Reduction of the water equivalent of a snow cover by melting, evaporation, wind and avalanches.

Absolute annual range of temperature

Difference between the highest and the lowest temperatures during a specific year.

Absolute drought

Drought period of at least 15 days without daily measurable precipitation. (Term used in British Climatology).

Absolute extreme

Highest or lowest value of a climatic element that is the greatest extreme for the whole period for which there are documented observations available.

Absolute humidity

The density of the water vapor component in a system of moist air; that is, the ratio of the water vapor mass present to the volume occupied by the mixture.

Absolute monthly maximum temperature

The highest monthly maximum temperature observed in a specific calendar month over a specific period of years.

Absolute monthly minimum temperature

The lowest monthly minimum temperature observed in a specific calendar month over a specific period of years.

Absolute standard barometer

Barometer which provides absolute measurements of pressure without having to be calibrated.

Absorbed solar radiation

Solar radiation absorbed by the atmosphere’s constituent gases, suspended material, clouds or by the Earth’s surface.


Process that result in the retention of the radiant energy (electromagnetic radiation) that strikes a substance.

Absorption band

Connected series of overlapping or barely spaced absorption lines.

Absorption line

Minute wavelength or frequency “range” in the electromagnetic spectrum within which radiant energy is absorbed by the medium through which it is passing.

Absorption spectrum

Array of absorption lines and bands resulting from the passage of radiant energy through a selectively absorbing medium.

Accessory cloud

Cloud accompanying another cloud, generally smaller that the latter, and separated from its principal part or sometimes partially merged with it. A specific cloud may be accompanied by one or several accessory clouds. Pileus, velum and pannus are accessory clouds.


Gradual adjustment of living organisms to climatic conditions other than those to which they are accustomed.


Growth of a cloud or precipitation particle by the collision and union of a frozen particle (ice crystal or snowflake) with a supercooled liquid droplet which freezes on impact.

Accumulated temperature

Sum of the departures of temperature (e.g., daily or monthly mean temperature) from a reference temperature for a specific period. Sometimes, it is also measured by the total number of days (or hours) since a given date during which the temperature was above a standard value.

Acid rain

Acid precipitation that occurs because of the emission of sulfur dioxide and nitrogen oxides into the atmosphere, which chemically transformed are absorbed by water droplets in clouds and fall to the surface as rain or snow. This affects the chemical balance of soil, lakes and streams.


Curve on a thermodynamic diagram which represents the temperature changes of a parcel of air subjected to an adiabatic process.

Adiabatic ascent

Air parcel rising with its relative humidity remaining below 100 percent; therefore, there is no moisture condensation and no heat is added.

Adiabatic atmosphere

An atmosphere which has a dry-adiabatic lapse rate.

Adiabatic process

Process in which a system (air parcel) transforms thermodynamically without exchanging heat with its surroundings. In this kind of process, any change in the internal energy of the mass is a consequence of working, not of its interaction with its surroundings by virtue of a temperature difference between them.


The transfer of air mass properties by velocity field of the atmosphere.


Statement issued by a weather service about weather situations that, if not observed, may lead to dangerous circumstances.


The study of the upper atmosphere, particularly in relation to its composition, properties, and relative motion, and the radiation received from outer space.


Colloidal system composed by tiny solid or liquid particles, whose diameters are less than one micrometer, dispersed in some gas, usually air. Examples are dust, soot, salt and other airborne particles.


The process in which solid precipitation particles combine in the atmosphere to produce larger particle, e.g. hailstones.

Agricultural drought

Drought conditions that result in poor or adverse crop behavior, usually because plants are unable to meet potential transpiration, as a consequence of soil moisture limitation or high atmospheric demand.

Air conductivity

Facility of the atmosphere to conduct electricity, expressed as the current density per unit electrical potential gradient in the direction of flow.

Air discharge

Lightning discharge between a cloud and a cloudless region of the atmosphere. The lightning occurs in the form of a sinuous streak, often forked, passing from a thundercloud to the air without striking the ground. The lightning often includes a log, quasi-horizontal part.


Quasi-permanent emission of radiation by the gases of the upper atmosphere distinguishable at night (nightglow) and presumed to occur also during the day (dayglow).

Air mass

Extensive body of air, relatively horizontally homogeneous in terms of temperature, pressure and moisture distribution. Temperature and humidity are characteristics of the region over which the air has been situated. It suffers specific modifications while drifting away from that source region.

Air mass source region

Region where an air mass is originated, acquiring its properties of temperature and humidity. The horizontal homogeneity of the air mass is caused by the sustained contact during days or weeks, with the surface below.

Air parcel

Imaginary volume of air to which thermodynamic and kinematic quantities may be assigned. A sample of air under study usually referred in comparison with its surroundings.

Air pollutant

Air shower

Auger shower, cosmic-ray shower, extensive air shower. Simultaneous appearance of downward ionizing particles, with or without photons, caused by a single cosmic ray.

Air stream / Air current

Generally, any moving body of air.

Air temperature

The temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation.

Air toxin

Dangerous air pollutant that is suspected or known to produce cancer or other serious diseases or adverse health effects.


Ratio of reflected radiation to incident radiation, referenced to some surface. It is the proportion of radiation that is reflected, expressed as a percentage. Solar albedo is the percentage of sunlight reflected from a surface.


Condition in which the body presents an exaggerated response to a certain substance.

Allobaric wind

The wind velocity resulting when the Coriolis force balances a locally accelerating geostrophic wind.


Vertical distance of a level, a point or an object in the air considered as a point, measured form mean sea level. In general, height of an object above the Earth´s surface or above mean sea level.

Altocumulus (Ac)

Abbreviated Ac. Principal cloud genus or type, middle cloud with a white and/or gray coloration, which forms as a layer or patch, with waved aspect and cumuliform structure. Appears as laminae, rounded masses, rolls, etc.

Altostratus (As)

Abbreviated As. Principal cloud genus or type, middle cloud with a stratiform structure, in the form of a grayish or bluish layer of striated, fibrous or uniform appearance. They are never white.

Amount of precipitation

Depth to which the precipitation would cover, in a liquid form, a horizontal projection of the Earth’s surface, in the absence of infiltration, runoff or evaporation, and if all solid precipitation was melted. It is usually expressed in inches or millimeters, equivalent to liters per square meter. Snowfall is also measured by the depth of fresh snow covering an even horizontal surface.

Ambient temperature

The temperature of the air surrounding a given object.

Ana cold front / Anafront

Front at which the warm air ascends the frontal surface up to high altitudes. Precipitation might occur to the rear of the front.

Anabatic wind

An upslope wind; it usually applies only to the wind blowing up a hill or mountain as the result of strong surface heating of the slopes.


General name for instruments that measure wind speed, either total wind speed or the one of the components of the wind vector.

Antarctic air

Mass of air from the Antarctic Continent which is very cold in the lower levels.

Antarctic anticyclone

Annual mean anticyclonic circulation affecting Antarctica in which the net inflow of air in the high troposphere and the outflow near the surface are linked by subsidence over the continent.

Antarctic Circumpolar Current (ACC) / West Wind Drift

The largest of all ocean currents, it is an eastward flowing current that circles the Antarctica and goes from the water surface to the ocean floor. A current comparatively slow in speed, although thanks to its depth, has a great volume transport.

Antarctic front

Front of great extent, in high southern latitudes, which separates cold Antarctic air from relatively wart polar air.

Antarctic stratosphere vortex

Persistent westerly stratospheric circulation during Winter in the Southern Hemisphere, which is most intense between latitudes 60 and 70°S and increases with height up to the stratopause.

Anticyclone / High pressure system, area or center

Same as high pressure system or center. Large body of air with a closed circulation, with descending air in its core, in which the atmospheric pressure is higher than the pressure of the surrounding air. The wind blows outward and clockwise around an anticyclone in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, due to Earth´s rotation and Coriolis Effect. They are associated with fair weather.

Anticyclonic circulation / Anticyclonic rotation

Systematic atmospheric circulation associated with an anticyclone, which is clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere.

Anticyclonic gloom

The low-visibility phenomenon accompanying a well established and almost stationary anticyclone and caused by the accumulation of dust and other pollutants between the Earth’s surface and the temperature inversion associated with the anticyclone.

Anvil cloud

A cirriform cloud with an anvil shape, which forms the upper part of a well developed cumulonimbus. Its glaciated top spreads out horizontally upon reaching the tropopause or by the action of the winds aloft.


Point of Earth´s orbit in which the planet is farthest from the Sun; nowadays, approximately July 1st, but the date advances slowly at a rate of about one half hour each year (opposite of perihelion).

Apparent Temperature Index

Measure of relative comfort or discomfort due to the combined effect of heat and humidity. It is based on physiological studies of evaporative skin cooling for different combinations of temperature and humidity. The index is found on a chart that has temperature in one axis and humidity in the other.

Arcs / Arcs of Lowitz

Photometeor of the halo family consisting of two luminous arcs that adopting an oblique position, sometimes connect parhelia or paraselene with a small halo.

Arctic air

Mass of air in the Arctic regions, very cold in the lower levels, which invades lower latitudes at irregular intervals.

Arctic front

Front which separates cold arctic air originating at high northern latitudes from relatively warm polar air.

Arctic haze

Haze in the Arctic regions which reduces horizontal and oblique visibility and which may extend to a height of about 6 miles (10kms). It appears blue-gray when viewed away from the Sun, and reddish-brown toward it.

Arctic high / Arctic anticyclone, polar high or polar anticyclone

A weak high which appears over the Arctic Basin during late spring, summer and early fall. It develops in association with persistent negative net radiation values and migrates southeastward toward continental interiors.

Arid zone

1. Zone in which precipitation is so deficient that irrigation must be practiced to support cultivation.
2. Zone in which evaporation always exceeds precipitation.

Assimilation/ Data assimilation

Combination of diverse data into a unified and consistent description of a physical system, such as the state of the atmosphere. Transformation of research data into a language that numerical weather models can process.


Any small celestial body that revolves around the sun. Asteroids orbits generally lie generally between Mars and Jupiter, with diameters between a few to several hundred kilometers.


Zone of the Earth´s mantle that consists of several hundred kilometers of deformable rock; it is situated beneath the lithosphere.


Gaseous envelope that surrounds the Earth or other celestial objects, gravitationally bound to them.

Atmospheric absorption

Transformation of radiant energy into thermal, mechanical, electrical, etc. energy, through its absorption by atmospheric constituents.

Atmospheric disturbance

1. Any interruption of a state of equilibrium of the atmosphere.
2. Minor depression or meteorological conditions in an area showing signs of a developing cyclonic circulation.

Atmospheric dynamics

The study of atmospheric motions, including their time variations which are involved in weather phenomena on all scales, by means of the principles of thermodynamics and fluid dynamics.

Atmospheric Effect

Atmospheric electricity

The various electrical phenomena which occur naturally in the atmosphere.

Atmospheric impurities

Particles or gases which are alien to the normal constituents of the air and which occur in it in greatly varying quantities.

Atmospheric model

Description of the atmosphere giving a simplified or schematic outline of its key structures and processes via representative diagrams (conceptual), systems of mathematical equations or numerical approximations.

Atmospheric pressure

Force or pressure exerted by the atmosphere on a given unit area, as a consequence of the gravitational attraction exerted by the planet upon the column of air situated directly above it. It is one of the basic meteorological elements, measured by a barometer and expressed usually in millibars.

Atmospheric radiation

Long-wave radiation emitted by and propagated through the atmosphere.

Atmospheric tide / Solar atmospheric tide

Periodical oscillations of the atmosphere, caused by the gravitational action of the moon and the Sun.

Atmospheric turbulance

Apparently random atmospheric motions; they are effective in transferring heat, moisture and momentum but are so complex that only their statistical properties can be recognized and studied.

Atmospheric wave

Generally, any pattern with some roughly identifiable periodicity in time and /or space. In meteorology: waves in the horizontal flow pattern (e.g., Rossby wave, long wave, short wave).

Atmospheric window

Spectral region in which the atmosphere absorbs very little terrestrial radiation.


Region of light surrounding the sun; it is the exterior ring in a series which is nearest the luminary in a corona. It is bright and faintly colored, reddish or chestnut. The aureole appears when there is a broad range of droplet sizes.


Radiant emission from the upper atmosphere over middle and high latitudes, produced by interactions of rapid charged particles with the ionospheric gas.

Aurora Australis / Southern lights

Aurora of the southern latitudes.

Aurora Borealis / Northern lights

Aurora of the northern latitudes.

Auroral arc

Auroral light in the form of a regular bow which extends east-west at right angles to the magnetic meridian.

Auroral bands

Auroral structure, either as “homogeneous bands” or as “rayed bands”, resulting when an auroral arc loses its regular shape.

Auroral corona

Auroral rays extending to the magnetic zenith, in parallel lines along the geomagnetic lines of force, and appearing by perspective to diverge from the magnetic zenith.

Auroral draperies / Auroral curtains

Auroral bands of rayed structure in which the ways are exceptionally long and therefore give an appearance of draperies or curtains.

Auroral rays

Lines of auroral light, usually sharply defined, extending along the geomagnetic lines of force.

Auroral storm

A rapid succession of auroral sub-storms, occurring in a short period, of the order of the day, during a geomagnetic storm.

Auroral sub-storm

Small perturbation of the aurora.

Auroral zone

Region situated about 10-15° from either geomagnetic pole, within which the aurora is most frequent and active.

Automated weather station

Location where meteorological observations are automatically taken by proper devices.

Autumnal equinox

One of the two points of intersection of the Sun´s apparent annual path and the plane of the Earth´s equator (celestial equator). Approximately on September 22nd in northern latitudes, the sun crosses the celestial equator from the Northern to the Southern Hemisphere. In the Southern Hemisphere, this same phenomenon occurs approximately on March 21st.


Mass of snow moving rapidly down a mountain slope. Avalanches can be dry or wet, according to the nature of the snow; usually dry snow forms loose avalanches and wet snow forms slabs. It has a considerably destructive force.

Avalanche wind

Rush of air produced by an avalanche or landslide.

Aviation weather observation

Evaluation of those weather elements that are fundamental for aircraft operations. This is done according to a set procedure.

Axis of anticyclone

The line joining the points of maximum pressure at the surface and the upper levels.

Axis of depression

Line joining the points of maximum pressure at the surface and the upper levels.