COPY CTJH. USE. [Crown Copyright Reserved.} M.O. 225 ii STRY METEOROLOGICAL OFFICE THE METEOROLOGICAL GLOSSARY SECOND EDITION Entirely rewritten In continuation of The Weather Map Published ly the Authority of the Meteorological Commiteee LONDON: PRINTED AND PUBLISHED BY HIS MAJESTY'S STATIONERY OFFICE To be purchased directly from H.M. STATIONERY OFFICE at the following addresses: Adastral House, Kingsway, London, W.C.2 ; 120, George Street, Edinburgh; York Street, Manchester; i, St. Andrew's Crescent, Cardiff; 15, Donegall Square West, Belfast; or through any Bookseller. 1930 Price 4J. 6d. Net. 1-3-7-201i METEOROLOGICAL GLOSSARY CONTAINING INFORMATION IN EXPLANATION OF TECHNICAL METEOROLOGICAL TERMS. The first edition of the Meteorological Glossary was issued in 1916, and was reprinted with additions in 1917 and again in 1918. Since 1918 there have been many advances in meteorology, for example in connexion with the study of weather maps, and the present edition has been almost completely re-written, the task being apportioned among the Professional Staff of the Office. The opportunity has been taken, by increasing the size of the page, to bring the Glossary into conformity with the other handbooks published by the Meteorological Office. In accordance with the practice of the Oxford Dictionary the initial word of each article is in black type. Words in small capitals in the body of the text are the subjects of articles in another part of the Glossary. International definitions of cloud forms are distinguished by the use of italic type. The International Meteorological Committee at the meeting in London in 1921 passed a resolution requesting the inclusion, in a future edition of the Meteorological Glossary, of the equivalents in various languages of the words defined. In this edition it has been possible to give effect to this resolution by the inclusion at the end of the work of equivalents in Danish, Dutch, French, German, Italian, Norwegian, Portuguese, Spanish and Swedish. Thanks are due to the Directors of the Meteoro­ logical Services of the countries concerned for their assistance in providing these translations. Meteorological Office, A ir Ministry, June, 1930. METEOROLOGICAL GLOSSARY Absolute Extremes. See EXTREMES. Absolute Humidity is the mass of aqueous vapour, expressed in grammes per cubic metre of air. The term is sometimes incorrectly applied to the pressure of the aqueous vapour in the air. In accordance with Dalton's Law the water vapour in the atmosphere exerts the same pressure as it would if the air were not present. Knowing the vapour pressure e, we therefore obtain 8, the mass of vapour in a cubic metre of moist air, from the formula 8= fcojrx-f-e -^o where 80 is the density of aqueous vapour under standard pressure and temperature P0 and T0 e whence 8 = 216-7 -~- grams per cubic metre. Absolute Temperature. The temperature of the centigrade thermo­ meter, increased by 273, more properly called the temperature on the absolute, or thermodynamic scale. The absolute scale is formulated by reasoning about the production of mechanical work at the expense of heat (which is the special province of the science of thermodynamics, see ENTROPY), but for practical purposes the scale may be taken as identical with that based on the change of volume and pressure of one of the permanent gases with heat. For thermometric purposes aiming at the highest degree of refinement the hydrogen scale is used, but for the purposes of meteorological reckoning the differences of behaviour of the permanent gases, hydrogen, oxygen, nitrogen are unimportant. In physical calculations for meteorological purposes the absolute is the natural scale ; the densities of air at any two temperatures on the absolute scale are inversely proportional to the temperatures. Thus the common formula for a gas, P _ Po p (273 + t) po (273 + <0) where p is the pressure, p the density and t the temperature centigrade of the gas at one time, pa, p0, t0 the corresponding values at another, becomes p0r0' where T and T0 are the temperatures on the absolute centigrade scale. Its most important feature for practical meteorology is that from its definition there can be no negative temperatures. The zero of the absolute scale is the temperature at which all that we call heat would have been spent. In the centigrade scale all temperatures below the freezing point of water have to be prefixed by the negative sign . This is very incon­ venient, especially for recording observations in the upper air, which never gives temperatures above the freezing point in our latitudes at much above 4 kilometres (13,000 ft.), and often gives temperatures below the freezing point nearer the surface. The absolute temperature comes into meteorology in other ways ; for example, the rate at which heat goes out into space from the earth depends, according to Stefan's Law, upon the fourth power of the absolute tempera­ ture of the radiant substance. See RADIATION. On the absolute scale defined by Lord Kelvin, the freezing point is at 273- 1, and the boiling point at 373- 1. The name tercentesimal has been given to the scale of temperatures obtained by adding 273 to the centigrade temperature. This scale is a sufficiently close approximation to the absolute thermodynamical scale for all practical purposes in meteorology. Temperatures on this scale are frequently represented by the sign °A. ; thus 10 °C. = 283° A. This convention has been adopted throughout the present edition of the " Glossary." (38822) Wt. 13652/684/1053 5,000 8/30 Harrow G.l/27 ABSORPTION 4 Temperatures can also be expressed in an absolute scale of Fahrenheit degrees (A.F.) of which the zero is approximately 459° below the Fahrenheit zero. Some common temperatures on the absolute scales and their equivalents in Centigrade and Fahrenheit are : Centigrade. Fahrenheit. °A. °C. °F. AF. The boiling point of helium . 4 -269 -452-2 7-2 The boiling point of nitrogen . 77 -196 -320-8 138-6 The freezing point of mercury 234-2 - 38-8 - 37-8 421-6 The freezing point of water . 273 0 32 491-4 The mean temperature of 282-7 9-7 49-5 508-9 London. " Temperate " as shown on an 285-8 12-8 55 514-4 ordinary thermometer. The best temperature for a 290 17 62-6 522-0 living room. A hot summer day 300 27 80-6 540-0 The temperature of the human 310 37 98-6 558-0 body. The temperature of the sun . 6,000 __ 10,000 Absorption (Atmospheric). See RADIATION (radiation and the atmos­ phere). Accumulated Temperature. A term used to describe the excess or defect of temperature in relation to a selected base level, prevailing over a more or less extended period of time, e.g. a day, week, month or year. The aggregate areas above and below the selected base line bounded by (1) a curve showing the variation of temperature with time, (2) the base line, and (3) the two ordinates from the base to the curve at the beginning and end of the period of time considered, are measures of the accumulated temperature above or below that base line. Values of accumulated temperature are intended to afford a better means of tracing the influence of temperature on crops during critical periods of their growth than mean temperature, the variations of which from year to year are not so marked. Accumulated temperature is expressed in day-degrees, a day-degree Fahrenheit signifying 1 °F. of excess or defect of temperature above or below the selected base line continued over a period of 24 hours, or any other number of day-degrees for an inversely proportional number of hours. The base temperature adopted by the Meteorological Office is 42°F., as being nearly equivalent to 6°C., the temperature considered by several Continental investigators* to be the critical value above which the growth of vegetation in an European climate commences and is main­ tained. It follows from the definitions that to compute the accumulated temperature accurately it would be necessary to integrate the excess or defect of temperature above or below the critical limit by utilising thermo­ graph records. As few observing stations are equipped with thermographs an approximation is made. Formulae for computing approximate values of accumulated temperature from daily values of maximum and minimum temperature were devised by the late Lieutenant-General Sir Richard Strachey (Quarterly Weather Report, 1878, Appendix II) and modified by him so as to be applicable to weekly means of maximum and minimum temperature, in which form they have been used in computing the totals of accumulated temperature published in the Weekly Weather Report for individual stations from 1878 to 1914 and for districts from 1878 to 1927. While Strachey's formulae as applied to weekly means give reasonably accurate results when the daily extremes are both on the same side of the * A. Angot: " Etude sur la marche des phenomenes de la vegetation en France pendant les annees 1880 et 1881." Paris, Ann. Bureau, Central Mtttorologique, 1882. Part I, p. B. 17. 5 ADIABATIC base line, recent investigation has shown that no formulae applicable to weekly means can give results comparable in accuracy with formulae designed to apply to daily values of maximum and minimum temperature, in those weeks in which the temperature fluctuates about the base line ; e.g. the mean of the seven readings of minimum temperature during a given week may be above 42°F. even though some of the readings pass below that limit. In such cases Strachey's formulae as applied to weekly means give zero accumulated temperature below 42 °F., which is obviously incorrect. Accordingly in the Weekly Weather Report (for 1928-9) daily values of maximum and minimum temperature have been utilised in the computation of accumulated temperature, in accordance with formulae designed to apply to daily values of maximum and minimum temperature.