4. This is succeeded by an evening maximum, generally occurring between 8 P.M. and 11 P.M.

5. Pressure then decreases gradually to the nocturnal minimum.

These phenomena are much more marked in low than in high latitudes, and near the equator they are so pronounced and so slightly interfered with by the passage of temporary atmospheric disturbances that the hour of the day can frequently be read off by a glance at the barometer.

17. Now Buchan's explanation of these diurnal variations of pressure as caused indirectly by the action of dust particles seems the most plausible. During the night the dust particles lose heat rapidly by radiation into space. They lower the temperature of the surrounding air below the dew-point, and consequently decrease the vapour-tension. This decrease is greatest just before dawn, because then the work of radiation has reached its climax; thus the pressure at the earth's surface is lowest at this time. After the sun has risen vapour-tension is again increased by the evaporation of moisture from the surface of the dust particles; the latter also get superheated by the sun's rays, and help to increase the temperature and the elastic force of the air itself. This double increase is sufficient to affect the barometric column producing the morning maximum at the hour when the lower strata of air are increasing in temperature with the greatest rapidity; for the tendency to lift the whole atmosphere without affecting the pressure at the earth's surface is not

sufficient to overcome the inertia and viscosity of the higher layers, especially as those layers have much fewer dust particles, and are not similarly affected.

Now, as insolation upon the surface of land and water increases, temperature and evaporation also increase, and form strong ascending currents which overcome the resistance of the higher layers mentioned above. Thus pressure again decreases to the afternoon minimum. These descending currents are broken up over land surfaces, and are partially compensated for over each locality by descending currents on all sides of them. It should be observed that dust particles being more numerous and of larger size in the lower strata of the atmosphere than in the higher, the whole operation which results in the afternoon minimum is confined to the lower and middle layers.

After the afternoon minimum caused by the superheating of the lower atmosphere has passed away from the longitudes affected by it, barometric pressure at the earth's surface commences to rise until the evening maximum is attained, which is usually the case after sunset. This maximum may be attributed partly to the overflow of the atmosphere still rising in the west, and partly to descensional currents established in the rear of the upward current.

18. These fluctuations of pressure are, at least over land surfaces, intimately connected with certain definite cloud-formation, especially where there are no very strong horizontal currents. These will be treated of when we come to speak of the cloud-forms

of the globe. It will, however, be seen here that the morning and evening maxima will generally coincide with the least amount of cloud, because in the first the increase of vapour-tension is not rapid enough to enable the resistance of the upper strata to be overcome by an ascensional current, and because in the second case descending currents exist which cause evaporation of water particles. The afternoon minimum, however, frequently coincides, over those land surfaces which are not extremely arid, with the maximum formation of "Inversion" clouds, and therefore of precipitation.

It may also be mentioned that wherever there are, owing to geographical position, upper currents of considerable velocity, unaffected by the heating of the lower layers of air, the rapid nocturnal cooling of the middle and upper strata tends to produce high night cloud of Inclination and Interfret types. In such cases we frequently observe the greatest amount of cloud and also of precipitation in the hours of the night. This is particularly the case upon and in the neighbourhood of high mountains, the nocturnal radiation from which tends to lower the temperature of the surrounding atmosphere.

CHAPTER II

CLASSIFICATION, NOMENCLATURE, AND DESCRIPTION

OF CLOUDS

19. THE task attempted in the following chapter possesses certain intrinsic difficulties which must be immediately faced. In the first place, the objects of our research being for the most part inaccessible, we rely to some extent upon theory, while our ends are in the main practical. In the second place, it must be distinctly understood that many types of cloud merge imperceptibly into one another. The last, but not least, of the obstacles which beset us lies in already existing nomenclatures, not only differing from one another, but also running counter here and there to those broad distinctions of physical structure which observers should take as their basis of classification.

It seems undesirable to sweep away all existing nomenclature, not only because many of the divisions hitherto adopted appear to coincide with those differences of structure which in their results have presented themselves to the eye of every good observer, but because we cannot expect men to abandon all the associations connected with those characteristics of sky to which certain of the best cloud-names have been given. To

the aim of making the weather-watcher as competent as possible in the identification of cloud structures, which is at present the principal object to be attained, a certain portion of the logic of nomenclature has with reluctance to be sacrificed. Thus, in compound names the termination of the affix sometimes signifies that the whole name denotes an actually compound cloud, sometimes that it denotes the primary kind of cloud, while the suffix expresses some important modification of structure capable of being readily identified.

20. In Table I., to every scientific name there is appended an English title in the second column, in the third an abbreviation, and in the fourth a symbol, which will be found of utility in charts, and for special purposes.

In Table II. certain sub-species or varieties are similarly treated.

it

One of the objections offered to this classification is that it is too simple. Another, more repeatedly urged, is that it is too complex, and that what is most needed, if a science of clouds is to be popularised, is the cutting down of the number of names, and the general simplification of nomenclature. Few men, is contended, will be induced so to take up the study of cloudland as to care to learn what any particular cloud looked at should be named. Some authorities on this subject, to whom I am indebted for valuable observations, would reduce the divisions of clouds to three, or even two. By doing this we might greatly amplify the number of observers; but the mass of observations so collated would be of relatively small