FOR LOAN Met. O. 985 METEOROLOGICAL OFFICE Meteorological Glossary Sixth edition LONDON: HMSO UDC 551.6(02): 551.501.1 3 8078 0001 4455 2 ) Crown copyright 1991 Applications for reproduction should be made to HMSO First published 1916 Sixth edition 1991 ISBN 0 11 400363 7 HMSO publications are available from: HMSO Publications Centre (Mail and telephone orders only) PO Box 276, London, SW8 5DT Telephone orders 071-873 9090 General enquiries 071-873 0011 (queuing system in operation for both numbers) HMSO Bookshops 49 High Holborn, London, WC1V 6HB 071-873 0011 (Counter service only) 258 Broad Street, Birmingham, Bl 2HE 021-643 3740 Southey House, 33 Wine Street, Bristol, BS1 2BQ (0272) 264306 9-21 Princess Street, Manchester, M60 8AS 061-834 7201 80 Chichester Street, Belfast, BT1 4JY (0232) 238451 71 Lothian Road, Edinburgh, EH3 9AZ 031-228 4181 HMSO's Accredited Agents (see Yellow Pages) and through good booksellers FOREWORD In preparing this new addition I have attempted to correct all the misprints and other minor errors in the last printing of the fifth edition, to revise entries in the light of recent advances where this seemed appropriate, and to include the new terms introduced since the last edition that a meteorologist might encounter in the scientific and technical meteorological and climatological literature, apart from those used only by a handful of expert specialists. In the execution of this task I am glad to acknowledge the generous help of many of my former colleagues at Bracknell and the outstations who have provided critical comments on the entries in the old edition and suggested many new items for inclusion (often supplied with suitable wording). Any remaining defects are, of course, due only to me. A great deal of Dr Mclntosh's work remains unaltered, including the general structure and lay-out. However, I have decided to eliminate the climatological tables as being unsuitable for a glossary; such material, in much greater detail, is better sought in specific climatological publications. The Office has long ceased to have any active interest in either seismology or geomagnetism; entries relevant to those topics have therefore been either eliminated or drastically reduced. Similarly, certain entries dealing with technical, scientific or statistical matters that are not in themselves specifically meteorological have been abbreviated or eliminated. Acronyms form a vexed issue; they proliferate, they are often ephemeral and related to temporary units of administration or policy steering committees, and are sometimes ambiguous and often ugly; they have been kept to a minimum. SI units have been used almost exclusively in this edition except for those still used operationally in meteorology such as the knot and the millibar. R.P.W. Lewis PREFACE TO THE SIXTH EDITION In 1916 the Meteorological Office published two pocket-sized companion volumes, the Weather map to explain how weather maps were prepared and used by forecasters, and the Meteorological glossary to explain the technical terms employed. That first edition of the Meteorological glossary contained 302 terms; the fifth edition, published in 1972, had something like seven times as many, and still the number of terms employed in meteorological research and day-to-day forecasting continues to increase so that it has been necessary to introduce well over 100 completely new entries in this sixth edition. The first three editions of the Meteorological glossary (published in 1916, 1930 and 1939) were written collectively by the professional staff of the Office and this had led to some unevenness of treatment. When, in the mid 1950s, it was decided to have a new edition prepared incorporating information on the considerable advances that had taken place during and after the Second World War, it was thought that the unified approach to be expected from a single author was called for, and the Office was singularly fortunate in being able to secure the services of Dr D.H. Mclntosh of the University of Edinburgh. Dr Mclntosh is a physicist and meteorologist of wide experience who was himself a member of the Office for 15 years and was willing to consult and collaborate with the scientific staff of the Office to make the new edition (the fourth) as complete, informative and authoritative as possible. In 1967 Dr Mclntosh agreed to undertake the preparation of the fifth edition with co-operation from within the Meteorological Office, and it appeared in 1972 containing numerous revisions, new entries, and deletions of obsolete terms. In 1986 Mr R.P.W. Lewis, a recently retired member of the Meteorological Office, was asked to prepare a further edition, incorporating the advances of the preceding 15 years. Mr Lewis was ideally qualified for this task, having been head of the Office's Library and Publications Branch for the last 13 years of a career that took him into a wide variety of areas of meteorology. His considerable experience and expertise is reflected in this latest edition which maintains the standards and updates the contents of its predecessors. Meteorological Office, 1991 LIST OF SYMBOLS 1 . Scalar quantities a radius of the earth Bv, radiance of black body c phase speed; specific heat cp specific heat at constant pressure Cv specific heat at constant volume e vapour pressure / 2ftsin</> (Coriolis parameter) g gravitational acceleration H density scale height h pressure scale height K coefficient of eddy viscosity k wave number / mixing length M mean molecular weight of air N Brunt-Vaisala frequency n refractive index of a medium p pressure ps surface pressure q quasi-geostrophic potential vorticity; specific humidity R specific gas constant; Bowen ratio R* universal gas constant Ra Rayleigh number Re Reynolds number Ri Richardson number Ro Rossby number r humidity mixing ratio 5 entropy T temperature t time U relative humidity u, v, w velocity components in x, y, z directions Ug, Vg components of geostrophic wind V wind speed x, y, z rectangular coordinates: eastward, northward and vertical in a plane tangential to the earth's surface Z geopotential height a specific volume (p"1) /? df/dy Fd dry adiabatic lapse rate Ts saturated adiabatic lapse rate 7 ratio of specific heats (CP/CV); lapse rate £ vertical component of relative vorticity £, vertical component of absolute vorticity rj viscosity 8 potential temperature 0W wet-bulb potential temperature 0SW pseudo wet-bulb potential temperature A longitude v dynamic viscosity II Exner function p density a P/Pi, —a(d9/dp) (stability) T Reynolds stress $ geopotential <f> latitude i/f stream function fl angular velocity of the earth a> dp/dt (vertical component of velocity in pressure terms) 2. Vector quantities i, j, k unit vectors in direction of axes V wind velocity Vg geostrophic wind VT thermal wind £ vorticity = curlV Q angular velocity of the earth's rotation 3. Operators -77 differentiation following a fluid element, <9 , d , d , d i.e. 37+W3~ + u3~ + vv-:r- ot ox ay az (/)\d//G V del-operator V,2 Laplacian operator ablation. The disappearance of snow and ice by melting and evaporation. The chief meteorological factor which controls the rate of ablation is air temperature; subsidiary factors are humidity, wind speed, direct solar radiation and rainfall. The rate of ablation of a snow-field is also affected by such non-meteorological factors as size, slope and aspect of the snow-field, depth and age of snow, and nature of the underlying surface. abroholos. A violent squall on the coast of Brazil, occurring mainly between May and August. absolute extremes. See EXTREMES. absolute humidity. An alternative for VAPOUR CONCENTRATION. absolute instability. See STABILITY. absolute instrument. An instrument with which measurements may be made in units of mass, length, and time (or in units of a known and direct relationship to these) and against which other non-absolute instruments may be calibrated. See also STANDARD. absolute momentum. The vector (M,N) where M = v + fx N = -u + fy (u,v) being the horizontal velocity and/the CORIOLIS PARAMETER in the x,y directions. On the surface of the rotating earth, in the absence of forcing terms such as pressure gradients, M and N are the appropriate conserved quantities that replace the velocities v and u. The term 'pseudo-angular momentum' is also sometimes used. absolute stability. See STABILITY. absolute temperature. A temperature measured with respect to ABSOLUTE ZERO. absolute vorticity. See VORTICITY. absolute zero (of temperature). A temperature of 273.15 °C, the zero on the kelvin (absolute) scale of temperature. absorption. Removal of radiation from an incident beam, with conversion to another form of energy electrical, chemical or heat. The absorption of radiation by gases is highly selective in terms of wavelengths and may depend also on pressure and temperature. Numerical expression is given by the law, variously known as Bouguer's or Lambert's law, applicable to monochromatic radiation: I =_ 7T0 e-am where /o is the intensity of incident radiation, /the intensity after passing through mass m of the absorbing substance and a the absorption coefficient. An alternative expression is where x is the path length through the absorbing substance, a has dimensions of M for a mass absorption coefficient or L'1 for a length absorption coefficient. The effectiveness of a gas as an absorber of solar or terrestrial radiation depends on the width and strength (absorption coefficient) of the absorption lines and bands, the concentration of the gas, and the wavelength positions of the bands relative to the maximum of the Planck curve (see RADIATION) at solar or terrestrial temperature, respectively. The relative energies involved in atmospheric absorption processes are represented in Figure 1, in which the Planck curves appropriate to solar and terrestrial radiation temperatures (6000 K and 245 K, respectively) are shown with equal areas to represent net balance of the two fluxes.