Barometric pressure

“ Both Espy and Hann have abundantly shown that the formation and downfall of rain do not produce any low barometric pressure unless they produce a whirling action of the wind—that, in fact, the latent heat evolved by the condensation of vapour into rain may so warm up the cloud as to produce a temporary rise in pressure even at the surface of the ground, due to the outward push produced by the sudden expansion of the cloud. ”

“ Thus, far back in the interior—back to the South Geographical Pole itself—across one thousand six hundred miles of lofty plateau—is a zone of high barometric pressure, while to the north lies the D'Urville Sea and beyond it the Southern Ocean—a zone of low pressure. As if through a contracted outlet, thereby increasing the velocity of the flow, the wind sweeps down over Adelie Land to equalize the great air-pressure system. And so, in winter, the chilling of the plateau leads to the development of a higher barometric pressure and, as the open water to the north persists, to higher winds. ”

“ It tends on the contrary to be low on days of fog or rain.
The results obtained as to the relation between dissipation and barometric pressure are conflicting. At Kremsmünster, Zölss (42) found dissipation vary with the absolute height of the barometer, a± having a mean value of 1·36 when pressure was below the normal, as against 1·20 on days when pressure was above the normal. He also found a± on the average about 10% larger when pressure was falling than when it was rising. ”

“ It is not proper to say that by warming the air, thereby diminishing its specific gravity and causing it to rise, so that colder air flows in to take its place, we thereby diminish the barometric pressure. It is easily seen that in the expression p=RT/v, which, as we have before said, is the law of elasticity, T and v may so vary as to counterbalance each other, and allow the pressure p to remain the same. ”

“ The upward ascent of warm, humid air is accompanied by a decrease of pressure and consequent expansion and cooling which leads to rapid condensation and a heavy downpour of rain; the change of the moisture from a vaporous to a liquid form liberates heat, which serves to perpetuate the upward flow of air, and thus prolongs the life of the storm. During the passage of the central area of low barometric pressure over a given locality the clouds frequently part and portions of the clear sky may be seen. ”

“ These equations are applied to cases in which masses of air of different temperatures and, moisture’s are superposed and then left free to assume stable equilibrium. It results in every case that there is no free energy developed. Any condensation of moisture by expansion is counterbalanced by redistribution of potential energy and by the work done in the interchange of locations. The idea that barometric pressure gradients make the storm-winds is seen to be erroneous and the primary importance of gravity gradients is brought to light. ”

“ As a rule these phenomena correspond to fluctuations in barometric pressure, and therefore are, in a rough way, indicative of changes in weather. It is obvious that a body of air inclosed in the earth and communicating by one or a few small openings with the air above will set up outdrafts and indrafts in adjusting its tension to that of the latter. ”

“ The problem of the relation between wind and pressure in the free atmosphere is more complex than this, and involves the consideration of the inertia of the masses of air that are in motion with the earth around its axis. The air is so extremely mobile that it moves quickly in response to slight differences in pressure that cannot be detected by ordinary barometric measurement. The gradients or differences of pressure that are shown on meteorological charts are not directly, but only very indirectly, due to buoyancy, as caused by heat and moisture. ”

“ They survive because they are true. Mariners at sea and shepherds on land learned their lessons well; neither the one nor the other was possessed of a daily weather map. The wind was a fair barometer; the blinking of the stars was an excellent hygrometer. The discovery of the underlying principles of barometric pressure was the beginning of modern meteorology. The use of the barometer quickly appealed to sailors, and practically every deep-water vessel in the world is equipped with one. Transportation companies, lighting companies, farmers and manufacturers find it a necessity. ”

“ Barometric pressure is not an element of weather, in the ordinary sense of the term, since the fluctuations of pressure that occur in the human environment are entirely inappreciable to the senses. We have seen, however, that pressure is intimately related to wind, which is a weather element of much importance. ”

“ In this country, alterations in the barometric pressure are chiefly valuable as indicating an approaching change in the wind, and as well as of the amount of moisture in the air; hence the instrument is often called "the weather glass." A sudden diminution in the atmospheric pressure is likely to be attended with an escape of ground air from the soil, and therefore to cause injury to health, especially among the occupants of basement rooms, unless the whole interior of the building be covered with concrete. ”

“ Besides, it may not unfrequently happen, that, while effects are witnessed at the place of observation, the cause which produced them may be so far removed as not to influence the barometer. We are, therefore, led to infer that these phenomena result, not from the prevalence of the winds acting on the water, accumulating it at one point and depressing it at others, but from sudden and local changes in the pressure of the atmosphere, giving rise to a series of barometric waves. ”

“ Whenever there is an area over which the barometric pressure is less than the normal, it is an indication of an ascending current of air, and wherever there is an ascending current of air there is a probability of rainfall, though if the air be very dry it may not be carried to a sufficient height to be cooled below its dew point. ”

“ This pressure varies according to the state of the weather, and the changes are indicated by an instrument called a barometer. Generally speaking, the falling of the mercury in the tube of the barometer indicates rain, and its rise heralds clear weather. Sometimes the rise is followed by cold winds, frost and ice. What these changes really indicate, however, can be determined only by comparing the barometric changes, at certain hours, in a number of places very far apart. This is done by the Signal Service. ”

“ Thus the forces that are conspiring to increase the energy of the storm are powerfully assisted by the presence and condensation of aqueous vapor, and the increasing updraught and rarefaction are at once marked by the decreasing barometric pressure at the center. ”

“ The physics of the atmosphere has not yet been explored so exhaustively as to explain fully these three systematic barometric variations, but neither have we as yet any necessity for appealing to some unknown cosmic action as a possible cause of their existence. The action of the solar heat upon the illuminated hemisphere, and the many consequences that result therefrom, may be expected to explain the barometric periods. The variations of sunshine and cloud must inevitably produce periodic variations of temperature, moisture, pressure and motion, whose exact laws we have not as yet fathomed. ”

“ Table I. is calculated from the formula, height in feet = 60,200 (log. 29·922 - log. B) + 925; where 29·922 is the mean atmospheric pressure at 32° F., and the mean sea-level in latitude 45°; and B is any other barometric pressure; the 925 being added to avoid minus signs in the Table.
Table II. contains the correction necessary for the mean temperature of the stratum of air between the stations of observation; and is computed from Regnault’s co-efficient for the expansion of air, which is ·002036 of its volume at 32° for each degree above that temperature. ”

“ This is never done, but the heat which has been boxed up [1] in the air is lost by radiation, and we have lost power. Let us see to what extent this takes place.
Thirteen cubic feet of free air at normal temperature and barometric pressure weigh about one pound. We have seen that 116 degrees of heat have been liberated at half stroke. The gauge pressure at this point reaches 24 pounds. According to Mariotte's law, "The temperature remaining constant, the volume varies inversely as the pressure," we should have 15 pounds gauge pressure. ”

“ During the season of rising temperature the surface of the land warms more quickly, and becomes hotter than that of the sea, and during the season of falling temperature the reverse is the case. Barometric pressure tends to be higher over the colder region than over the warmer, and there is accordingly a tendency for air to flow, in the lower levels of the atmosphere, from the former towards the latter. ”

“ In very small storms, such as tornadoes, waterspouts, etc., this process gives rise to very rapid uprising currents, a very rapid whirl around the central axis and a very low barometric pressure at the centre, but in extensive storms the vertical current is not so conspicuous, although the buoyancy of the central air tends very strongly to maintain the disturbance. ”

“ The most serious defect in such instruments, even when kept in good working order, is a want of sensitiveness commensurate with the desired openness of scale. It is very important that a fraction of a minute of time should be as recognizable as one-tenth of a degree of temperature; one thousandth of an inch of barometric pressure, and velocities of one hundred miles per hour, as well as rapid changes in all these elements, must be measurable. ”

“ Observing every day the accidental variations of the atmosphere, we find it difficult to persuade ourselves that in a great space of time the effects of these variations mutually compensate each other: that in a long succession of years the averages of the temperature of the humidity, and of the barometric pressure, differ so little from month to month ”

“ Under any circumstances, and in any locality, a high barometer not less than a low one should demand particular attention, and if possible, hourly readings taken some time before and after the passage of the maximum: this will be referred to more particularly under the next head.
Preceding and Succeeding Accumulations of Pressure.—Mr. Redfield has shown in his Memoir of the Cuba Hurricane of October, 1844, that two associated storms were [Pg 47] immediately preceded by a barometric wave, or accumulation of pressure, the barometer rising above the usual or annual mean. ”

“ This is practically never the case except when travelling along the sea-shore or the margin of a great lake the elevation of which has been determined. But, meteorology apart, barometric readings in any little known country are of value, because by comparing them with simultaneous readings taken at a neighbouring fixed station, new data as to the altitude of the country may be obtained. While in camp, it would be an extremely useful thing to make barometer readings, even with an aneroid, every two hours, in order to get some information as to the normal daily range of atmospheric pressure. ”

“ Stand with your back to the wind, and the lowest barometer, or centre of depression, will be to your left in the northern hemisphere (in the southern hemisphere to the right) ; this rule holds universally. (2.) The force of the wind is proportional to the barometric gradient, or the quotient of the dis tance between two places stated in miles by the difference of pressure stated in inches of mercury as observed at the two places. Hence, in the Channel, where the isobarics are close together, winds are high, but in the north of Scotland, where the isobarics are far apart, winds are light. ”

“ In barometric leveling, differences in height are determined by measuring the differences in atmospheric pressure at various elevations. Air pressure is measured by mercurial or aneroid barometer, or a boiling point thermometer. ”

“ Since atmospheric pressure is a force, the practice of measuring it in units of length is rather like measuring time in bushels or potatoes in hours. The inconsistency is serious from a scientific point of view, because it divorces barometric measurements from other physical measurements, in which pressures are measured in units that have nothing to do with length; viz., dynes per square centimeter. ”

“ Some whirlwinds are accompanied by rushes from the upper atmosphere, from the colder regions, which, mingling with warmer and moister air near the sea, cause dense clouds. About their centre it sometimes happens that the barometer falls as much as two or three inches, showing a diminution of atmospheric pressure by nearly a tenth part; when it should be expected, from physical considerations alone, that very dense clouds would be formed. ”

“ The kind of mercury barometer that you buy.
Weather men do not have to wait for an actual storm to be reported. If the reports from the west show that the air is rising as it swirls along—that is, if the barometer readings in the west are low—they know that this low-pressure air is approaching them. And they know that low pressure usually means air that is rising and cooling and therefore likely to drop its moisture. ”

“ As a consequence of the lateral expansion there follows a transference of part of the air from the warm to the cold hemisphere along the earth’s surface; and, as a consequence of the vertical expansion, an overflow in the upper regions of the atmosphere in the same direction. Hence, in so far as the dry air of [Pg 16] the atmosphere is concerned, the atmospheric pressure will be least in the summer and greatest in the winter of each hemisphere. But the production of aqueous vapor by evaporation being most active in summer, the pressure on the barometer will be much increased from this cause. ”