Wind pressure

“ The gradient of pressure in the direction of the wind overcomes resistances, but the gradient of pressure, perpendicular to the direction of the wind, is far greater than that in the direction of the wind, and is that which produces the areas of decided low pressures that appear as storm centres on the daily weather map. Therefore, in general, the wind cuts across the charted isobars in oblique directions and at angles which are nearly 90° for the feeble winds far removed from the centres, but which are almost zero for the most violent winds near the low centre. ”

“ In general, the wind may be considered to be the result of a change of pressure and temperature in the atmosphere at the same level. The air of a warmer region, being lighter, ascends, and gives place to a current of wind from a colder region. These two currents—the higher and the lower—will continue to blow until there is equilibrium. ”

“ Wind tends to blow from an area of high pressure to one of low pressure, but due to rotation of the Earth, wind is deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere. ”

“ If we should be in the track of a moving storm so that its center passed over us the winds at the beginning would blow in one direction and then there would come a subsidence until it had moved forward through the quiet zone, when we should feel the wind in the opposite direction until the area of low pressure had moved forward into the region of high pressure. The velocity of the wind will be determined by the difference of pressure between [Pg 96] the areas and by the distance that the areas of high and low pressure are apart. The steeper the grade the more rapidly the fluid will flow. ”

“ Assuming the wind speed at any instant to vary by one sixth of the mean, its impactual pressure will then vary by thirty-six per cent of the pressure of the mean wind, remembering that the pressure varies as the square of the speed. ”

“ When heated by the sun, it expands; and, in doing so, it rises because it is less dense and therefore lighter. As soon as the pressure of the air above is lessened, air rushes in below from all the cooler regions around. This rushing of air we call a wind. If the low pressure lies to the north of us, the air rushes northward over us to fill it, and we say the wind is from the south; if the air is flowing to the south of us, we say the wind is from the north. ”

“ A rise from a very low pressure indicates wind and clearing weather—the more rapid the rise, the stronger the wind. Rapid changes in pressure indicate early and marked changes in the weather.
A sudden rise in pressure indicates as great a change as a sudden fall.
The wind is apt to blow hardest when, after having been very low, the barometer begins to rise. ”

“ Thus the meridional currents produced by overheating combine with the currents embracing the whole [p. 46] wind system of the earth, with the result of disseminating the excess of temperature and humidity of the torrid zone over the temperate and arctic zones, thereby producing the prevailing winds.
8. This is accomplished by the production of alternating local depressions and elevations of barometric pressure by the disturbance of indifferent equilibrium in the upper layers of the air. ”

“ The position of least pressure must be carefully considered; as, in accordance with the law, the wind will blow around that locality. The same remark applies to areas of high pressure, which, however, very rarely occur in a well-defined manner over the British Isles.”
Referring to the table on page 76, the scale 0 to 6 was formerly used by meteorological observers at land stations, and it was intended to express, when the square of the grade was obtained, the pressure of the wind as given in the second column. ”

“ The velocity with which the wind moves from the High toward the Low depends on differences in air pressure, modified in the lower stratum by the friction offered in passing over surfaces of varying degrees of roughness, the speed being greater over a water surface with the same difference in air pressure than over a level unwooded prairie, and greater over the open prairie than over an irregular wooded area. ”

“ Under the most favourable conditions, i. e., when the places of high and of low barometer are in immediate juxtaposition, as on the inside and outside of an air-pump, an effective difference of 0.006 inch in the barometric pressure would create a breeze with a velocity of seven miles the hour. Such a wind is capable of exerting a horizontal pressure of 0.2 lb. ”

“ Wind blows from regions where the barometer is high to where it is low, and with a force proportioned to the difference of the pressures, and places between very high and very low pressures feel most the violence of the resulting storm, and not those where the pressure is absolutely the lowest.
If the wind being N. passes to N.E. we get clear weather; the air is dry, the barometer high, and in winter a considerable degree of cold follows. If the wind gets on to the E. the barometer will fall, and the sky become more or less overcast. Snow and S. wind may then be expected. ”

“ There is a variety of wind called mountain winds that arise from different causes. As has been stated in a former chapter, under ordinary conditions the air is more dense at sea-level than at any point above, and the density is constantly changing from denser to rarer the higher we ascend. Suppose at a certain point, say halfway up a mountain side, the air has a certain density, and if it is at rest the lines of equal density or pressure will seek a level, just as water would under the same conditions. ”

“ But each storm is in reality a law unto itself; and while we know something of the relations between pressure and flow of the air; as yet we know very little about the relations of wind and weather. The problem is [Pg 35] complicated by the behavior of the load of water vapor.
Fig. 5. Euros—The Southeast Wind
The Chief Forecaster of one of the great national weather services recently wrote:
"Despite the fact that maps have now been drawn day by day for over half a century, we may safely say that no two maps have been identical." ”

“ When the barometrical readings taken simultaneously at stations distributed over a wide area are compared, the distribution of atmospheric pressure can be ascertained, and it is possible to tell from this the force and direction of the winds prevailing within this area, and generally also the weather which is likely to be experienced. The greater the inequality of pressure, the greater will be the rush of air to the centre of depression, and the stronger will be the wind. The wind, however, does not flow in a straight line from a region of high to a region of low pressure. ”

“ It is from these local irregularities using the word local in a very wide sense in the distribution of atmos pheric pressure, whether the irregularities originate in the temperature or aqueous vapour, that all winds, from the lightest breeze to the most destructive hurricane, take their rise ; for, as already stated, wind is merely the flowing away of the air from where there is a surplus of it to where there is a deficiency. ”

“ The fact is that the wind is continually varying in force, and while the ordinary pressure plate is admirably adapted for measuring the force of a steady and uniform wind, it is entirely unsuitable for following the rapid fluctuations of the natural wind. To make matters worse, the pen which records the motion of the plate is often connected with it by an extensive system of chains and levers. A violent gust strikes the plate, which is driven back and carried by its own momentum far past the position in which a steady wind of the same force would place it ”

“ 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. ”

“ 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. ”

“ 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. ”

“ WIND AT 125 MILES AN HOUR. “The wind at 125 miles an hour is something awful. I could neither hear nor see when it was at its height and it was difficult to breathe. I am nearly six feet in height and estimating the surface of my body exposed to the wind at five square feet, my body sustained at that time a pressure of 390 pounds. ”

“ The air is thus forced up the wind-trunks into an air-chamber called the wind-chest, the pressure of it being equalized by a reservoir, which receives the excess of wind through an aperture, and permits escape, when above a certain pressure, by a discharge valve or pallet. ”

“ It is not enough that we have the evaporation of moisture that will ascend into the higher regions of the air and there be condensed into cloud and possibly rain, but we must have the means for distributing these conditions over a large area, and for this purpose we have the phenomenon of wind. Why the winds blow can be accounted for to a certain extent,—we might say to a large extent,—but there yet remain many unsolved problems relating to wind and weather. Of the phenomena of wind we will speak more fully in a future chapter. ”

“ All winds are caused by the disturbances of indifferent equilibrium, and the motion of the air is to restore equilibrium.
2. These disturbances are caused through overheating of the layers of air near the surface of the earth by insolation, through unsymmetrical cooling of the higher layers by radiation, and through the heaping up of air masses caused by obstructions.
3. The disturbances are adjusted by ascending currents, wherein the particular species of acceleration occurs in which the increase of velocity is proportioned to the diminution of pressure. ”

“ When any portion of the earth’s surface is periodically more heated above its normal temperature, or average for the year, than the neighboring region, the resulting abnormal temperature gradient causes a periodic surface wind tending toward the excessively heated place, and a counter wind above. That is, the cooler and heavier column of air sinking and uplifting the lighter, results in a lowering of the common center of gravity of the two 386columns of air, and thus furnishes the driving power of the wind. ”

“ When isolated tracts of the atmosphere have been rendered rare and light under the concentrated warmth of the sun, it is simple weight—the weight of colder and heavier portions of the air—that makes winds rush into the spots where the deficient downward pressure is, and that causes the sails of innumerable windmills to whirl before the impulse of the breeze. ”

“ It is quite erroneous to imagine that the low pressures in storm areas and in the polar regions, and especially the belt of low pressure at the equator are due simply to the diminution of the density and weight of the air by the action of its warmth or its moisture, or to the abundant rainfall as relieving the atmosphere of the weight of water. ”

“ A current of air increases in velocity where the space of its passage is straitened: the same wind which was moderate in an open plain becomes violent in passing through a narrow passage in a mountain, or between two [100] lofty buildings; and its most violent action is at the top of these structures or mountains, for air being compressed by these obstacles, its density and mass becomes increased, and as the velocity remains, the force or momentum of the wind naturally becomes much stronger. ”

“ Heat and cold are thus combined; the equilibrium is distributed by a shock, the wave of wind issues forth and is distended, scattered and broken up in every direction in fierce streams. The dispersion of the gusts shakes the streaming locks of the wind upon the four corners of the horizon. ”

“ The amount, taken by itself, of vapour, moisture, wet, rain, or snow, in the wind, or current of air (direction and strength remaining the same) seems to cause a change amounting, in an extreme case, to about half an inch.
3rd. The strength or force alone of wind, from any quarter (moisture and direction being unchanged) is preceded, and foretold, or accompanied by a fall or rise, according as the strength will be greater or less, ranging, in an extreme case, to more than 2 inches. ”

“ The velocity of these vapors, though not necessarily in reality greater than that of the central clouds, appears greater, owing to their proximity, and, of course, also to the usual presence of a more violent wind. They are also apparently much more in the power of § 4. They are subject to precisely the same great laws.the wind, having less elastic force in themselves; but they are precisely subject to the same great laws of form which regulate the upper clouds. They are not solid bodies borne about with the wind, but they carry the wind with them, and cause it. ”

“ The force of the wind on a surface placed squarely across the flow of air increases roughly with the square of the wind speed. For this reason, it is a good approximation to say that an eighty-mile wind is four times as destructive as a forty-mile wind. ”