GEOG 204 / AGRO 244 - Glossary of Terms

Covers lectures 2 to 11, 16 to 20, and 28 to 30.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |


A

Aerodynamic resistance (ra) - a parameter (in s m-1) that relates the turbulent flux densities measured over a layer in the atmosphere to the gradient of the corresponding property in the same layer.

Absorption - the process in which incident radiant energy is retained by an object or a substance (e.g. gas).

Absorptivity (aλ) - the relative fraction of the irradiance reaching a surface that is absorbed. Values for absorptivity range between 0 (no absorption) to 1 (black body).

Adiabatic process - a thermodynamic change of state of a system in which there is no transfer of heat or mass across the boundaries of the system. Compression always results in warming, expansion in cooling.

Air mass - a large body of air in the order of thousands of km across with roughly uniform physical properties (temperature, humidity). We can distinguish between maritime (m) or continental (c), polar (P) or tropical (T) air masses.

Albedo (α) - the ratio of the amount of short-wave radiation reflected by a surface to the short-wave irradiance (amount incident upon it).

Amplitude - half the magnitude from crest-to-trough of a wave.

Anabatic wind - an upslope wind due to local surface heating typically during daytime.

Atmospheric boundary layer (ABL) - the layer of the atmosphere from the surface to the level where we find a diurnal course of temperature, humidity etc. (same as Planetary Boundary Layer, PBL).

Atmospheric 'window' - part of the long-wave specturm where we observe a relative gap in the absorption for atmospheric gases (between 8 µm and 13 µm).


B

Beer's Law - Physical law that describes the decay of a radiant flux density as a function of distance when transmitted through a homogeneous medium.

Black body - an hypothetical body which absorbs all of the radiation striking it, i.e. allows no reflection or transmission. A black body has an emissivity of 1.

Buoyancy - the upward force exerted upon a parcel of fluid by virtue of the density difference between itself and the surrounding fluid.


C

Cavity zone (also 'quiet zone') - A zone immediately before and after a barrier (e.g. wind break) where the wind is significantly reduced and turbulence from the top of the barrier is small.

Cirrus - High-altitude, thin and delicate ice clouds often appearing as veil-like patches or thin wispy fibres.

Cold air drainage - see katabatic wind.

Cold front - a front where a fast cold air mass undercuts a slower warmer air mass hence lifting the warmer air up. Typically associated with heavy showers - sometimes thunderstorms.

Collision-coalescence process - Raindrop formation in warm clouds (i.e. >0°C) in which large cloud droplets collide and join with smaller ones to form raindrops.

Condensation - the process by which vapour becomes a liquid.

Conduction - the transfer of energy in a substance by means of molecular motions without any macroscale motion.

Continental air mass (prefix 'c') - an air mass that stayed for a long time over land and is therefore typically drier than a maritime air mass.

Convection - mass motions within a fluid resulting in transport and mixing of properties (e.g. energy and mass).

Convergence - the condition when wind is flowing to the same area from different directions simultaneously (net inflow of air). This condition will result a vertical movement of air leading typically to cloud formation and precipitation.

Coriolis Force (Fc) - an apparent force that apparently deflects objects in a rotating frame of reference such as the Earth's surface.

Cosine law of illumination - The cosine law of illumination describes how the irradiance caused by a direct beam decreases as the beam's angle varies from overhead (normal) to a more slanted path relative to the surface.

Cumulative leaf area index - The total leaf area index (LAI) from canopy top down to a given depth in the canopy.

Cumulus - Billowy individual clouds that have flat bases.

Cyclone - A low-pressure system with rotating winds, typically containing warm and cold fronts.


D

Damping depth (D) - The depth in m in a soil where the amplitude of an ideal temperature wave has decayed to e-1 (37%) of its value at the surface.

Density (ρ) - Mass per volume in kg m-3

Diffuse short-wave radiation (D) - short-wave radiation reaching the Earth's surface after having been scattered from the direct-beam by molecules, aerosols or cloud-droplets in the atmosphere.

Displacement zone - A zone around a barrier (e.g. wind break) where the air is displaced upward and accelerated as it moves over the barrier.

Direct-beam short-wave radiation (S) - that portion of short-wave radiation received in a parallel beam 'directly' from the Sun.

Down-slope wind - same as katabatic wind.

Down-valley wind - A local wind circulation that is present during night in mountain areas and developed due to thermal differences between the riges in the back of a valley (cool) and the valley floor (warm). The Down-valley wind drains cooler (and denser) air close to the ground down the valley and there is a recirculation well aloft. The opposite is the up-valley wind..

Dry adiabatic lapse rate (Γd) - A process lapse rate that describes how a parcel of dry air changes its temperature with height under adiabatic conditions. See also saturated adiabatic lapse rate


E

Eddies - Coherent parts in a turbulent flow that have roughly same properties.

Eddy covariance method - An approach to directly measure flux densities by tracking the vertical movement of eddies and the properties they carry. This is practically implemented with fast responding ultrasonic anemometers and potentially gas analyzers.

Eddy diffusivity (K) - The constant of proportionality (in m2 s-1) in the K-Theory that relates a gradient of of anything to the corresponding flux density.

Emission - the process in which an object or surface actively releases radiant energy.

Emittance - the radiant flux density in W m-2 a surface releases by emission.

Emissivity (ε) - the ratio of the total radiant energy emitted per unit time per unit area of a surface at a specified wavelength and temperature to that of a black body (emissivity = 1) under the same conditions.

Energy flux - rate of flow of energy in Joules per second (J s-1).

Energy flux density - rate of flow of energy per unit surface area in Joules per second and square meter (J s-1 m-2 = W m-2).

Environmental lapse rate (ELR) - the decrease of ambient air temperature with height in the atmosphere.

Erectophile - Predominantly vertical arrangement of leafs in a plant or crop. Opposite: planophile.

Evaporation - (or vaporization) the process by which a liquid is transformed into a gas, in the atmosphere usually water changing to water vapour.

Extinction coefficient (k) - Parameter in Beer's Law that describes the magnitude of the decay of a radiant flux density when transmitted through a homogeneous medium.

Extra-terrestrial irradiance (KEx) - short-wave radiant flux density received at the 'top' of the Earth's atmosphere from the sun.


F

Far infrared radiation - the same as long-wave radiation.

Ferrel cell - The mid-latitude cell in the general global circulation.

Flux - rate of flow of some quantity (example: energy flux, mass flux).

Flux density - the flux of any quantity through a unit surface area (examples: energy flux density ).

Forced convection - same as mechanical convection.

Fourier's Law - A physical law that relates a temperature gradient in a body to the heat flux density by conduction. The constant of proportionality between temperature gradient and heat flux density is the thermal coductivity.

Free convection - same as thermal convection.

Frequency (ν) - number of crests of an electromagnetic wave that pass by per second (in s-1).

Friction (Ff) - The force decellerating air moving over a rigid surface that slows down the wind.

Front - a boundary between two contrasting air masses.


G

Gradient - The change of anything with height (or horizontal distance). Example: Temperature gradient.

Greenhouse effect - refers to the fact that certain atmospheric gases (greenhouse gases) absorb long-wave radiation that is primarely emitted from Earth's surface. The absorption by these gases heats up the atmosphere. The warmer atmosphere then emits even more radiation back to the surface where it can be again absorbed and increases the overall surface temperature. The (natural) greenhouse effect increases the average surface temperature of the planet by 33 K. Increased greenhouse gas concentrations due to human activities enhance the greenhouse effect (=enhanced greenhouse effect).

Greenhouse gases - atmospheric gases that cause the greenhouse effect, i.e. absorb long-wave radiation. The most important greenhouse gases are water vapour, carbon dioxide, methane, ozone, and nitrous oxide.

Grey body - In contrast to a black body a body with an emissivity < 1. A grey body absorbs only a fraction of the radiation striking it, i.e. shows reflection and/or transmission.


H

Hadley cell The first, low-latitude cell in the general global circulation characterized by updraft along the ITCZ and subsidence in the subtropics.

Heat - Thermal energy content of a body. Expressed as J kg-1 or J m-3.

Heat capacity (C) - the amount of energy absorbed (or released) by unit volume of a system for a corresponding temperature rise (or fall) of 1 Kelvin in J m-3 K-1. See also: specific heat.

Heat conduction - see conduction

Heat flux - rate of flow of sensible or latent heat in Joules per second (J s-1).

Heat flux density - A particular form of energy flux density. Rate of flow of sensible or latent heat per unit surface area in Joules per second and square meter (J s-1 m-2 = W m-2).

High ('H' on weather map) - An area of high pressure and characterized by rotating winds and subsiding air.


I

Irradiance - total radiant flux density received by a surface in W m-2 in a given wavelength band.

Irradiation - same as irradiance.

Isobar - a line on a weather map that connects points of equal surface pressure.

Intertropical convergence zone (ITCZ) The zone of general convergence, uplift and therefore high precipitation between the two Hadley cells on the Northern and Southern hemisphere.


J

Jet streams - Ribbons of high wind speeds encircling the globe at altitudes of 10 to 15 km.

Joule (J) - SI-unit for energy (1 J = 1 N m = 1 kg m2 s-2).


K

K-Theory An approach that relates a flux density to a measured gradient by an eddy diffusivity.

Katabatic wind - any wind blowing down an incline, often due to cold air drainage during nighttime.

Kelvin (K) - SI base unit for thermodynamic temperature (T (K) = T (°C) + 273.15).

Kinetic energy - the energy a body possesses as a consequence of its motion, defined as one-half the product of its mass and the square of its speed.

Kirchhoff's Law - says that the absorptivity of a body is equal its emissivity in a wavelength. It assumes no transmission.


L

Laminar flow - a flow in which the fluid moves smoothly parallel; in contrast to turbulence.

Land breeze - A local wind circulation that is present during night and developes due to thermal differences between the water body (warm) and the land (cool). The land breaze blows air close to the ground from the land to the sea (or lake) and from the sea to the land well aloft. The opposite is the sea breeze..

Lapse rate - See environmental lapse rate and process lapse rate.

Latent heat - the energy released or taken up per unit mass by a system in changing phase.

Latent heat of fusion of water (Lf) - the energy taken up to melt one kilogram of ice. This is equal the energy released when 1 kilogram of water freezes (0.33 MJ kg-1 at 0 °C).

Latent heat of vaporization of water (Lv) - the energy needed to evaporate one kilogram of water. This is equal the energy released when 1 kilogram of water vapour condenses (2.50 MJ kg-1 at 0 °C).

Leaf area index (LAI) - The one-sided leaf area per unit ground area in m2 m-2.

Lifting condensation level (LCL) - The base of clouds where rising air parcels reach the dew point temperature and start to condense.

Long-wave radiation (=far infrared radiation or thermal infrared radiation) - radiation with a wavelength between 3 µm and 100 µm. Long-wave radiation in the climate system originates from emission of objects on Earth (Surface, Atmosphere, Clouds etc.).

Low ('L' on weather map) - An area of lower pressure typically associated with a cyclone.


M

Maritime air mass (prefix 'm') - an air mass that stayed for a long time over an ocean and is therefore typically more humid than a continental air mass.

Mass flux - rate of flow of mass in kilogram per second (kg s-1).

Mechanical convection (also 'forced convection') - Turbulent motion induced by mechanical forces such as deflection or friction.


N

Near infrared radiation (NIR) - Radiation in the range 0.7 µm to 3 µm. Part of the short-wave spectrum that is beyond the part visible to the human eye.

Neutral - The situation when the environmental lapse rate is roughly equal the dry adiabatic lapse rate. Practically this means that any vertical motion is neither enhanced nor suppressed due to density differences.

Net all-wave radiation (Q*) - The net radiant flux density in W m-2 when calculating the radiation balance, i.e. short-wave irradiance minus short-wave reflectance plus long-wave irradiance minus long-wave surface emittance / reflectance.

Net long-wave radiation (L*) - The net long-wave radiant flux density in W m-2, i.e. long-wave irradiance minus long-wave surface emittance / reflectance.

Net short-wave radiation (K*) - The net short-wave radiant flux density in W m-2, i.e. short-wave irradiance minus short-wave reflectance.

Newton (N) - The SI unit for a force (1 N = 1 kg m s-2).


O

Occlusion - a front formed in a cyclone when the cold front overtakes the warm front.

Orographic clouds - Clouds that are formed when air is forced to rise as it approaches a mountain range.


P

Pascal (Pa) - The SI unit for pressure (1 Pa = 1 N m-2 = 1 kg m-1 s-2).

Photosynthetically active radiation (PAR) - Radiation in the range 0.4 µm to 0.7 µm which is roghtly the visible part of the short-wave spectrum. PAR can be converted to chemical energy through the process of photosynthesis.

Photon - the elementary quantity of radiant energy.

Planetary albedo - The spatially and temporally averaged albedo of a planet including its atmosphere. Earth's planetary albedo is 0.31.

Planetary boundary layer (PBL) - same as Atmospheric Boundary layer

Planck's Law - A basic radiation law that describes the emittance from a black body as a fuction of wavelength and absolute temperature in Kelvin (Planck curves). Integrating a Plack curve over all wavelengths results in the Stafan Boltzmann Law. The location of the maximum of a plack curve is descibed by Wien's Displacement Law.

Planophile - Predominantly horizontal arrangement of leafs in a plant or crop. Opposite: erectophile.

Polar air mass (symbol 'P') - an air mass that stayed for a long time in high latitudes and is therefore typically cool.

Polar cell - The third, most poleward cell in the general global circulation characterized by subsidence over the poles and NE-winds close to the surface.

Polar front theory - A simplified theory explaing the formation and decay of mid-latitude cyclones.

Porosity - The volume fraction in a soil that is occupied either by water or air in m3 m-3.

Potential energy - the energy which a body possesses as a consequence of its position in a field of gravity.

Potential temperature - the temperature a parcel of dry air would have if brought adiabatically from its present position to a standard pressure of 100 kPa.

Pressure - The force exerted on a surface by the ambient air in Pa.

Pressure Gradient Force (Fp) - the pressure-equalizing force that acts on an air parcel due to an existing horizontal pressure gradient.

Process lapse rate - the way the temperature or an air parcel changes if it changes its height.

Projection coefficient (G) - same as extinction coefficient if Beer's Law is applied to plant canopies.

Pyranometer - An instrument to measure the short-wave irradiance (or short-wave reflectance if installed upside down) of a surface.

Pyrgeometer - An instrument to measure the long-wave irradiance (or long-wave emittance plus reflectance if installed upside down) of a surface.


R

Radiant energy - the energy of any type of electromagnetic radiation.

Radiant flux - the energy flux of radiant energy (in J s-1).

Radiant flux density - the energy flux density of radiant energy (in J s-1 m-2 = W m-2).

Radiation - the process by which radiant energy is propagated through free space by virtue of joint undulatory variations in the electric and magnetic fields in space.

Reflection - the process in which incident radiant energy is returned back where it originated (mirrored or diffused).

Reflectance - the radiant flux density in W m-2 a surface returns back by reflection.

Reflectivity (αλ) - the relative fraction of the irradiance reaching a surface of an object that is reflected back. Values for reflectivity range between 0 (no reflection) to 1 (e.g. a perfect mirror).

Reynolds notation - The mathematical concept of splitting a time series into its average and the deviation thereof.


S

Saturated adiabatic lapse rate (Γs) - A process lapse rate that describes how a parcel of air changes its temperature with height under adiabatic conditions when it is condensing water and therefore releasing additionally the latent heat of vaporization. See also dry adiabatic lapse rate.

Scattering - the process of diffusing a portion of the incident radiation in all directions by molecules, air droples or aerosols.

Sea breeze (or lake breeze) - A local wind circulation that is present during day and developes due to thermal differences between the water body (cool) and the land (warm). The sea breaze blows air close to the ground from the sea (lake) to the land and from the land to the sea well aloft. The opposite is the land breeze.

Sensible heat - that heat (energy) able to be sensed (e.g. with a thermometer). Used in contrast to latent heat.

Shear stress (τ, same as 'Momentum flux density') - The flux density of momentum in (kg m s-1) (m-2 s-1) = N m-2.

Short-wave radiation (=solar radiation) - radiation with a wavelength between 0.15 µm and 3 µm. Short-wave radiation in the climate system originates from the sun. Short-wave radiation is further separated into ultraviolet radiation, photosynthetically active radiation (visible), and near infrared radiation.

Skin friction - The process when air is retarded close to a surface or object. May create mechanical convection.

Sky view factor (Ψsky) - the fraction of the radiant flux leaving a certain surface (or object) that is intercepted by the sky (as opposed to by other ground objects).

Soil heat flux plate - A sensor to directly measure the soil heat flux density A thermopile measures the temperature gradient across a thin plate of known thermal conductivity.

Soil heat flux density (QG) - The flux density of sensible heat transferred by conduction in the soil in W m-2 = J s-1 m-2.

Solar constant (S0 same as I0) - solar (= short-wave) radiant flux density at the top of the atmosphere normal to the solar beam at Earth's mean distance from Sun. Its value has been determined by satellite measurements to 1366 W m-2.

Solar radiation same as short-wave radiation.

Solar zenith angle (θ) - vertical direction of the Sun relative to the zenith expressed in degrees or radians.

Sonic anemometer - same as ultrasonic anemometer

Specific heat (c) - the amount of energy absorbed (or released) by unit mass of a system for a corresponding temperature rise (or fall) of 1 Kelvin in J kg-1 K-1. See also Heat capacity.

Stable - The situation when the environmental lapse rate is less than the dry adiabatic lapse rate. Practically this means that any vertical motion is suppressed and repelled due to density differences.

Stefan Boltzmann Law - A basic radiation law that describes the integral emittance of a black body as a fuction of its absolute temperature in Kelvin.

Stratus - Sheet or layer-shaped unstructured low clouds that cover much or all of the sky.

Sublimation - the transition of a substance directly from the solid to the vapour phase or vice versa.

Subsidence - Sinking or descending of air on a large scale such as in a high-ressure system

Système International d'Unités (SI) - international scientific system of units which is mandatory in climatology. The SI utilises seven base units from which other derived units can be obtained.


T

Temperature gradient - The change of temperature with height or distance in K m-1

Thermal admittance (µ) - a surface thermal property that governs the ease with which it will take up or release heat. It is the square root of the product of the thermal conductivity and heat capacity.

Thermal belt - The regions half-way up the slopes in a valley that have typically the higest air temperatures during night.

Thermal conductivity (k) - a physical property of a substance describing its ability to conduct heat by molecular motion across a given temperature gradient according Fourier's Law. Its units are W m-1 K-1.

Thermal convection - motion caused only by density differences in a fluid (e.g. due to surface heating).

Thermal diffusivity (κ) - the ratio of the thermal conductivity to the heat capacity of a substance. It determines the rate of heating due to a given temperature distribution in a given substance. It tells us how fast temperature waves will travel in a substance. Its units are m2 s-1.

Thermal infrared radiation (TIR) - the same as long-wave radiation.

Torque - the product of a twisting or rotational force and its distance to the center of the rotation.

Transmission - the process in which incident radiant energy is passing through an object.

Transmissivity (τλ) - the relative fraction of the total irradiance on the surface of an object that is transmitted through the object. Values for transmissivity range between 0 (no transmission, opaque body) to 1 (perfect transmitter).

Tropical air mass (symbol 'T') - an air mass that stayed for a long time in lower latitudes and is therefore typically warm.

Troposphere - the lowest 10-20 km of the Atmosphere, characterized by decreasing temperature with height, appreciable water vapour and vertical motion, and weather.

Turbulence (also: turbulent flow or turbulent motion) - a state of a flow in which the instantaneous velocities exhibit irregular and apparently random fluctuations. These fluctuations (eddies) are capable of transporting atmospheric properties through convection at rates far in excess of molecular processes. Used in contrast to laminar flow.


U

Ultraviolet radiation (UV) - Radiation in the range 0.15 µm to 0.4 µm of the short-wave spectrum. It is not visible to the human eye.

Ultrasonic anemometer - An instrument that outputs all wind components at high frequency by measuring of the speed of sound.

Unstable - The situation when the environmental lapse rate is larger than the dry adiabatic lapse rate. Practically this means that any vertical motion is enhanced and accelerated due to density differences.

Up-slope wind - same as anabatic wind.

Up-valley wind - A local wind circulation that is present during day in mountain areas and developed due to thermal differences between the lower part of a valley (cool) and the ridges in the back of a valley (warm). The up-valley wind blows air close to the ground up the valley and there is a recirculation in the opposite direction well aloft. The opposite is the down-valley wind.


V

Vaporization - same as Evaporation.

View factor - fraction of one object's view (usually hemispherical) that is occupied by another object (or the sky, see sky view factor).

Volume fraction (θ) - The volume that a soil compound (organic material, mineral material, water, air) occupies per total soil volume in m3 m-3.

Volumetric water content (θw) - The volume fraction of a soil that is currently occupied by water in in m3 m-3. It strongly influences the heat capacity of the soil.


W

Wake zone - A zone in the lee of a barrier (e.g. wind break) extending 10 to 25 times the height of the barrier where the wind is still reduced and turbulence is increased.

Warm front - a front where a faster warm air mass rides up a cooler slower air mass. Typically associated with long-lasting moderate precipitation.

Watt (W)- The SI unit for power (1 W = 1 J s-1 = 1 kg m2 s-3).

Wegener-Bergeron-Findeisen Process - Raindrop formation in cold clouds (i.e. <0°C) in which ice cristals grow on the expense of liquid water droplets to form large ice cristals that then fall to graound and typically melt on the way down.

Wien's Displacement Law - A basic radiation law that describes the wavelength of maximum emittance of a black body as a fuction of its absolute temperature in Kelvin.

Wind direction - The horizontal angle from North (clockwise) from which wind is blowing.


Z

Zenith - that point in the sphere surrounding an observer that lies directly above him.

 

Last additions: September 28, 2011. Selected definitions are from T. R. Oke (1987): 'Boundary Layer Climates', p. 400 to 405, with permission from the author.

space