Rayleigh-Jeans law that the energy radiated by a blackbody at extremely short wavelengths is extremely large, and the total energy radiated is infinite, whereas in reality it must be finite.

upwelling

The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean. Upwelling occurs most commonly among the western coastlines of continents, but may occur anywhere in the ocean. Upwelling results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Subsurface water of greater density and lower temperature replaces the surface water, and exerts a considerable influence on the weather of coastal regions. Carbon dioxide is transferred to the atmosphere in regions of upwelling. This is especially important in the Pacific equatorial regions, where 1-2 GtC/year may be released to the atmosphere. Upwelling also results in increased ocean productivity by transporting nutrient-rich waters to the surface layer of the ocean.

water vapor

The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration, ti contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

Wein's displacement law

This radiation law suggests that the wavelength of maximum emission of any body is inversely proportional to its absolute temperature.

western boundary currents

The strong, permanent, moving currents found in all five major oceanic gyres: the Brazil, East Australian, Kuroshio, and Aguilhas currents, and the Gulf Stream; the East Africa Coast Current is a western boundary current in the small gyre of the North Indian Ocean during the northern hemisphere summer, but it virtually disappears in winter.

wind stress

A drag or tangential force imposed on Earth's surface by the motion of an adjacent body of air.

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