The Origin of Cyclones of Middle Latitudes

Like the Norwegian scheme of construction of the cyclone, Exner's scheme attaches great importance to the phenomena of ascent and descent of cold air at fronts, but Exner does not regard the cyclone as of purely dynamical origin. Among recent impor tant papers by Austrian writers on this and kindred subjects, refer ence should be made to papers by Exner in Sitzungsber. Wiener Akad. (from 1906 onwards), Geografiska Annaler (192o) and Annalen Hydrog. u. Mat. Met. (i919); by Ficker in the Mete orologische Zeitschrift (from 1910 onwards) and by A. Wegener in the same journal (1921). A paper by Ficker in Met. Zeit. (March, 1923) gives a bibliography of the more important papers up to that date.

The Amalgamation of Vortices.

In a series of papers (Q.J.R. Met. Soc., vols. xlvii. and xlix.) Fujiwhara developed a theory of the origin of cyclones which he based upon observations of the behaviour of vortices in water. The usual hydrodynamical treatment of vortical motion in fluids leads to the conclusion that two vortices with the same sense of rotation should repel each other, while two vortices with opposite senses of rotation should approach each other. Fujiwhara's observations gave precisely the opposite result, that vortices rotating in the same sense attract each other and eventually amalgamate, and that vortices rotating in opposite senses should repel each other. Thus when a number of vortices having the same sense of rotation exist in proximity to one another, they tend to approach one another, and to amalga mate into one intense vortex. When a large vortex absorbs a smaller one, its intensity is increased. These results described in Fujiwhara's papers, are by no means new. Similar results are described by Mrs. Ayrton ("On a new method of driving off poisonous gases," Proc. Roy. Soc. A., 1919), Ahlborn, (Phys. Zeit. xxiii.) and others.

The Travelling Anticyclone.

The anticyclone is to a cer tain extent the converse of the cyclone, in that it is a centre of high pressures and has a system of winds which blow round this centre in a clockwise sense relative to the earth. It is also com monly regarded as a region of descending air, but this is only true when the unit of time is the day or the week. The estimates which have been made by Shaw of the rate of descent of air in anticy clones give velocities of the order of a few hundred metres per day.

No satisfactory theory of the origin of the travelling anticyclone has been evolved. Exner (Dynamische Meteorologie ate. Aufl., p. 358) suggests that its origin may be due to the motion of currents of air from low latitudes which bring with them part of the stratosphere above them. Hanzlik (Denkschriften Wiener Akad., 1908) investigated a number of anticyclones and classified them as "warm" and "cold." The cold anticyclone is shallow and does not extend to the stratosphere. Its motion is usually rapid. The

warm anticyclone extends to higher levels, and its motion is slight. An anticyclone may frequently arrive over Europe as a cold anti cyclone, but if it becomes stationary, then according to Hanzlik it may become warmer and more intense.

The line-squall, waterspout, and tornado are discussed in some detail by M. A. Giblett on "Line-Squalls," Journal Roy. Aero. Soc. 1927. Tropical cyclones (hurricanes) are described in any textbook (e.g., Geddes' Meteorology or Hann's Lehrbuch der Meteorologie). Exhaustive studies of these phenomena are given by Mrs. E. V. Newnham in Geophysical Memoir No. 19, and by Cline in Tropical Cyclones. A fully illustrated article on "Torna does" by R. de C. Ward will be found in the Quarterly Journal R. Met. Soc., 1917.

Elastic Oscillations of the Atmosphere.—Mathematical discussions of the elastic oscillations of the Atmosphere have been given by Rayleigh (Collected Papers, vol. iii. p. 335, and Theory of Sound § 333), and by Margules (Sitzungsber. Wiener Akad. ci part 2a, cii part 2a. An abstract of these papers was given by Trabert in Met. Zeit., 1903).

At any station in the tropics a barograph trace shows two com plete waves each day, maxima occurring approximately at io A.M. and io P.M., and the minima at 4 A.M. and 4 P.M. local time. The amplitudes of these waves are greatest at the equator, and dimin ish with increasing latitude. Beyond latitude 50°, this double wave becomes indistinct, and in latitudes above 70°, the nature of the phenomena changes. Near the poles the maxima of the pressure waves occur everywhere at approximately the same abso lute time, between 10.3o and 13.3o Greenwich mean time (Simp son, "The Twelve-Hourly Barometer Oscillation," Q.J.R. Met. Soc., 1918).

Radiation in the

Earth's Atmosphere.—The sun has an "effective" temperature of about 6,000° C and the position of maximum intensity in the band of wave lengths covered by the solar spectrum is at about 0.7 1. Such radiation is only very slightly absorbed on its passage through the atmosphere, and so reaches the earth's surface with only a slight diminution of in tensity by absorption, though with some loss by reflection from clouds. At the earth's surface, and particularly over the sea, some of the incoming beam of solar radiation is reflected upwards, while the remainder is absorbed. The radiation reflected back from clouds or the earth's surface passes outwards with very slight loss by absorption, and is therefore of no effect in heating the earth or its atmosphere. Aldrich (Annals Astroph. Observatory iv., App. 2, 1922) computes the fraction of radiation lost by reflection by clouds, the earth's surface, dust and other causes as .43.