Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/133

Rh OSCILLATIONS.] METEOROLOGY 123 to latitude and geographical position. This maximum is therefore caused by accessions to the mass of the atmosphere overhead, contributed by the ascending currents from the longitudes of the afternoon low pressure immediately to westward. As midnight and the early hours of morning advance, these contributions become less and less and at length cease altogether, and pressure continues steadily to fall. But between the time when the increase of pressure from the overflow through the upper regions of the atmosphere ceases and the time when pressure increases from the heat rays, direct or indirect, of the returning sun, or during the hours of the night when the effects of nocturnal radia tion are the maximum, pressure is still further reduced from another cause. Radiation towards the cold regions of space takes place, not only from the surface of the globe, but also directly from the molecules of the air and its aqueous vapour. The effect of this simultaneous cool ing of the atmosphere through its whole height is neces sarily a diminution of its tension. Since this takes place at a more rapid rate than can be compensated for by any mechanical or tidal movement of the atmosphere from the regions adjoining, owing to the inertia and viscosity of the air, pressure continues to fall to the morning minimum. This minimum is thus due, not to the removal of any of the mass of air overhead, as happens in the case of the afternoon minimum, but to a reduction of the tension or pressure of the air consequent upon a reduction in the temperature through radiation from the aerial molecules towards the cold regions of space. In the open ocean the morning minimum is largest in the equatorial regions, and it diminishes with latitude ; but the rate of diminution with latitude through anticyclonic and other regions is generally less and more uniform than in the case of the afternoon minimum. The amplitude and times of occurrence of the phases of the diurnal barometric tides are subject to great modifica tions over the land. The amplitude of the oscillation from the morning maximum to the afternoon minimum is greatest where the atmosphere is driest and the sky clearest, and least where the atmosphere is highly saturated and the sky more frequently and densely covered with clouds, being thus generally the reverse of what is observed to take place over the open sea. The meteorology of India affords the most striking illustrations of this remark. At Bombay in April during the dry atmosphere and clear skies of the north-east monsoon, the oscillation is O llS inch ; but in July during the humid atmosphere and clouded skies of the south-west monsoon it falls to 067 inch. In the Punjab, where the air is drier, it is much greater, rising in exceptional years, such as 1852, to 187 inch. The much greater amplitude of this oscillation on land as compared with the open sea is entirely due to the heating of the earth. By this heating of the surface the lower strata of the air become also highly heated and the tension is increased ; and, since the air does not expand freely, vertically and laterally, from its inertia and viscosity, the barometer rises. When, however, the resistance is overcome, the ascending current which sets in is stronger owing to its higher temperature. Since this higher temperature which has its origin in the superheated surface is in addition to the direct heating of the air by the heat rays of the sun as they pass through it, the morning maximum and the afternoon minimum over land are both more extreme than over the open sea. It follows that this oscillation is much larger over land, and largest in climates where insolation is strongest. In places already referred to where the morning maxi mum is greatly retarded, such as Helder, Sitka, Yalentia, and Falmouth, the afternoon minimum in the summer months is singularly small, so small indeed that it does not fall so low as the mean pressure of the day. This peculiarity in the diurnal barometric tide is in all prob ability due to their insular position to the westward of a more or less extensive tract of land, by which a tidal overflow is propagated through the upper regions from the continental towards the insular situations. This tidal overflow receives its impulse from the ascending current from the land, which rises sooner and stronger from inland than from insular situations. On the other hand, on the open sea, and away from land in regions where the morn ing maximum and afternoon minimum are both small, the minimum always falls below the mean of the day, and the time of occurrence of the maximum is not retarded as is the case in insular situations. A map of deviations from the daily mean pressure of the morning minimum in summer shows, as regards the middle and higher latitudes, that it is greatest near the sea, and least in inland con tinental situations. Indeed in the interior of the Old- World continent the dip in the curve in the early morning is so small that the minimum does not fall below the daily mean pressure, but at most places remains considerably above it. The same relations are seen in north-western Europe, where the morning minimum is - 020 inch at Valentia and Falmouth, -0*018 inch at Helder, and -0 &quot;01 2 inch at Amsterdam, whilst at Kew it is only - 002 inch. From its compact form and relations to the surrounding ocean, the Spanish Peninsula well illustrates the peculiarities of this phase of the pressure. The deviations from the daily mean pressure of the morning minimum are at Lisbon - 022 inch and Coimbra - O Oll inch, but at Madrid in the interior + 00 9 inch, pressure in the last case, just as happens in the interior of Asia, not falling so low as the daily mean. The larger minimum near the sea arises from the higher temperature there during the night as compared with more inland situations, from which results a tidal overflow through the upper regions from the sea towards the land, I during the night. The effect of this overflow is to reduce the pressure over those regions whence it proceeds and to I increase it in those regions over which it advances. The shallowing of the morning minimum is greatest in the higher latitudes of continental climates and most complete at great elevations, where in some cases the minimum vanishes, in other words, where the amount of aqueous vapour is small and the time is short during which no part of the atmosphere overhead is touched by the sun s rays. Since the peculiarity is observable in the curves over nearly the whole continent, appearing even in the low latitudes of Calcutta and Madras, it might be suggested whether AVC have not evidence here of a vast tidal movement propagated through the higher regions towards that trough-like section of the atmosphere as it moves westwards over the continent where the temperature of the lower strata of the air is about the minimum of the day and pressure also about the minimum. Reference has been made under ATMOSPHERE to the smallness of the range from the A.M. maximum to the P.M. minimum in the North Atlantic during summer. This phase in the diurnal distribution of pressure is represented in fig. 3, which shows for June the mean amount of the oscillation by lines of 10, 20, 40, 60, 80, and 100 thousandths of an inch, or O OIO inch, 020 inch, &c. This abnormality begins in March, attains the maximum in June, and terminates in October. It is thus confined to the warmer months of the year, and, unlike most meteorological phenomena, is not cumulative, but follows July as that of the temperature of the air, or in August as
 * as the temperature of the latter falls lower than the sea
 * the sun, so that its maximum occurs in June, and not in