M.O.225 ii (A.P.897) AIR MINISTRY METEOROLOGICAL OFFICE THE METEOROLOGICAL GLOSSARY THIRD In continuation of the Weather Map^» B R A Published by the Authority of the Meteorological Committee L07VDOAT HER MAJESTY'S STATIONERY OFFICE 1939 (Reprinted 1961 incorporating Amendment Lists Nos. 1-6) Decimal Index S5I-S Crown copyright reserved Printed and published by HER MAJESTY'S STATIONERY OFFICE To be purchased from York House, Kingsway, London w.c.2 423 Oxford Street, London w.i I3A Castle Street, Edinburgh 2 109 St. Mary Street, Cardiff 39 King Street, Manchester 2 50 Fairfax Street, Bristol i 2 Edmund Street, Birmingham 3 80. Chichester Street, Belfast i or through any bookseller Price £i is. od. net. Printed in England S.O. Code No. 40-63-1-61* 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 second edition issued in 1930 was almost completely re-written, the task being apportioned among the Professional Staff of the Office. In this second edition the opportunity was 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 the second edition effect was given 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. The progress of meteorological knowledge since 1930 has again made a number of revisions necessary, and the publication of a third edition has given an opportunity of incorporating these. Thanks are due to all those, both the staff of the Meteorological Office and others, who have given help in the revision. Meteorological Office, Air Ministry. November, 1938. (81689) A2 METEOROLOGICAL GLOSSARY Absolute Extremes. See EXTREMES. Absolute Humidity, also called vapour density*, is the mass of aqueous vapour per unit volume of air, and is usually 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 where d0 is the density of aqueous vapour under standard pressure and temperature P0 and T0 whence d = 216-7 grams per cubic metre. e being measured in millibars and T in absolute degrees on the tercentesimal scale. See AQUEOUS VAPOUR. Absolute Temperature. The absolute scale of temperature is formulated by reasoning about the production of mechanical work at the expense of heat (which is the special province of thermodynamics, see ENTROPY). The absolute temperatures corresponding with 0° C. and 100° C. are 273-16 and 373 16 respectively. 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. It is moreover sufficiently accurate to replace it by a scale (to which the name TERCENTESIMAL has been given) obtained by adding 273 to the Centigrade temperature. It is usual in meteorology to use temperatures on this scale in formulae which are strictly true only for absolute temperatures, such temperatures being frequently indicated by °A. (thus 10° C. = 283° A.), a convention that has been adopted in the present edition of the Glossary. For thermometric purposes aiming at the highest degree of accuracy 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) DO (273 + *0) where p is the pressure, p the density and t the temperature Centigrade of the gas at one time, pQ, p0, t0 the corresponding values at another, becomes JL = £o ? T Po To 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 inconvenient, especially for recording observations in the upper air, which never gives temperatures above the freezing point in our latitudes at much above 4 kilometres ( 1 3,000 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 temperature of the radiant substance. See RADIATION. * Recently the World Meteorological Organization has adopted " vapour concentration " ABSORPTION 4 Absolute temperature on the Fahrenheit scale is degrees Fahrenheit plus 459-4. The Absolute Fahrenheit scale is sometimes referred to as the Rankin Scale. Absorption (Atmospheric). See RADIATION (Radiation and the atmosphere). Acceleration the rate at which velocity changes with time. Any convenient unit such as the mile per hour per hour or the centimetre per second per second may be used, its dimensions being length/square of time. Acceleration, like velocity, is a vector having both magnitude and direction. Thus in uniform circular motion, although the speed does not change the direction does, and the motion has an acceleration directed towards the centre of rotation the so-called centripetal acceleration. In meteorology we are mainly concerned with the acceleration of the moving atmosphere and, in accordance with the fundamental principle of dynamics, we may write down an expression for the acceleration by equating it to the vector sum of all the forces acting upon unit mass of air. The only real forces concerned are the force of gravity and the gradient of pressure both of which are theoretically determinable by observation ; but, in small scale motions, such as convection currents and squalls, where vertical accelerations are significant, the difficulties are such that no attempt has yet been made to solve the equations of motion even approximately. Large scale motions are more amenable to mathematical analysis because then the vertical acceleration is entirely negligible compared with the acceleration of gravity ; the force of gravity may be cancelled against the vertical gradient of pressure. The only acceleration of significance is in that case the horizontal component and the only remaining force is the horizontal gradient of pressure. There are however two decisive reasons why we cannot determine the acceleration from the force in the atmospheric problem. First there is the effect of turbulence. The motion at any level is affected, through the process of mixing, by the motion both above and below. The result is in a sense analogous to that of viscosity in a non-turbulent fluid and may be allowed for by introducing a " frictional term " into the equation of motion. Near the earth's surface this term is important and gives a large contribution to the acceleration, but within the free atmosphere it may justifiably be ignored except perhaps in regions of rapid shear. The second factor is the effect of the rotation of the earth. The observed acceleration is referred to axes rotating in space, and this must be allowed for by including the " geostropbic acceleration " which is proportional to the velocity and acts at right angles to the direction of motion. Ignoring turbulence the apparent acceleration of the air may be equated to the acceleration due to the pressure gradient plus that due to the earth's rotation. We may write = - ilV ........ (1) dt p v ' where V is the vector wind, G is the horizontal gradient of pressure taken positive towards the low pressure, p is air density, I the geostrophic factor 2o> sin <j> (see GRADIENT WIND) and i signifies rotation of the vector IV through one right angle in the cyclonic direction. The vector equation may be expressed by the triangle of accelerations shown in Fig. 1 where V is the, velocity, / V is at right angles to V, G/p is directed towards the low pressure and is the acceleration. dt This equation, it should be noted, does not provide a method of determining acceleration from the pressure gradient, but merely gives a relationship between acceleration and velocity ; theoretically, for a given gradient, the acceleration may have any arbitrary value if the velocity is suitably adjusted. To make any progress it is therefore necessary first to make some assumption as to the nature of the motion.