Calendaricity / Singularity

A characteristic meteorological condition (i.e. warm or cold spells) which tends to occur at a fixed, or almost fixed, calendar date at a particular location.


Absence or air motion or wind with a speed of less than 1 knot (Beaufort scale wind force 0).

Calvus (cal)

Cumulonimbus in which at least some protuberances of the upper part are beginning to lose their cumuliform outlines but in which no cirriform parts can be distinguished. The protuberances and sprouting tend to form a whitish mass, with more or less vertical striations.

Capacity of the wind

Total weight of material particles which may be lifted per unit volume of air by a wind of a specific speed.

Cap cloud

Stationary cloud on or above an isolated mountain peak; it is a particular case of pileus.

Capilatus (cap)

Cumulonimbus characterized by the presence, mostly in its upper portion, of distinct cirriform parts of a clearly fibrous or striated structure, frequently having the form of an anvil, a plume or a vast, more or less disorderly mass of hair. This type of cloud is usually accompanied by a shower, or by a thundershower, often with squalls and sometimes with hail; it frequently produces very well defined virga.

Castellanus (cas)

Clouds which present, in at least some portion of their upper part, cumuliform protuberances in the form of turrets which generally give the clouds a crenellated appearance. The turrets, some of which are taller than they are wide, are connected by a common base and seem to be arranged in lines. The castellanus character is especially evident when the clouds are seen from the side. This term applies to Cirrus, Currocumulus, Altocumulus and Stratocumulus.

Ceiling (US) / Height of cloud base

1. Height above the Earth’s surface of the base of the lower cloud layer whose amount exceeds a specific value.
2. Vertical visibility in a surface-based layer which completely obscures the whole sky.

Cellular circulation

Circulation in which the air particles in motion appear to be approximately confined in cells.

Cellular cloud pattern

Mesoscale organization of convection in the form of a quasi-regular pattern of cloud cells. Such patterns may be composed of open or closed cells, or both.


Temperature scale in which zero is the freezing point of water and one hundred is the boiling point, at standard atmospheric pressure.

Central depression

Large depression, often stationary or quasi-stationary, within which one or several smaller depressions circulate.

Center of action

Large depression, often stationary or quasi-stationary, within which one or several smaller depressions circulate.


Abbreviation of chlorofluorocarbons, also called freons. Synthetic compounds that were used in industrial applications throughout most of the 20th century. Owing to their long life in the atmosphere, they cause ozone depletion.

Change of state

Changes of atmospheric pressure, density, temperature and humidity in an atmospheric process, these properties being interrelated by the equation of state and by thermodynamic relations.

Chaos Theory

Theory that describes the behavior of certain nonlinear dynamical systems, which under certain circumstances and conditions show a phenomenon known as chaos. One of its characteristics is its sensitivity to initial conditions, which popularly receive the name of butterfly effect. The atmosphere is a typical example of a chaotic system.

Characteristic of pressure tendency

Shape of the curve recorded by a barograph during the three-hour period preceding an observation, represented on the synoptic chart by a symbol with a similar shape.


Level in the upper-atmosphere constituting the upper boundary of the chemosphere.


Vaguely defined region of the high atmosphere in which molecular dissociation and recombination occur during the day and night respectively, under the influence of ultraviolet radiation.


East to south-east wind blowing from the desert in Morocco.


Foehn type wind that blows, generally from the west or southwest, on the plains to the lee or eastern side of the Rocky Mountains, in U.S. and Canada. Its direction may be changed by topography.


Technique typically used for weather forecasts that consists in the removal of a color from one image to reveal another “behind” it. The removed color becomes “transparent” and the weather map appears where the screen was green or blue.


Thin layer above the photosphere and below the corona of the Sun, in which predominates gases as hydrogen and helium. It is best observed during total solar eclipse.

Circle of inertia

Nearly circular trajectory of an air particle moving horizontally at constant speed relative to the Earth’s surface in the absence of a horizontal pressure gradient. Balance is achieved between the Coriolis and centrifugal forces.

Circulation index

Parameter indicating the intensity of the atmospheric circulation in a particular region.

Circulation model

Simplified representation of atmospheric flow used to study its principal characteristics.

Circulation type

Pattern of atmospheric circulation with definite characteristics over a specific region.

Circumhorizontal arc

A halo phenomenon consisting of a colored arc, red on its upper margin; it extends of about 90° parallel to the horizon and lies about 46° below the Sun.

Circumzenithal arc

Photometeor consisting of a luminous arc of a circle situation in a horizontal plane and centered at the zenith. It sometimes happens that circumzenithal arcs are seen when a large halo is not visible.

Cirrocumulus (Cc)

Abbreviated Cc. Principal cloud type or genus that appears as a thin, white patch, sheet or layer of cloud, without shading, composed as tiny elements in the form of grains, ripples, etc., with a more or less regular arrangement.

Cirrostratus (Cs)

Abbreviated Cs. Principal cloud type or genus, that appears as a transparent, whitish cloud veil of fibrous or smooth appearance, covering the sky in total or part. They often produce halo phenomena.

Cirrus (Ci)

Abbreviated Ci. Principal cloud type or genus, composed of detached clouds in the form of white and delicate filaments, white patches or narrow bands. They may have a hair like appearance or a silky sheen, or both.

Clearing / Clearance

1. Decrease of total cloud amount from an initial cloudy state.
2. Time in which this decrease takes place.
3. Gap in a cloud layer covering the entire sky.

Clearing Index

Air quality index used to regulate air quality decisions. It is defined as the mixing depth multiplies by the transport wind.


Synthesis of weather conditions prevailing over time at a given point; it is the aggregate of the varying aspects of the atmosphere-hydrosphere-land surface system, in a given area, characterized by long-term statistics (mean values, variances, probabilities of extreme values) of the meteorological elements in that area, for a period of 30 years or more.

Climate change / Climatic change

Any systematic change in the long-term statistics of climate elements (e.g. temperatures, pressure or winds) sustained over several decades or longer. Generally speaking, it refers to any kind of climatic inconsistency regardless of their statistical factor, if it is due to natural external forcing, such as internal processes of the climate system, or anthropogenic forcing. In a more restricted sense, it denotes a significant change in the mean values of a meteorological element (such as amount of precipitation) in the course of a certain period of time (a decade or longer).

Climatic classification

Division of the Earth’s climates into a worldwide system of contiguous regions, each of which is defined by the relative homogeneity of its climatic elements. Examples are Köppen’s and Thornthwaite’s climate classifications.

Climatic divide

Discontinuity between regions with different climates.

Climatic element

Any one of the properties or conditions of the atmosphere which together define the climate of a place (e.g. temperature, humidity, precipitation).

Climatic factors

Physical conditions (other than the climatic elements) which control the climate (latitude, elevation, topography, distribution of land and sea, ocean currents, etc.).

Climatic optimum

The period of history from about 5000 to 2500 BC during which temperatures were higher than at present in nearly all parts of the world.

Climatic oscillation

A fluctuation in which the variable tends to move gradually and smoothly between successive maxima and minima.

Climatic periodicity

A rhythm in which the time interval between successive maxima and minima is constant or very nearly constant throughout the record.

Climatic region

Region in which there is a fairly uniform climate according to specific criteria.

Climatic revolution

Climatic variation of a very long period (more than 106 years), possibly due to geotectonic activity such as continental drift, orogeny and large-scale changes in land and water distribution, and to solar variations.

Climatic rhythm

An oscillation or a vacillation in which the successive maxima and minima occur at approximately equal intervals of time.

Climatic risk

The degree of probability that unfavorable weather will occur over a certain period of time.

Climatic trend

A climatic change characterized by a smooth, monotonic increase or decrease of average value in the period of record. Not restricted to a linear change with time, but characterized by only one maximum and one minimum at the end points of the record.

Climatic type

Climatic of a region, characterized by a series of means and annual variations of temperature, precipitation and other elements. The types are often designated by letters, as in Kӧppen’s classification.

Climatic vacillation

A fluctuation in which the climatic variable tends to dwell alternately around two or more average values, and to drift from one to the other at regular or irregular intervals.

Climatic variation

A fluctuation, or a component thereof, whose characteristic time scale is sufficiently long to result in an appreciable inconstancy of successive 30-year averages (normal) of the variable. It is often used to designate common natural variations from one year to the next, or changes from one decade to the next.

Climatic zone

Zone defined by the latitudinal distribution of the climatic elements. The expressions polar, temperate, subtropical, tropical and equatorial climate, are used to indicate the climatic zones which succeed each other from the pole to the equator.

Climatological division

The division of climate according to different space scales (macro, meso, micro, crypto, etc.) or different time-scales (Pleistocene, etc.).

Climatological front

Mean or characteristic geographical position of a front in a specific region of the Earth.

Climatological standard normal

Average of climatological data calculated for the following consecutive 30-year periods: 1 January 1901 to 31 December 1930; 1 January 1931 to 31 December 1960; etc..


Description and scientific study of the mean physical state of the atmosphere, which is climate, together with its statistical variations in time and space as reflected in weather behavior over a specified period of time –many years-.


Search for laws offering precise explanations of climatic phenomena.

Closed cells

Mesoscale organization of convective cloud, having the appearance of cloud patches of roughly equal size separated by cloud-free rings.


Hydrometeor consisting in a visible aggregate of minute water droplets and/or ice particles in the atmosphere, above Earth´s surface, suspended in the free air and usually not touching the ground. It may also include non-aqueous particles, such as fume, smoke or dust.

Cloud amount / Cloud cover

Portion of the sky covered by clouds of a certain genus, species, variety, layer or combination of clouds- usually measured in tenth or eighth of the sky cover.

Cloud band

Dense aggregate of cloud extending along a straight or curved line.

Cloud bank

A fairly well-defined mass of clouds observed at a distance; it covers an appreciable portion of the horizon sky but does not extend overhead.

Cloud bar

Dense bank of clouds which appears on the horizon with the approach of an intense tropical cyclone; also, any long, narrow unbroken band of cloud.

Cloud base

The lowest level of a cloud or cloud layer.

Cloud classification

System of distinguishing and grouping clouds according to one or more of the following criteria: appearance, height, process of formation and particulate composition.

Cloud cluster

Convective cloud system often associated with low altitude convergence zones and covering an area of 190 to 580 square miles (500 to 1500 km2).

Cloud discharge

Lightning discharge which occurs within a thundercloud and procedures a diffuse illumination usually without a distinct channel being seen.

Cloud droplet / Cloud drop

Drop, supercooled or not, belonging to a cloud and having a diameter of about 4 to 100µm.

Cloud electrification

Process whereby different parts of a cloud become positively or negatively charged, especially in a thunderstorm.

Cloud forest

The appearance which the upper surface of a layer of clouds assumes when it has protuberances and sprouting in the form of towers.

Cloud genera

Main characteristic, mutually exclusive forms of clouds, constituting the basis of the cloud classification included in the International Cloud Atlas from 1956.

Cloud layer

Arrangement of clouds, continuous or composed of separated elements whose bases are at the same level and whose thickness are approximately the same.

Cloud luminance

Luminance determined by the amount of light scattered, reflected and transmitted by the particles which constitute a cloud. This light comes mainly from a luminary or from the sky; an appreciable part may also come from the Earth’s surface.

Cloud particles

The particles of water, either drops of liquid water or ice crystals, which form a cloud.

Cloud physics

The study of the physical and dynamical processes governing the structure and development of clouds and of the associated precipitation.

Cloud sector

Differentiated zone of a cloud system in which the general appearance of the sky, as a whole, displays marked peculiarities.

Cloud seeding

Weather modification technique in which active agents (aerosol, small ice particles) are added to clouds basically in order to simulate precipitation processes, dissipate clouds or fog and suppress hail. The most common used agents are carbon dioxide, silver iodide and sodium chloride.

Cloud sheet

Particular arrangement of clouds forming a continuous and relatively thin layer of great horizontal extent.

Cloud streaks

Aggregates of cloud distributed along narrow paths, straight or curved.

Cloud street

Clouds arranged in lines roughly parallel to the wind direction and appearing, on account of perspective, to converge towards a point or two opposite points on the horizon called the radiation point(s). The cloud most frequently appearing in cloud streets is Cumulus mediocris.

Cloud system

Distinct and lasting grouping of clouds, generally comprising several differentiated zones (cloud sectors) which are assembled in a characteristic way and in each of which the general aspect of the sky as a whole displays marked peculiarities.

Cloud system of a depression

Cloud system typically associated with a low-pressure system.

Cloud-to-cloud discharge

Lightning discharge occurring between a positively charged center and a negatively charged center, these two centers being located in different clouds.

Cloud top

The highest level in a specific cloud or cloud layer where the air contains a perceptible quantity of cloud particles.

Cloud top temperature

The temperature of a cloud top or of the air near that level.

Cloud tracer

Cloud whose movement is used as a measure of air motion.

Cloud veil

Cloud sheet sufficiently transparent to allow the position of the Sun or the Moon to be detected.

Cloud winds

Wind estimates based on the observed movements of identifiable cloud elements over short time intervals.

Clouds from volcanic eruptions

Clouds produced by volcanic eruptions generally appear like strongly developed cumuliform clouds with rapidly growing protuberances. They may spread out at a high altitude over vast areas, so that the sky assumes a peculiar tint which may persist for several weeks.

Cloudy skies

Sky with a total cover greater than 4 oktas.


In a colloidal system or in a system considered analogous (droplets in a cloud), -the joining of small particles in suspension into larger particles, forming a precipitate.


Process of formation of a single liquid water drop by the union of two or more colliding drops.

Coalescence efficiency

The fraction of all colliding drops which coalesce to form larger drops.

Coarse mesh

In a numerical forecasting model, mesh used over a large area (often a hemisphere) to provide boundary conditions for the fine-mesh calculations.

Coastal climate

The climate in coastal regions resulting from the modification of the macroclimate due to the discontinuity in surface roughness at the coastline and to the different thermal and moisture properties of sea and land.

Coefficient of transparency

Coefficient, appearing in the mathematical expression of the Bouguer-Lambert law, expressing the fraction of the direct solar radiation which arrives at the Earth’s surface when the sun is at the zenith.


Saddle/backed region with a weak pressure gradient which appears between two depressions and tow anticyclones arranged alternately in a cross.

Cold-air drop

Vast volume of cold air separated from the mass of cold air at higher latitudes during the formation of a cut-off low.

Cold anticyclone

Anticyclone which is cold relative to surrounding areas at the same levels; usually only the lower layers are considered.

Cold dome

Dome-shaped mass of cold air, bounded laterally and above by a frontal surface, which occurs mainly in the rear of a depression.

Cold front

Any non-occluded front which moves so that cold air relatively replaces warmer air; that is, the leading edge of a relatively cold air mass.

Cold low

Depression which is cold with respect to its surroundings at the same levels.

Cold occlusion

Occlusion in which the cold air behind the front is colder than the air in advance of it.

Cold pole

The hemispheric location which has the lowest mean annual temperature.

Cold tongue

Marked extension of an area of cold air equatorwards.

Cold wave

Marked cooling of the air, or the invasion of very cold air, over a large area.

Collision-coalescence process

Process that produces precipitation by the collision and coalescence between cloud droplets, drizzle drops and rain drops. Precipitation is produced by the growth of liquid particles by virtue of the impact of two water drops and their merging into a single larger one.

Columnar snow crystals

Fairly short prismatic ice crystals either solid or hollow, the ends of which may be plane, pyramidal, truncated or hollow.

Comfort index

An index which expresses the combined effects of temperature and humidity on bodily comfort and which can be used to indicate, by reference to a table, graph or monogram, whether the conditions specified are broadly speaking comfortable or not.

Complex low

An area of low pressure containing more than one low-pressure center.

Compression wave

Wave generated by variations in the atmospheric pressure which are propagated because of the compressibility of the air.

Computer model

Data-driven system that uses a incorporated set of rules to predict the results of a process or to create a simulation about how a given set of conditions will change over time. In this encyclopedia, refers to a system that uses a built-in set of equations to simulate the atmospheric behavior and then generate information to produce weather forecasts.


In general, physical process that results in the transition from the gaseous to the liquid state; opposite of evaporation. In meteorology, the term applies to the transformation of water vapor into dew, fog or cloud droplets.

Condensation level

Level at which the air subjected to a lifting process becomes saturated.

Condensation nuclei / Condensation nucleus

Nuclei, that is, aerosol particles on which water vapor can condensate.

Condensation trail / Contrail / Adiabatic trail

Cloud which forms in the wake of an aircraft when the air at flight is sufficiently cold and moist.


Transfer of energy as a sole consequence of random molecular collisions. The conduction of heat energy is result of temperature gradients.

Congestus (con)

Cumulus cloud which are markedly sprouting and are often of great vertical extent; their building upper portions frequently resemble a cauliflower.

Continental air

Mass of air which remains over a continent for several days and which, therefore, has fairly low moisture content.

Continental anticyclone

Area of high atmospheric pressure which remains over a continent during the cold season.

Continental climate

Climate characteristic of the interior of a continent marked by large annual and daily temperature ranges, low relative humidity and moderate or small irregular rainfall.


Degree to which a climate is affected by continental influences; the inverse of “oceanity”.


Organized molecular motions within a fluid -layer of air- leading to vertical transport and mixing of the properties of that fluid –e.g.: heat-. In free convection, motion is cause solely by density differences within the fluid. In forced convection, motion is caused by mechanical forces, such as orography.

Convection cell / Convective cell

A mass of air in a well-organized process of convection, with a systematic pattern of internal motion and little or no mixing with neighboring masses.

Convection cloud / Convective cloud

Cumuliform cloud which forms in an atmospheric layer made unstable by heating at the base or cooling at the top.

Convection current / Convective current

1. Any current of air involved in convection such as a thermal or an up-draught in a Cumulus cloud.
2. Any net transport or electric charge effected through mass motions of some charged medium (e.g., eddy convection).
3. Any electric current induced by other than electrical forces (e.g., falling charged precipitation particles).

Convection Zone

Mass of air in a well organized process of convection, with a systematic pattern of internal motion and little or no mixing with surrounding masses.

Convective cluster

An identifiable field of convective clouds.

Convective condensation level – CCL

Condensation level attained as the result of convection of air from the Earth’s surface.

Convective index / Stability index

A measure of the static stability of an atmospheric layer.

Convective or local storm

Storm caused by convective motion in the atmosphere; meso-meteorological scale storms. Usually, thunderstorms, squalls and tornadoes are put in this category.

Convective precipitation

Precipitation caused by convective motion in the atmosphere.

Convective region / Adiabatic region

Region in which the vertical distribution of temperature is controlled by convection.

Convective thunderstorm

Thunderstorm produced by a convective cloud.

Convective zone

Is located in the Suns outer layer, between the Sun’s surface (the photosphere) and the inner radiative zone. Here the energy is transferred by means of convection. Hotter gasses coming from the radiative zone expand and rise, then reach a point of equilibrium before cooling down and beginning to sink back toward the Sun’s surface. As the gasses sink closer to the radiative zone, they reheat and begin to rise again.  This repeating process creates thermal column which forms a pattern on the surface of the Sun, known as granulation.


Contraction of a vector field. Convergence is a divergence of negative sign.

Convergence line

Line along which the horizontal convergence is a maximum.

Cooling degree-day

A degree-day used to estimate the energy requirements for air-conditioning or refrigeration. One cooling degree-day is counted for each degree that the daily mean temperature is higher than a specific base temperature, e.g., 25°C.

Core sample

Sample of material obtained by driving a hollow tube into the undisturbed medium (rock, soil, snow, etc) and withdrawing it with its contained sample or core.

Coriolis Effect

Deflection resulted from Coriolis force.

Coriolis force

Apparent force, due to Earth´s rotating motion, relative to Earth´s surface, that produces a deflection of moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.


Photometeor formed by one or more sequences (usually not more than three) of colored rings, of small angular radii, centered on the Sun or the moon, when veiled by a thin cloud. Outer part of the gaseous envelope that surrounds the Sun; it is easily visible during total solar eclipses or with a coronagraph.

Corona (of the Sun)

Outer part of the gaseous envelope that surrounds the Sun; it is easily visible during total solar eclipses or with a coronagraph.

Coronal Mass Ejection (CME)

Massive burst of solar wind and radiation, originated from the Sun’s surface during the Solar Maxima period that occurs every 11 years. CMEs are sometimes associated with flares but usually occur independently.

Cosmic dust

Atmospheric dust originating directly from extra-terrestrial sources (comets, meteors, etc.).

Cosmic radiation

Radiation of a very high energy and great penetrative power which emanates from cosmic regions.


Faint spot of light, round or elongated, in the night sky at the antisolar point, linking the east and west zodiacal lights.

Countergradient wind

Component of the wind in the direction opposite to that of the horizontal pressure gradient.


Cardiopulmonary Resuscitation

Crepuscular rays

Same as shadow bands. Dark blue bands alternation with light ones, situated on or below the horizon, radiating from the sun, in a fan-like disposition during twilight.

Crest cloud

Stationary cloud on, or slightly above or downwind from, a ridge of mountains.

Cretaceous warming period

Warming period within the Cretaceous Period, which is one of the major divisions of the geologic timescale. The Mid-Cretaceous Period extends from 120 to 90 million years before present. Climatic-wise, it was characterized by very warm conditions, with no ice at the poles, a much higher than today sea level and vast continental areas covered by shallow seas. Sediment cores show that temperatures in the deep ocean were 15-20° C higher than today.

Criteria pollutant

Pollutants that can produce health injury, environmental harm or property damage. In United States, the Criteria Pollutants are: carbon monoxide, lead, ozone, particulate matter with aerodynamic size less than or equal to 10 um and sulfur dioxide.

Cross-isobar angle

Angle between the wind vector and an isobar, at any level. Such an angle is most conspicuous within the friction layer where the wind very commonly has a component from high to low pressure.

Cross wind

Wind which blows in a direction perpendicular to that of the motion of an object relative to the Earth’s surface.


Outermost solid layer of a planet.


The climate of small enclosed spaces (e.g. a house); the smallest scale of climate.

Cumuliform cloud

Cloud with the bulging appearance of a Cumulus. When such clouds, arranged in lines and joined by a common base, possess protuberances giving them a turreted appearance, they are classed in the species castellanus. When they constitute elements separated into tufts they are classed in the species floccus.

Cumulonimbus (Cb)

Abbreviated Cb. Principal cloud type or genus vertically developed and exceptionally dense, occurring in the form of isolated clouds or as a line or wall of clouds with separated upper portions. At least part of its upper portion is usually smooth, fibrous, or striated and nearly almost flattened as it approaches the tropopause.

Cumulus (Cu)

Abbreviated Cu. Principal cloud type of genus, in the form of individual, detached elements that are generally dense and with non-fibrous outline sharps. They are clouds arranged in lines and joined by a common base, relatively dark and horizontal in the base and brilliant white in the sunlit parts of these clouds, with protuberances giving them a turreted appearance.

Cut-off high

Warm anticylone which has been displaced poleward from the middle-latitude belt of westerly winds.

Cut-off low

Cold low which has been displaced equatorward out of the basic westerly current in which it was located.


Process of initiation or intensification of a cyclonic circulation.


Process for weakening or terminating of a cyclonic circulation.

Cyclone / Depression, Low pressure area

Closed circulation in which fluid moves in the same sense as that of the earth, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere, and undefined at the equator. It is a region of the atmosphere in which the pressure is lower than that of the surrounding region at the same level. On a synoptic chart, it is represented by a system of isobars at a specified altitude level, enclosing relatively low values of pressure.

Cyclone family / Family of depression

A series of lows forming successively along a cold front.

Cyclone movement

The spatial displacement of a center of low pressure caused by the local redistribution of mass in the atmosphere. The trajectory of the center is often altered by heating or cooling of the air column. This can result from thermal fluxes at the surface or latent heat transformations associated with cloud formation and precipitations, These processes change the temperature distribution in the air column, resulting in density changes which modify the surface pressures.

Cyclonic circulation

Atmospheric circulation associated with a depression. It is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Cyclonic or depression storm

Any storm associated with a cyclonic circulation, or within it.

Cyclonic precipitation

Any precipitation associated with, or within the circulation of a cyclone.

Cyclonic rotation

Rotation along a streamline or a trajectory which, viewed from above, is a counterclockwise in the Northern hemisphere and clockwise in the Southern Hemisphere.

Cyclonic shear

1. Horizontal shear such that the wind speed increases from left to right in the Northern hemisphere and from right to left in the Southern Hemisphere.
2. Vertical, shear such that the rotation from the wind vector to the shear vector at a particular point is cyclonic in sense.

Cyclonic wave / Frontal wave

Undulation on a surface front corresponding to the first stage of development of an extra-tropical depression.

Cyclostrophic wind

Theoretical wind which results from the equilibrium between the horizontal pressure force and the centrifugal force due to air motion on a curved path, assuming that only these forces act on the air.