METEOROLOGICAL GLOSSARY M.O. 729 A.P.827 Meteorological Office Meteorological Glossary Compiled by D. H. Mclntosh, M.A., D.Sc. London: Her Majesty's Stationery Office: 1963 Decimal Index 551.5(038) First Published 1916 Fourth Edition 1963 © Crown copyright 1963 Published by HER MAJESTY'S STATIONERY OFFICE To be purchased from York House, Kingsway, London w.c.2 423 Oxford Street, London w.l 13A Castle Street, Edinburgh 2 - 109 St. Mary Street, Cardiff 39 King Street, Manchester 2 50 Fairfax Street, Bristol 1 35 Smallbrook, Ringway, Birmingham 5 80 Chichester Street, Belfast 1 or through any bookseller Price £1 12s. 6d. net Printed in England for Her Majesty's Stationery Office by Dawson & Goodall Ltd., Bath. PREFACE In 1916, during the directorship of Sir Napier Shaw, the Meteorological Office published two pocket-size companion volumes, the 'Weather map' to explain how weather maps were prepared and used by the forecasters, and the 'Meteorological glossary' to explain the technical meteorological terms then employed. With the advance of the science and the elaboration of its techniques the publications have been in continuous demand, many times reprinted and on several occasions completely revised. The second edition was in 1930, the third in 1938-39, and soon after World War II it was obvious that radical revision was necessary once again. In 1956 the fourth edition of the 'Weather map' was issued but, for the first time in forty years, it was not found possible to prepare simultaneously a new edition of the 'Meteoro­ logical glossary', which had become very much out-of-date and in need of a complete remodelling. For earlier editions the task had been shared amongst the professional staff of the Office and the result was an interesting and up-to- date volume containing much useful information, although the freedom allowed to the many contributors had led to a unique volume of quite uneven character with articles varying from brief dictionary definitions to encyclo­ paedic essays. For the fourth edition the number of new entries was to be larger than ever before and it was decided that the need was now for a more systematic reference work containing a brief definition of all the terms in ordinary use rather than for a compilation of miscellaneous articles giving information which would more properly be looked for in one of the many modern textbooks. For this purpose a single author, assisted if need be by expert referees, would, it seemed, be advantageous and the Office was fortunate in finding in Dr. D. H. Mclntosh of the University of Edinburgh a physicist and meteorologist of wide experience willing to undertake the major task. Before being passed for printing every article has been read critically by more than one member of the scientific staff of the Office and Dr. Mclntosh has shown a remarkable readiness to compromise. In this way it is hoped that the excellence of the author's original draft has been fully retained while providing a work which will adequately meet the needs of the official service. It would, however, be too much to hope that no further improvement will be possible and the Office will be pleased to receive from any source criticisms and suggestions calculated to increase the value of the work, not only for the professional meteorologist, but for interested people everywhere. Meteorological Office, Bracknell, Berkshire. 1962. ACKNOWLEDGEMENTS The Meteorological Office is indebted to the following for permission to reproduce many of the illustrations appearing in this publication: Plates G. V. Black, Esq. 2 H. A. D. Cameron, Esq. 3 M. W. Dybeck, Esq. 22 J. Paton, Esq. 1,18 Rev. Fr. C. Rey, S.J. 23 J. Steljes, Esq. 19, 21 J. E. Tinkler, Esq. 20 The Royal Meteorological Society 5, 6, 8, 9, 10, 11, 12, (Clarke-Cave Collection) 13, 15, 16 The United States Navy 26 The remaining Plates, Nos. 4, 7, 14, 17, 24, 25, 27 and 28 are all Crown copyright. 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, rainfall. The rate of ablation of a snow-field is also affected by such non-meteorological factors as size, slope and aspect of snow-field, depth and age of snow, nature of underlying surface. abroholos: A violent squall on the coast of Brazil, prevalent from May to 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 stability: See STABILITY. absolute temperature: See TEMPERATURE SCALES. absolute vorticity: See VORTICITY. absolute zero (of temperature): Temperature of 273-15° C, the zero on the Kelvin (absolute) scale. See TEMPERATURE SCALES. absorption : Removal of radiation from an incident solar or terrestrial beam, with conversion to another form of energy electrical, chemical or heat. The absorption of radiation by the gases of the atmosphere is highly selective in terms of wavelength and may depend also on pressure and temperature. Numerical expression is given by the law, variously known as Beer's, Bouguer's, or Lambert's law, applicable to monochromatic radiation: / = 70e- am where 70 is the intensity of incident radiation, / the intensity after passing through mass m of absorbing substance and a the absorption coefficient. An alternative expression is I = /0e-«* where x is the path length through the absorbing substance. 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° and 250° K, respectively) are shown with equal areas to represent over-all balance of the two fluxes. Fine structure of the absorption bands is omitted. The main constituents of the atmosphere, N2 and O 2, are almost completely transparent except in the far ultra-violet: minor constituents I'O 2 50 Wavelength in FIGURE 1 Relative importance of various absorptions in the atmosphere (a) Black-body emission for 6000° and 250°K. (b) Atmospheric absorption spectrum for a solar beam reaching ground level. (c) Atmospheric absorption spectrum for a solar beam reaching the temperate tropopause. p, 9, fy, Q. and x are the near infra-red bands of water vapour. In (b) and (c) no fine structure is shown although all the infra-red bands have in fact a very complex structure. Most of the bands shown have a broad doublet structure, but even this is smoothed out. It has been assumed that the atmosphere contains 3 mm of ozone at S.T.P., all above 11 km; that it contains 2 gm/cm2 of water vapour above ground level and 10-3 gm/cm2 above 11 km; that carbon dioxide and nitrous oxide are mixed in equal proportions with the atmosphere at all heights. The small window (dotted) at 0-2(j. and the two ozone bands marked (?) have not yet been observed at the two levels concerned. (GOODY, R. M. and ROBINSON, G. D. ; Radiation in the troposphere and lower stratosphere. Quart. J.R. met. Soc., London, 77, 1951, p. 153.) such as O3, CO 2, H2O, N2O have intense absorption bands, mainly in the infra-red and longer wavelengths. The product of (a) and (c) in Figure 1 represents the energy absorbed by the stratosphere, and the product of (a) and ((b)—(c)) that absorbed by the troposphere. Since water has an appreciable absorption coefficient in solar radiation wave­ lengths greater than about one micron, thick clouds are able to absorb over 20 per cent of incident solar radiation. Towards terrestrial radiation, clouds and fog behave as almost perfect black bodies. The surface of the earth is very variable in its absorption of solar radiation (see ALBEDO) but absorbs nearly all incident terrestrial radiation. ABTOP: In weather messages, a code word indicating that a report of upper air temperature and wind, in abridged form, follows in figure code. See 'Handbook of weather messages.'* acceleration : Rate of change of velocity with respect to time. Like velocity, accelera­ tion has both magnitude and direction: thus uniform circular motion, involving change of direction without change of speed, implies acceleration ('centripetal acceleration'). The dimensions are L T~ 2. The 'absolute acceleration' of the air (ya), i.e. acceleration measured relative to axes fixed in space, is equal to the force acting per unit mass of air (Newton's second law of motion). The important forces per unit mass in air motion are the PRESSURE GRADIENT FORCE ( v/>)> the Newtonian force of GRAVITY directed P towards the earth's centre (ga), and the 'frictional' forces arising from eddy VISCOSITY or molecular viscosity (F). Thus we have the 'equation of absolute motion': \ra = — w + & + F P Va comprises the 'relative acceleration' (y), i.e. acceleration measured with respect to the rotating earth, and two other accelerations: (a) the 'centripetal acceleration' of the coinciding point of the earth (y<>); (b) the 'Coriolis acceleration' or 'geostrophic acceleration' (2SI A V where SI is the earth's angular velocity and A denotes the vector cross product).