Atmospheric pressure

“ It is obvious that changes in the general distribution of atmospheric pressures over extended areas, of the great centers of high and low pressure, are closely associated with fluctuations in temperature and rainfall. ”

“ Subterraneous Effects of Atmospheric Pressure.—Hardly sufficient account has been taken of the variations in the pressure of the atmosphere as a force competent to produce important effects within the earth and on its surface. The pressure on a man's body amounts to thirty thousand pounds, and that exerted upon a table ten feet long and five feet wide is equivalent to more than one hundred thousand pounds. ”

“ Pressure Variation Pressure varies with altitude and temperature of the air as well as with other minor influences which we neglect here.
Altitude
As we move upward through the atmosphere, weight of the air above becomes less and less. If we carry a barometer with us, we can measure a decrease in pressure as weight of the air above decreases. Within the lower few thousand feet of the troposphere, pressure decreases roughly one inch for each 1,000 feet increase in altitude. The higher we go, the slower is the rate of decrease with height. ”

“ But the attractive energy of the sun (at his surface) exceeds the force of the earth's gravity about twenty-seven times. "We may safely infer, then, that an atmosphere such as the corona was supposed to be, would cause a pressure exceeding the atmospheric pressure we experience some thousands of times. The gas forming the prominences would be correspondingly compressed under these circumstances. But as a matter of fact the pressure at the very base of the colored prominences appears to be a mere fraction of that which our own air exerts at the sea-level. ”

“ Let us take, for instance, the pressure of the air as indicated by the height of the barometer, and show that the oscillations of the mercury must be due to the sun. Imagine for a moment that the sun were to be extinguished, one consequence of the cessation of the arrival of heat at the earth would be that winds would blow no longer. There could be hardly any movements whatever in the air except such as might arise from atmospheric tides. ”

“ Any cause which produces changes in atmospheric pressure invariably puts it in motion, giving rise to gentle airs or furious gales, according to degree; and here, at least, we have a relation between the movements in the air and the movements of the needle so close that it is difficult to say which is cause, which effect, or whether the two be not the effects of a common cause. ”

“ In the absence of precise knowledge of the composition of the primitive planetary atmospheres, equilibrium concentrations with a wide range of temperatures, pressures, and elemental compositions are being investigated by NBRF and NBS. These investigators have postulated that the maximum atmospheric pressure may have approached 100 atm if the primitive Earth was sufficiently hot and if an appreciable portion of the water on Earth's surface today was present on primitive Earth. ”

“ It follows from what has been stated that much which has been written regarding these fluctuations, and in ex planation of them, does not rest on facts; and nearly everything yet requires to be done in the way of collect ing data towards the representation and explanation of the daily oscillations of atmospheric pressure which are, as regards two-thirds of the globe, perhaps, as already stated, the most regular of recurring phenomena, and an explanation of which cannot but throw much light on many of the more important and difficult problems of the atmosphere. ”

“ Pressure Altitude In the standard atmosphere, sea level pressure is 29.92 inches of mercury or 1013.2 millibars. Pressure falls at a fixed rate upward through this hypothetical atmosphere. Therefore, in the standard atmosphere, a given pressure exists at any specified altitude. Pressure altitude is the altitude in the standard atmosphere where pressure is the same as where you are. Since at a specific pressure altitude, pressure is everywhere the same, a constant pressure surface defines a constant pressure altitude. ”

“ We may show further that, at the sea level, spirits of wine may be pumped higher according to its less specific gravity; and that if we attempt to pump water at successive altitudes above the sea level, we can only raise it to less and less heights, corresponding with the lessened atmospheric pressure at those altitudes, where the column of air producing the pressure is shorter. ”

“ These strips of accumulated pressure are doubtless crests of atmospheric waves rolling forwards. In some cases a ship in its progress may cut them transversely in a direction at right angles to their length, in others very obliquely; but in all cases, whatever section may be given by the curve representing the observations, too much attention cannot be bestowed on the barometer, the wet and dry bulb thermometer, the direction and force of the wind, the state of the sky, and the appearance of the ocean during the ship's passage through such an accumulation of pressure. ”

“ During the coldest hours of the day a reverse movement takes place and forms a corresponding trough of pressure. This diurnal maximum and minimum of pressure is practically a raising and lowering of the center of mass. ”

“ 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. ”

“ Distribution of Pressure.—The movements of the air caused by heating, cooling, expansion and contraction include the general or planetary movements, as well as the massing of the air in one locality and the counterbalancing depressions formed in another. ”

“ Suppose the entire atmosphere to be, from the bottom to the top, nearly of the same temperature, and in a perfect state of the quiescent equilibrium, and that from some cause a certain volume of air above has its specific gravity so changed that it commences to descend. As it descends the pressure upon it increases—and air, being compressed, contracts and gives out heat. A like volume ascends to take its place, and in ascending it expands and grows cool. ”

“ As the air rises over the point of low storm area, or, in other words, the area of low pressure, and travels into the atmosphere, it is not permitted to rise to any great height, because it is always acted upon by the force of gravity and is being pulled back to earth again. ”

“ Taking the analogy of our atmosphere in its relation to the earth, we know that when heat is absorbed by it, it expands, the result being that a greater pressure is exerted by the expanding atmosphere, than would be exerted if it remained at the same temperature all the time. ”

“ 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. ”

“ That quantity of water which had been frozen by the mixture of snow and salt, was almost as much like snow as ice, such a quantity of air-bubbles were contained in it, by which it was prodigiously increased in bulk.
The pressure of the atmosphere assists very considerably in keeping fixed air confined in water; for in an exhausted receiver, Pyrmont water will absolutely boil, by the copious discharge of its air. ”

“ And since the weight of the vapors which ascend must continually equal that of the products of condensation which are falling, it is further evident that the upward currents, rushing through contracted channels, must move with enormous velocity, and therefore, of course, that the pressure and temperature must rapidly increase from the free surface downward. ”

“ If we ascend into the air 5000 feet, it is perfectly evident that there are 5000 feet less of atmosphere pressing upon us than at the point from which we started. This diminution of pressure is often measured by the temperature at which water boils at different heights. ”

“ Thus, at Mount Washington, and at other stations of considerable altitude, expansion due to temperature-increase raises the center of mass in summer; mean pressure, therefore, is raised. In winter, low temperature causes contraction, lowering the center of mass and therefore the pressure. In other words, while sea level and also the observer’s station are at fixed altitudes, the center of mass is a varying altitude; it is raised by increasing temperature and lowered by decreasing temperature. ”

“ 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. ”