DICTIONARY of WEATHER and CLIMATE

The Facts On File DICTIONARY

of WEATHER and CLIMATE

Revised Edition

The Facts On File DICTIONARY

of WEATHER and CLIMATE

Revised Edition Edited by

Jacqueline Smith

The Facts On File Dictionary of Weather and Climate Revised Edition

Copyright © 2001, 2006 by Market House Books Ltd All rights reserved. No part of this book may be reproduced or utilized in any

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PREFACE

This dictionary is one of a series designed for use in schools. It is intended for stu- dents of meteorology and climatology, but we hope that it will also be helpful to other science students and to anyone interested in science. Facts On File also pub- lishes dictionaries in a variety of disciplines, including astronomy, biology, chem- istry, computer science, Earth science, physics, mathematics, forensic science, and marine science.

This second Facts On File edition has been extensively revised and extended. The dic- tionary now contains over 2,000 headwords covering the terminology of modern meteorology and climatology. A totally new feature of this edition is the inclusion of over 800 pronunciations for terms that are not in everyday use. The Appendix con- tains a chronology of important events and discoveries in the subject, a bibliography, and relevant Web sites, as well as a number of useful conversion tables. A guide to using the dictionary has also been added to this latest version of the book.

We would like to thank all the people who have cooperated in producing this book.

A list of contributors is given on the acknowledgments page. We are also grateful to the many people who have given additional help and advice.

ACKNOWLEDGMENTS

Editors (previous editions)

Stella Stiegeler B.Sc. John O. E. Clark B.Sc.

Contributors

B. M. Abbott B.Sc., Ph.D., F.G.S. Anna Clyde B.Sc. Andrew Hill B.Sc., Ph.D., F.G.S.

I. P. Joliffe B.Sc., M.Sc, Ph.D., C.I.C.E., F.C.S., F.R.G.S. R. B. Lanwarne B.Sc. P. A. Smithson B.Sc., Ph.D., F.R.Met.S. T. J. Speechley B.Sc.

F. A. Sultan B.Sc., M.Sc., D.I.C. S. D. Weaver B.Sc., Ph.D., F.G.S.

A. C. Wornell B.A.

Pronunciations William Gould B.A.

CONTENTS

Preface v Acknowledgments

vi Guide to Using the Dictionary

viii Pronunciation Key

x Entries A to Z

1 Appendixes

I. Chronology 245

II. Conversion Tables 253

III. Temperature Conversions 258

IV. Web Pages 259 Bibliography

GUIDE TO USING THE DICTIONARY

The main features of dictionary entries are as follows.

Headwords

The main term being defined is in bold type:

aggregation The process by which ice crystals grow to form snowflakes through collision and clumping together.

Variants

Sometimes a word has a synonym or alternative spelling. This is placed in brackets after the headword, and is also in bold type:

coreless winter (kernlose winter) A unique feature of the climate in Antarctica, the condition in which surface tempera- tures remain almost the same from the au- tumn equinox to the end of winter.

Here, ‘kernlose winter’ is another term for ‘coreless winter’. Generally, the entry for the synonym consists of a simple cross-reference:

kernlose winter See coreless winter.

Abbreviations

Abbreviations for terms are treated in the same way as variants:

actual evapotranspiration (AE) The true amount of water vapor returned to the atmosphere through evaporation from a ground surface and transpiration from its plant cover where the water supply is lim- ited... .

The entry for the abbreviation consists of a reference:

AE See actual evapotranspiration.

Multiple definitions

Some terms have two or more distinct senses. These are numbered in bold type

offshore current

1. A current in the

sea or ocean that flows parallel to the coast.

2. Any current that flows away from shore.

Cross-references

These are references within an entry to other entries that may give additional useful information. Cross-references are indicated in two ways. When the word appears in the definition, it is printed in small capitals:

anticyclonic rotation The direction of the flow around an ANTICYCLONE (high).

In this case there is a cross-reference to the entry for ‘anticyclone’. Alternatively, a cross-reference may be indicated by ‘See’, ‘See also’, or ‘Compare’, usu-

ally at the end of an entry:

degree

1. A unit of measure on a tem-

perature scale. See Celsius scale; Fahren- heit scale; Rankine scale.

Hidden entries

Sometimes it is convenient to define one term within the entry for another term:

pilot balloon

A small meteorological

balloon, inflated with hydrogen or helium, and used for visually observing the upper wind speed and direction above a station. ... . A pilot balloon observation (pibal), the measurement and calculation of the wind speed and direction, is made by reading the elevation and azimuth angles of a theodo- lite while ... .

Here, ‘pilot ballon observation’ and ‘pibal’ are hidden entries under ‘pilot baloon’, and are indicated by italic type. Individual entries consist of a simple cross-reference:

pibal See pilot balloon.

Pronunciations

Where appropriate pronunciations are indicated immediately after the headword, en- closed in forward slashes:

halocarbon /ha-loh-kar-bŏn/

A chemi-

cal compound consisting of carbon, possi- bly hydrogen, and any of the halogens, i.e. chlorine, fluorine, bromine, or iodine. See also chlorofluorocarbon; halon; hydro- chlorofluorocarbon.

Note that simple words in everyday language are not given pronunciations. Also head- words that are two-word phrases do not have pronunciations if the component words are pronounced elsewhere in the dictionary.

Pronunciation Key

A consonant is sometimes doubled to prevent accidental mispronunciation of a syllable resembling a familiar word; for example, /ass-id/ /acid/,rather than /as-id/ and /ul-tră- /sonn-iks / /ultrasonics/, rather than /ul-tră-son-iks/. An apostrophe is used: (a) between two consonants forming a syllable, as in /den-t’l/ /dental/,and (b) between two letters when the syllable might otherwise be mispronounced through resembling a familiar word, as in /th’e-ră-pee/ /therapy/ and /tal’k/ /talc/. The symbols used are:

/a/ as in back /bak/, active /ak-tiv/ /nk/ as in rank /rank/, bronchus /bronk-ŭs/ /ă/ as in abduct /ăb-dukt/, gamma /gam-ă/

/o/ as in pot /pot/ /ah/ as in palm /pahm/, father /fah-ther/,

/ô/ as in dog /dôg/ /air/ as in care /kair/, aerospace /air-ŏ-

/o/ as in buttock /but-ŏk/ spays /

/oh/ as in home /hohm/, post /pohst/ /ar/ as in tar /tar/, starfish /star-fish/, heart

/oi/ as in boil /boil/ /hart/

/oo/ as in food /food/, croup /kroop/, fluke /aw/ as in jaw /jaw/, gall /gawl/, taut /tawt/

/flook/

/ay/ as in mania /may-niă/ ,grey /gray/ /oor/ as in pruritus /proor-ÿ-tis/ /b/ as in bed /bed/

/or/ as in organ /or-găn/, wart /wort/ /ch/ as in chin /chin/

/ow/ as in powder /pow-der/, pouch /d/ as in day /day/

/powch/

/e/ as in red /red/ /p/ as in pill /pil/ /ĕ/ as in bowel /bow-ĕl/

/r/ as in rib /rib/ /ee/ as in see /see/, haem /heem/, caffeine

/s/ as in skin /skin/, cell /sel/ //kaf-een/,/ baby /bay-bee/

/sh/ as in shock /shok/, action /ak-shŏn/ /eer/ as in fear /feer/, serum /seer-ŭm/

/t/ as in tone /tohn/ /er/ as in dermal /der-măl/, labour /lay-ber/

/th/ as in thin /thin/, stealth /stelth/ /ew/ as in dew /dew/, nucleus /new-klee-ŭs/

/th/ as in then /then/, bathe /bayth/ /ewr/ as in epidural /ep-i-dewr-ăl/

/u/ as in pulp /pulp/, blood /blud/ /f/ as in fat /fat/, phobia /foh-biă/, rough

/ŭ/ as in typhus /tÿ-fŭs/ /ruf/

/û/ as in pull /pûl/, hook /hûk/ /g/ as in gag /gag/

/v/ as in vein /vayn/ /h/ as in hip /hip/

/w/ as in wind /wind/ /i/ as in fit /fit/, reduction /ri-duk-shăn/

/y/ as in yeast /yeest/ /j/ as in jaw /jaw/, gene /jeen/, ridge /rij/

/ÿ/ as in bite /bÿt/, high /hÿ/, hyperfine /hÿ- /k/ as in kidney /kid-nee/, chlorine /klor-

per-fÿn /

een /, crisis /krÿ-sis/ /yoo/ as in unit /yoo-nit/, formula /form- /ks/ as in toxic /toks-ik/

yoo-lă /

/kw/ as in quadrate /kwod-rayt/ /yoor/ as in pure /pyoor/, ureter /yoor-ee- /l/ as in liver /liv-er/, seal /seel/

ter /

/m/ as in milk /milk/ /ÿr/ as in fire /fÿr/ /n/ as in nit /nit/

/z/ as in zinc /zink/, glucose /gloo-kohz/ /ng/ as in sing /sing/

/zh/ as in vision /vizh-ŏn/

AAO See Antarctic Oscillation. ablation The disappearance of snow

and ice through the processes of melting,

height h

temperature of air parcel

evaporation, or sublimation; the term may also be applied to the calving of glaciers,

removal of snow and ice by the wind (de- flation), and avalanching. The rate of abla- D A

tion is controlled principally by air L R

EL

temperature; solar radiation, humidity, temperature R

rainfall, and wind speed also contribute to of environment the process. Ablation usually refers to sur-

face snow and glacier ice but is sometimes temperature applied to ice crystals and snowflakes in

the atmosphere. Absolute instability absolute drought In the UK, a (9.8°C km –1 ). Consequently, at any height

DROUGHT in which there is a period of at

h the air parcel will be warmer and less least 15 consecutive days, none of which

dense than its surroundings, and so will have recorded 0.25 mm (0.01 in) of rain or

continue to rise, leading to absolutely un- more. This definition is inappropriate for

stable air, and often to the formation of dissimilar climates; it was originally de-

clouds of great vertical depth. fined in 1887 by the former British Rainfall Organisation, but is no longer much used.

absolutely stable air See absolute sta- See also partial drought.

bility.

absolute humidity

absolutely unstable air See absolute the amount of moisture ( WATER VAPOR ) in

A measurement of

instability.

the atmosphere, usually expressed as the mass (grams) of water vapor per cubic

absolute momentum The sum of the meter of moist air. In contrast to RELATIVE

momentum of a particle relative to the HUMIDITY , absolute humidity expresses the

Earth and the momentum of the particle amount of water in the air regardless of the

due to the Earth’s rotation. temperature. It is sometimes referred to as vapor concentration. See also humidity;

absolute stability The atmospheric specific humidity.

state in which the observed decrease in temperature with height, as defined by the

absolute instability The atmospheric ENVIRONMENTAL LAPSE RATE (ELR), is less state in which the observed decrease in

than the SATURATED ADIABATIC LAPSE RATE temperature with height, as defined by the

(SALR). Consequently, at any height h the

ENVIRONMENTAL LAPSE

RATE (ELR), is air parcel will be cooler and therefore greater than that of a dry air parcel cooling

denser than its surroundings. If an air par- adiabatically, as defined by the dry adiabat

cel is perturbed in this situation, it will re- cel is perturbed in this situation, it will re-

continuance, and can either be attached or separate from it. Examples include PILEUS

height h temperature (cap cloud), a smooth ‘cap’ cloud of small

of environment horizontal extent that forms just above cu- muliform clouds, PANNUS , and VELUM ,

h which are all generally separate from the

main cloud, and ARCUS (shelf and roll

clouds), which are attached. See also cloud temperature SA

classification.

LR of air parcel

/ă-klÿ-mă-tă-zay- temperature

acclimatization

shŏn / The process of adaptation made by organisms, such as animals (including hu-

Absolute stability mans) and plants, to a change in climate and/or environment, which may result, for

turn to its original position because vertical example, from a long-term CLIMATE air motion cannot be sustained. Absolutely

CHANGE or from the organism being trans- stable air typically accompanies a summer

ferred to a new environment. ANTICYCLONE (high). accretion In cloud physics, the process absolute temperature See thermody- by which ice crystals in a cloud grow namic temperature.

through contact with supercooled water droplets, which freeze instantly on impact

absolute vorticity See vorticity. with the ice crystals to form rime ice. Grau- pel (soft hail) forms through the accretion

absolute zero The zero of THERMODY - of small supercooled droplets onto falling NAMIC TEMPERATURE . This is the lowest

ice crystals. If the graupel descends temperature theoretically possible and al-

through an area of plentiful droplets above though it cannot be reached in practice,

freezing level, a coating of clear ice may temperatures of 10 –3 K are used in low-

form through accretion to produce ice pel- temperature physics. Absolute zero, is 0 K

lets. Hailstones form from successive ac- = –273.15°C. See kelvin.

cretions of opaque and clear ice when the nuclei are swept up into areas of abundant

absorption In the atmosphere, the con- supercooled droplets. version of electromagnetic radiation into internal energy of molecules. The absorp-

accumulated temperature The sum of tion process depends on the energy levels of

departures (positive or negative) of temper- the molecule and on the wavelength of the

ature from a reference temperature for a incident radiation. For example, ozone ab-

specific period. These departures may be, sorbs ultraviolet radiation; carbon dioxide

for example, on a daily, monthly, or yearly and water vapor both absorb infrared radi-

mean basis. The accumulated temperature ation. Absorption occurs across the whole

can be represented by the number of de- spectrum for electromagnetic wavelengths,

gree-hours or the number of DEGREE - DAYS . although there are ‘atmospheric windows’

For a number of specified hours (X), the in which little absorption occurs, notably

temperature exceeds the reference temper- in the radio and visible regions of the spec-

ature by a number of degrees (Y), thus trum. Of the incoming radiation around

there are X × Y degree-hours and (XY)/24 19% is absorbed by the atmosphere: 16%

degree-days.

by gases and dust and 3% by clouds. accumulation The increase in the sur-

accessory cloud

A smaller cloud fea- face area of an environment by a covering ture that is related to a main cloud, depen-

of snow, firn, and ice. Accumulation dent on it for development and

sources include precipitation in the form of sources include precipitation in the form of

snow and freezing rain; hoar frost from ra- emissions, especially by the use of catalytic diative cooling at the surface; rime associ-

converters in vehicle exhaust systems and ated with supercooled water droplets in the

flue-gas desulfurization plants in power atmosphere; and mass movement transport

stations. In the US, Phase I of the Acid Rain processes, such as avalanches and land-

Program of the 1990 Clean Air Act slides.

Amendments set out to reduce SO 2 emis- sions by 10 million tons and nitrogen oxide

acid deposition The deposition of emissions by 2 million tons by 2000. This acidic substances from the atmosphere

target has been reached. Phase II, begun in onto the Earth’s surface. This may be

2000, tightens permitted limits and encom- through wet deposition of acidic sub-

passes a wider range of plants. The Acid stances from precipitation that is more

Rain Program is noteworthy for its re- acidic than ‘normal’ (the background level

of acidity of rain is around pH 4.8 to 5.6), placement of traditional command regula- through dry deposition in which dry acidic

tions by an allowance trading system particulates settle directly onto the surface

making use of market forces to reduce pol- (e.g. acid soot), or through acidic vapors or

lution.

gases reacting with plants, water, and other surfaces. The sources of acidic pollu-

actinometer /ak-tă-nom-ĕ-ter/ An in- tants are generally emissions from the com-

strument used for measuring electromag- bustion of fossil fuels and other industrial

netic radiation, especially that transmitted activities. See also acid rain; occult deposi-

by the Sun, scattered by the atmosphere, tion.

and reflected by the Earth’s surface. There are a number of different designs including

acid rain Precipitation in which acidity those that use bimetallic strips which, de- is above the natural level. Normal rainfall

pending on their heat capacity, surface is slightly acidic with a pH of about 5.5 as

area, and reflectivity, bend differently as a

a result of chemical reaction between natu- result of being heated by radiation. rally occurring CARBON DIOXIDE and water vapor in the atmosphere forming weak car-

active layer The upper surface layer in bonic acid. Precipitation with a pH value

periglacial areas that thaws and freezes of less than 5 is considered to be acidic.

seasonally and lies above permanently The burning of such fossil fuels as coal and

frozen ground (see permafrost). The depth oil in power stations, motor vehicles, and

of the active layer varies from a few cen- many industrial processes releases large

timeters to around 5 m (16 ft). Frost heav- quantities of SULFUR DIOXIDE and NITROGEN

ing (frost heave) occurs when the active OXIDES into the atmosphere. These pollu-

layer freezes in winter and when it thaws in tants can be carried considerable distances

summer the layer becomes unstable; in from their original source and react with

water vapor in the presence of sunlight to poorly drained silts and clays the soil may form weak solutions of sulfuric and nitric

flow above the permafrost table. acids.

When deposited, acid rain can cause se- active remote sensing The methods in

vere environmental problems, including REMOTE SENSING of collecting information, killing fish, damaging forests and crops,

for example, the measurement of atmos- and accelerating the decay of buildings; it

pheric properties, using instruments that may also be of harm to humans. First rec-

emit an artificial signal, which is then ognized as an environmental problem in

partly returned by the phenomena to the the 1950s, when increased acidity was ob-

instrument. Such an instrument is said to served in lakes in Canada and Scandinavia,

‘active’ because it actually generates the ar- it had reached serious levels by the late

tificial beam of radiation; LIDAR and RADAR 1970s. Since the 1980s various measures

are examples of active remote sensing. have been introduced to reduce polluting

Compare passive remote sensing.

actual evapotranspiration (AE) The true amount of water vapor returned to the atmosphere through evaporation from a ground surface and transpiration from its plant cover where the water supply is lim- ited. Compare potential evapotranspira- tion.

adiabat /ad-ee-ă-bat/

A line drawn on a

THERMODYNAMIC DIAGRAM along which an air parcel moves as it ascends or descends through the atmosphere, cooling or warm- ing adiabatically. There are two types of adiabat: the DRY ADIABAT and the SATU - RATED ADIABAT .

adiabatic /ad-ee-ă-bat-ik/ Denoting any process in which there is no change of heat. In meteorology it describes a thermody- namic process in which the temperature of an air parcel changes without any ex- change of energy with the surroundings. By the first law of thermodynamics:

∆Q = ∆U + ∆W, where ∆Q = heat added to the system (in this case the air parcel), ∆U = change in in-

ternal energy (a function of temperature) and ∆W = work performed against the sur- roundings. In an adiabatic process, ∆Q = 0; hence ∆U = –∆W. As an air parcel expands, its volume increases, so performing work against the surroundings. By the first law of thermodynamics, the internal energy (and therefore the temperature) of the air parcel must decrease during an adiabatic expansion process; conversely the air parcel’s temperature increases during an adiabatic compression process. Two types of adiabatic processes take place: (1) the dry adiabatic process, when the air parcel is unsaturated; and (2) the saturated adia- batic process, when the air parcel is satu- rated. A rising saturated air parcel cools more slowly with height than a dry parcel because the release of latent heat at the change of state from gas to liquid acts to buffer the temperature decrease that occurs as a result of the adiabatic expansion process.

adiabatic change The changes in the temperature, pressure, and volume of an

air parcel that occur as a result of an adia- batic process. See adiabatic.

adiabatic chart See thermodynamic di- agram.

adiabatic lapse rate The rate at which

a parcel of air changes temperature adia- batically as it moves vertically. In this case heat does not leave or enter the system: as an air parcel rises the reduction in sur- rounding pressure enables the air is to ex- pand, which results in its temperature falling; conversely, if the air parcel sinks and is compressed its temperature rises. The SATURATED ADIABATIC LAPSE RATE (SALR) and the DRY ADIABATIC LAPSE RATE (DALR) apply to the rate of temperature change of an air parcel, depending on the moisture content of the parcel.

adsorption The process by which com- pounds, molecules, or ions are attached to the surface of a particle by chemical or physical means to form a thin layer of liq- uid or gas.

advanced microwave sounding unit

(AMSU) An instrument carried on polar- orbiting satellites to measure global at- mospheric temperature profiles and atmospheric humidity. It replaced the orig- inal microwave sounding unit (MSU) that was carried on NOAA satellites. AMSU comprises two separate complementary units: AMSU-A is a 15-channel microwave sounding instrument that measures atmos- pheric temperature, even in cloudy condi- tions; AMSU-B is a 5-channel microwave instrument that measures profiles of at- mospheric humidity. NOAA-15 was the first satellite to fly AMSU; it has since been carried on NOAA-16 and NOAA-17 (see POES).

advanced very high resolution ra-

diometer (AVHRR) The imaging scan- ner widely used on many meteorological satellites. It has been used on board the US National Oceanic and Atmospheric Ad- ministration’s (NOAA) polar-orbiting satellites (see POES) since the launch of TIROS–N (1978).

actual evapotranspiration

Advanced Weather Interactive Pro-

cessing System (AWIPS)

A technologi-

cally advanced interactive computer system for information processing, display, and telecommunications developed for use by the US National Weather Service (NWS). The system integrates meteorolog- ical, hydrological, satellite, and radar in- formation and data enabling forecasts to

be prepared more accurately and swiftly. advection /ad-vwk-shŏn/ In meteorol-

ogy, the transfer of a property, such as heat or humidity, by motion within the atmos- phere, usually in a predominantly horizon- tal direction. Thermal advection, for example, is the transport of heat by the wind. Advection is most often used to sig- nify horizontal transport but can also apply to vertical movement, for example in

a HYDRAULIC JUMP . Large-scale horizontal advection of air is a characteristic of mid- dle-latitude zones and leads to marked changes in temperature and humidity across boundaries separating air masses of differing origins. See also cold advection. Compare convection.

advection fog

A fog produced when rel- atively warm moist air cools as it flows across a cooler surface. It occurs most com- monly over the sea when water tempera- ture is still cool in the spring and early to mid summer. Outbreaks of warm moist maritime tropical air stream poleward oc- casionally along the English Channel or across the Grand Banks off Newfound- land, for example. The chilly sea can cool the lowest layer of air to its dew-point tem- perature; further chilling will lead to con- densation of the fog droplets. California fog is advection fog that occurs along the coast of California when warm moist air from over the Pacific Ocean blows across the cooler California Current. Sea fog is a type of advection fog, which is always characterized by windy conditions. Over land, advection fog occurs sometimes when mild moist air blows across cooler surfaces. For example, it occurs in winter over the central US when moisture-laden air from the Gulf of Mexico flows north- ward inland.

AE See actual evapotranspiration. aeolian /ee-oh-lee-ăn/ See eolian. aerial photography Photography of

the Earth’s surface from airborne plat- forms, such as satellites, spacecraft, and aircraft; it is the oldest method of REMOTE SENSING . The photographs may be vertical or oblique (high-oblique or low-oblique) and are frequently taken in runs of over- lapping prints, which can be assembled into mosaics (print lay-down). Three- dimensional images can be viewed when pairs of photographs taken with a stereo- scopic camera are viewed through a stereoscopic viewer (stereoscope). Aerial photography has widespread applications within the earth sciences, e.g. in mapmak- ing and the revision of existing maps, and the production of three-dimensional mod- els of the land surface; in land-use planning and mapping; and in providing data on topography, geology, hydrology, and vege- tation and soils. Color infrared film, which is sensitive to near-infrared wavelengths and visible wavelengths, is used, for exam- ple, in vegetation and land-use studies.

aeroallergen /air-roh-al-er-jĕn/

A sub- stance or gas in the atmosphere capable of inducing an allergy in humans. For exam- ple, nitrogen dioxide is associated with al- lergen-induced nasal inflammation in asthmatics.

aerodynamic roughness The property of a surface, which may be solid or a layer of different density, that determines whether the flow of a fluid (e.g. air in the atmosphere) across it is turbulent or smooth. A surface is defined as aerody- namically rough when the fluid flowing over it is turbulent right down to the sur- face itself. In the atmosphere, this condi- tion is very common and all surfaces are ‘rough’ to some degree; how much depends on the height and spacing of the roughness elements.

aerological diagram /air-rŏ-loj-ă-kăl/ See thermodynamic diagram.

aerological diagram

aerology aerology /air-rol-ŏ-jee/ The study of the afterglow

1. A broad high arch of radi- atmosphere through its vertical extent,

ance sometimes observed in the western generally above the surface layers.

sky after sunset and during the deepening TWILIGHT .

aeronomy /air-ron-ŏ-mee/ The study of

2. See alpine glow. the upper atmosphere of a planet, such as the Earth, in which it is usually the study of

ageostrophic wind /ay-jee-oh-stroff- those layers above the lower stratosphere.

A theoretical wind; the vector differ- Dissociation and ionization are the main

ik /

ence between the actual observed wind and processes within these levels in the Earth’s

the GEOSTROPHIC WIND at a given level. It is atmosphere; phenomena include AIRGLOW

not, therefore, an actual wind that blows and the AURORA . Aeronomy is sometimes

but is a very important concept associated used for studies of all the atmospheric lay-

with CONVERGENCE , DIVERGENCE , and VER - ers.

TICAL MOTION , for example. These are sig- nificant in the production of cloud and,

aerosol

A dispersion of solid or liquid ultimately, precipitation. Surface friction is particles suspended in a gas. In meteorol-

one property that leads to an ageostrophic ogy, the gas is the atmosphere and the

wind.

prime sources (both natural and artificial) of the particles are at the Earth’s surface,

aggregation In cloud physics, the including sea salt particles generated by

process by which ice crystals clump to- breaking waves, carbon particles in smoke

gether, the branched shapes of the crystals and soot, fine wind-blown soil and clay

readily linking, to form snowflakes. The particles, and ash from volcanic eruptions.

process is especially likely to occur at tem- They are important meteorologically be-

peratures between –5 and 0°C (23 and 32°F).

cause some of these particles can act as See also Bergeron–Findeisen condensation nuclei (see cloud condensa-

process.

tion nucleus) and FREEZING NUCLEI ; in sig- nificant concentration they can influence

/ag-roh-klÿ-mă-tol-ŏ- the RADIATION BUDGET of the atmosphere

agroclimatology

jee / The study of the effects of CLIMATE , which may include climate variability and

through ABSORPTION or SCATTERING of solar change, on agriculture. It may include, for radiation.

example, the effects of climate on crop pro-

afforestation duction, the length of the growing season,

/ă-fô-rĕ-stay-shŏn/ The the soil climate, and the location and as- planting of trees to establish new forest,

pects of agricultural units, such as farms in often for commercial purposes but also to

relation to climatic variables. improve the environment, for conservation

purposes, or to provide a CARBON SINK . agrometeorology /ag-roh-mee-tee-ŏ-rol- Compare deforestation.

ŏ -jee / The study and use of weather and climate information in relation to agricul-

African monsoon The predominantly ture. Such information may be used, for northeastward invasion of moist air from

example, to expand and improve agricul- the Gulf of Guinea during the summer

tural production, to assess the impact of months (May–September) across West

weather on crop response and survival, Africa. The MONSOON air is accepted as

and forecast the occurrence of frosts. being present as soon as daytime relative

humidity has risen to 50%. Elsewhere over Agulhas Current /ă-gull-ăss/

A warm tropical Africa, complex topography and

surface ocean current, originating in the In- the pattern of land and sea, leads to much

dian Ocean from the South EQUATORIAL modified seasonal airflow patterns. This

CURRENT and also fed by the Mozambique means that only some components of the

Current. The Agulhas Current flows south- seasonal reversal of winds may be detected.

westward along the South-African coast- westward along the South-African coast-

line and joins the eastward flow between Africa and Australia; some may continue

COMPOSITION OF AIR

to flow westward round the Cape of Good (% by volume) Hope to join the Atlantic. The Agulhas Current is one of the fastest flowing cur-

nitrogen (N 2 ) 78.08 rents in any of the oceans, capable of

oxygen (O 2 ) 20.95 reaching a maximum velocity of 5 knots

carbon dioxide (CO 2 ) 0.03 (5.8 mph) off the southeast coast of South

argon (Ar) 0.93 Africa.

neon (Ne) 1.82 × 10 –3 helium (He) 5.24 × 10 –4

air The mechanical mixture of gases that methane (CH 4 ) 1.5 × 10 –4 make up the Earth’s atmosphere. The two

krypton (Kr) 1.14 × 10 –4 most abundant constituents are NITROGEN

xenon (Xe) 8.7 × 10 –5 (78.08%) and OXYGEN (20.95%); also pre-

ozone (O 3 )1 × 10 –5 sent are variable amounts of water vapor

nitrous oxide (N 2 O) 3 × 10 –5 (the amount depending on location, tem-

water (H 2 O) variable, up to perature, and time), and argon (0.93%),

1.00 while the TRACE GASES include significant

hydrogen (H 2 )5 × 10 –5 amounts of CARBON DIOXIDE and METHANE .

Aircraft Meteorological Data out to influence more extensive regions. Relay See AMDAR.

The nature of an air mass depends on the location of the anticyclone with which it is

AIREP (aircraft report) An internation- associated, the season, and, for areas out- ally agreed type of SYNOPTIC CODE used to

side this source region, the nature of the transmit a message relating to dry-bulb

air’s track once it moves away. Air masses temperature as well as wind speed and di-

are classified according to temperature (i.e. rection from a commercial aircraft in

tropical, polar, and Arctic or Antarctic) flight.

and on whether the surface where it origi- nates is maritime or continental. The SUB -

air frost The condition in which the air TROPICAL ANTICYCLONES over oceans act as temperature of the lower atmosphere, mea-

source regions for warm and humid air sured at Stevenson screen level of 1.25 m (4

that is known as a MARITIME TROPICAL (mT) ft) above the ground surface is at or below

air mass. The wintertime CONTINENTAL AN - 0°C (32°F), the freezing point of water. See

TICYCLONES act as source regions for cold also frost; ground frost.

and dry CONTINENTAL POLAR (cP) air masses, while the highest latitude anticy-

airglow

A faint radiant emission that clones are sources of very cold and dry occurs in the ionosphere of the upper at-

ARCTIC (A) and ANTARCTIC (A or AA) air mosphere. It results from the ionization of

masses. CONTINENTAL TROPICAL (cT) air atoms and molecules by radiation from the

masses originate in the continental sub- Sun.

tropical anticyclones. Additional classifica- tions sometimes include equatorial (E) and

air mass An extensive homogeneous re- Mediterranean air masses; suffixes are also gion of air, typically a few thousand kilo-

sometimes used to indicate whether an air meters across, that possesses broadly the

mass is colder than the surface over which same temperature and humidity character-

it is moving (k) or whether it is warmer istics, with generally weak horizontal gra-

(w).

dients of these properties. Air masses have The air that flows away from the anti- their SOURCE REGIONS in the locations of the

cyclonic source regions is modified to vary- permanent or semi-permanent ANTICY -

ing degrees, depending on its track. That CLONES (high-pressure zones) where winds

moving out from northern wintertime con- are light and from which surface air spirals

tinents (continental polar), for example, tinents (continental polar), for example,

often blows across the North Atlantic and dioxide, and nitrous oxide. The index has North Pacific Oceans to be strongly heated

a scale of 0 to 500: the higher the value, the and moistened, and transformed into MAR -

greater the level of pollution. Six levels are ITIME POLAR (mP) air. Where two different

specified: Good, 0–50; Moderate, 51–100; air masses are juxtaposed, the horizontal

Unhealthy for Sensitive Groups, 101–150; gradients of temperature and humidity be-

Unhealthy, 151–200; Very Unhealthy, 201– tween them are steep forming FRONTS .

300; Hazardous, 301–500; each level is also assigned a color for easy recognition.

air-mass thunderstorm

A short-lived

A mathematical or formed by local differential heating of an

and generally isolated thunderstorm

air-quality model

conceptual attempt to simulate the path- air mass, not associated with frontal or

ways of pollutants through the atmosphere synoptic features. It forms in warm,

in order to form a first step in understand- humid, and unstable air masses, for exam-

ing the likely impact of air pollutants on ple the maritime tropical (mT) air that ex-

humans and their environment. The mod- tends from the Midwest to the Gulf Coast

els may incorporate components of states in the US. Air-mass thunderstorms

physics, fluid dynamics, and chemistry are typically 1–10 km in diameter and may

within and above the BOUNDARY LAYER of last for up to about 60 minutes.

the atmosphere.

air parcel

A concept used most often in air-quality standard The prescribed the context of small volumes of air that

concentrations of air pollutants that may or may not ascend or descend through

should not legally be exceeded during a their surrounding ‘environmental’ air. The

given period at a specified location, usually ‘parcel’ is a distinct and ‘isolated’ bubble of

within an urban environment. Recom- air that may cool or warm (if ascending or

mended guidelines for short-term exposure descending respectively) at either the DRY

to pollutants are also provided. See also ADIABATIC LAPSE RATE or the SATURATED ADI -

National Ambient Air Quality Standards. ABATIC LAPSE RATE . Such an UNSTABLE parcel may well appear as a small CUMULUS cloud,

air–sea interaction See atmosphere– for example.

ocean interaction. air pollution The presence in the at- Aitken nuclei counter (Aitken dust

mosphere of gases, aerosols, particles, or counter) A laboratory instrument for de- energy above background levels in such

termining the dust or cloud condensation quantities and over such timescales that are

nuclei content of a sample of air. The sam- likely to cause harm to all or part of the

ple is drawn into an expandable chamber biosphere, deterioration in the fabric of

containing air almost saturated with water buildings and structures, or changes to the

vapor. Sudden expansion of the chamber way the atmospheric system operates. See

causes the air to cool adiabatically to also pollution.

below its dewpoint, and water droplets to form on the dust or nuclei. A proportion of

air pressure See atmospheric pressure. the droplets settle onto a graduated plate and can be counted. The instrument was

Air Quality Index (AQI) In the US, a invented by the Scottish physicist and me- measure of the daily air quality of an area.

teorologist, John Aitken (1839–1919). The system was developed by the Environ- mental Protection Agency (EPA) and is a

Aitken nucleus /ayt-kin/ The smallest revision of the earlier Pollution Standards

and most numerous type of solid particle in Index (PSI), which it replaced in 1999. The

the atmosphere ( AEROSOL ), with a radius of AQI is calculated for five pollutants: ozone

less than about 0.1 µm, originating mainly (ground level), particulates (particulate

from deliberate combustion. Aitken nuclei matter or PM), carbon monoxide, sulfur

are most highly concentrated in continen- are most highly concentrated in continen-

tal air. The nuclei are named for the Scot- Alberta low See Alberta clipper. tish physicist and meteorologist, John Aitken (1839–1919).

Aleutian Current /ă-loo-shŏn/ (Sub-Arc- tic Current) A comparatively cold ocean

Alaska Current

A comparatively warm current that flows between latitudes 40° current that flows counterclockwise in the

and 50°N in the north Pacific Ocean. Gulf of Alaska; it is an example of a GYRE .

A semi-permanent region albedo /al-bee-doh/ The ratio, usually of low pressure that appears on average expressed as a percentage, of the reflected

Aleutian low

monthly MEAN - SEA - LEVEL PRESSURE charts, solar radiation beam to the total energy of

located over the Aleutian Islands chain that the beam falling on to a surface as mea-

stretches westward across the North Pa- sured across the total spectrum of incom-

cific Ocean from Alaska. It is deeper in the ing solar radiation. The greater the albedo

winter than the summer and is not neces- value, the less energy is available for other

sarily present from day-to-day. It is the lo- energy transformations (e.g. sensible and

cation in which, on average, the traveling latent heat). Bright surfaces, such as fresh

FRONTAL CYCLONES in the North Pacific snow, reflect more than dark surfaces, such

reach their minimum mean-sea-level pres- as a coniferous forest, and consequently

sure value.

have a higher albedo value.

Alliance of Small Island States

albedometer /al-bĕ-dom-ĕĕe-ter/ An in- (AOSIS)

A coalition of small island and strument used for measuring the reflecting

low-lying countries that share concerns power, i.e. the ALBEDO , of a surface (e.g.

about their vulnerability to sea-level rise clouds, snow, water, or crops).

associated with global CLIMATE CHANGE . The alliance was initiated in 1990 to

Alberta clipper (Alberta low)

strengthen the states’ negotiating position ally wintertime fast-moving low-pressure

A gener-

when voicing their opinion on greenhouse system that forms or redevelops to the lee

emissions within the UNITED NATIONS of the Canadian Rocky Mountains, to the

FRAMEWORK

CONVENTION ON CLIMATE

east of the mountain chain under westerly CHANGE (UNFCC). It has a membership of flow. These LEE DEPRESSIONS track generally

43 states and observers, from all regions of eastward to often produce strong gusty

the world.

winds and snowy weather in the vicinity of the Canadian/US border. Alberta clippers

A warm wind that can travel as far as the Maritime Provinces

almwind /ahm-wind/

blows across the plains of southern Poland of Canada and the northeast US, occasion-

and occurs mainly during the spring. The ally intensifying off the east coast when

almwind is a FÖHN -type wind originating they encounter the relatively warm moist

from the forced ascent of a southerly air over the Atlantic Ocean, and spreading

airstream across the Tatra Mountains, heavy snow over land.

which form the Polish/Slovakian border and reach heights of up to 2655 m (8711

ft). This air stream warms rapidly on its de- Surface type

ALBEDO

% Reflected scent from the mountain range, often initi- ating avalanches in southern Poland.

fresh snow 90% water (Sun high

A wind sys- in sky)

along-slope wind system

tem that circulates air up, along, and down water (Sun low

the slopes of a valley. The system is com- in sky)

10–80% prised of ANABATIC and KATABATIC compo- grass

15–25% nents. Such flows are generally shallow, forest

with the maximum velocities occurring within a few meters of the slope surface.

along-valley wind system along-valley wind system

over any surface; it measures the time it tem that occurs in mountainous regions. It

A wind sys-

takes for a radio pulse or continuous signal is generated as a result of the differential

to be reflected by a surface and converts heating of surfaces during the solar radia-

this into a geometric height. tion cycle, this being induced by variations in slope aspect. An up-valley wind is most

/al-tă-ther-măl/ (hypsi- likely to develop on calm clear days. Valley

altithermal

thermal) A period during which the wind systems may have ANABATIC or KATA -

Earth’s climate was warmer than it is cur- BATIC components, but will always be con-

rently. In particular, altithermal refers to a ditioned by the dynamics of the

period about 8000–4000 years ago during synoptic-scale GRADIENT WIND flow in the

which, it is believed, zones of higher pre- free atmosphere.

cipitation were shifted poleward of their present position.

alpine glow The series of colors that ap- pears on mountains, especially when snow-

1. The vertical height of an ob- covered, as the Sun nears the horizon at

altitude

ject (e.g. an aircraft, meteorological bal- sunset. The mountain tops exposed to the

loon, or atmospheric layer) above a Sun are first yellow, then become pinkish,

reference level, which is usually mean sea and finally purple. The series of colors, but

level.

in reverse order, is also visible at sunrise on

2. (elevation) A horizontal coordinate; it is mountains facing the rising Sun. A purplish

the angular displacement of an object or glow – the afterglow – is sometimes visible

astronomical body above the horizon. It is after the Sun has gone below the horizon.

positive if the object is above the horizon, negative if below.

alternative energy Energy derived from nontraditional sources or sources that are

alto- In meteorology, a prefix related renewable and do not deplete finite min-

uniquely to MIDDLE CLOUDS that have bases eral resources. The majority of countries

typically between 2000 m and 7000 m are heavily dependent on fossil fuels as

(6500 ft and 23,000 ft) above the surface. sources of energy, but with the decline in

See also cloud classification. reserves and contribution made to the en- hanced GREENHOUSE EFFECT by releases of

altocumulus /al-toh-kyoo-myŭ-lŭs/ A carbon dioxide from fossil fuel combus-

type of MIDDLE CLOUD commonly consist- tion, alternative energy sources are being

ing of parallel bands or small rounded increasingly used and developed. Examples

masses that have angular widths between include wind power, solar power, wave

1° and 5°. The cloud can appear as a patch and tidal power, biomass energy, geother-

or a more extensive sheet of cloud, white or mal power, and hydroelectric power.

gray in color, and typically partly shaded. World primary energy sources include only

Altocumulus clouds usually form by con- about 7% from renewable sources, most of

vection in an unstable layer aloft. which is hydroelectric. altocumulus castellanus /kas-tĕ-lay- altimeter An instrument used to mea- nŭs /

A type of MIDDLE CLOUD ; a species or sure the altitude of an object with respect

special form of ALTOCUMULUS in which the to a fixed level, such as sea level. There are

cumuliform components are much taller two main types. The pressure altimeter, the

than they are wide and so appear to be tur- most common of which is the ANEROID

reted or castellated. They express the pres- BAROMETER , is used in aircraft; it works on

ence of overturning in their layer and are the principal of measuring falls in atmos-

often known to be useful harbingers of pheric pressure with altitude and shows the

thundery weather. height, in meters or feet, above sea level. The radio altimeter (electronic altimeter) is

A type of an absolute altimeter, providing an altitude

altostratus /al-toh-stray-tŭs/

MIDDLE CLOUD , most often with a grayish MIDDLE CLOUD , most often with a grayish

or bluish tone. It is usually a sheet or layer anabatic wind /an-ă-bat-ik/

A wind of fairly uniform appearance with no opti-

that blows up the slope of a valley, some- cal phenomena, and totally or partly cov-

times attaining a velocity of 12 knots (14 ers the sky. It is sometimes thin enough to

mph). It develops as a result of the greater view the Sun through it in which case the

insolation warming of air that is in contact solar disk appears as if through ground

with the valley slopes during the day in glass. The cloud may contain water

comparison to that of the air at the same droplets only, or a mix of water droplets

height but vertically above the valley floor. and ice crystals.

The heated air above the slope rises in the form of a convection current, with the air

Ambient Air Quality Standards of the anabatic wind moving in to take its (AAQS) See National Ambient Air Qual-

place. An anabatic wind typically begins ity Standards.

30 minutes after sunrise and ceases 30 min- utes before sunset. Compare katabatic ambient pressure The ATMOSPHERIC wind. PRESSURE , at any point in the atmosphere, which surrounds a physical entity, such as

ana-front /an-ă-/ A WARM or COLD

a cloud. Ambient pressure is most fre- FRONT characterized by ascent of the warm quently referred to in discussions of air

moist air over the colder drier air; it is thus STABILITY , when the temperature and pres-

active in terms of deep cloud and PRECIPI - sure of a theoretical air parcel are com-

TATION . Compare kata-front. pared to the pressure and temperature ( AMBIENT TEMPERATURE ) of the surrounding

analog data Data represented in a con- air.

tinuous form. In radar, a scheme in which the radiative power emitted from a scene

ambient temperature The temperature being imaged is converted into a proxy sig- of the area surrounding a specified object,

nal, such as an electrical current, before for example the temperature of the sur-

being transmitted for display on the rounding atmosphere, soil, or water.

ground.

AMDAR /am-dar/ (Aircraft Meteorologi- analog image In remote sensing, an cal Data Relay) The internationally

image in which the continuous variations agreed scheme for relaying automatically

in the scene being sensed is recorded as sensed and transmitted weather data from

continuous variations in image tone. For commercial aircraft in flight. The relay is

example, in a photograph the variations in handled by the telecommunications capa-

the scene are recorded directly as varia- bility of GEOSTATIONARY SATELLITES .

tions in tone by the grains of the photosen- sitive film. Compare digital image.

AMIP /am-ip/ See Atmospheric Model Intercomparison Project.

analog weather forecasting

A method of weather forecasting in which past

weather situations similar to the present ous layer of featureless low cloud from

amorphous cloud

A usually continu-

are used to predict that the weather will de- which rain falls.

velop in a similar way. The method is diffi- cult to use and is now little used.

amplitude In a wave, the height be- tween the crest of the wave and the mean

analysis In synoptic meteorology, an ex- level, or the depth between the lowest point

amination of the state of the atmosphere of the trough of the wave and the mean

based on synoptic weather observations level.

and used as the basis for weather forecasts. The process includes the preparation of

AMSU See advanced microwave sound- charts, such as isobaric charts, from the ing unit.

data obtained from many locations at the data obtained from many locations at the

same time, together with the interpretation convenience. This type of barometer con- of the weather elements plotted and dis-

sists of a flexible capsule (the aneroid cap- played on the weather chart. See also isen-

sule or cell), which is usually metallic and tropic analysis; isobaric analysis.

partially exhausted of air. The capsule is held extended by an internal spring and as

anchor ice See ground ice. atmospheric pressure changes, the walls of this capsule expand and contract. These

andhi /an-dee/ In the northwestern re- movements are transmitted either mechan- gion of the Indian subcontinent, a dust

ically to a pointer on a dial, which registers storm that accompanies a sudden increase

the atmospheric pressure on a scale, or in wind speed; it is caused by strong con-