Air pressure

“ 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. ”

“ INFLUENCE OF FLUCTUATIONS IN TEMPERATURE AND PRESSURE ON THE APPARENT VOLUME OF AIR IN THE SYSTEM.
The air, being confined in a space with semi-rigid walls, is subjected naturally to variations in true volume, depending upon the temperature and barometric pressure. If the air inside of the chamber becomes considerably warmer there is naturally an expansion, and were it not for the tension-equalizer there would be pressure in the system. Also, if the barometer falls, there is an expansion of air which, again, in the absence of the tension-equalizer, would produce pressure in the system. ”

“ In the same way, when you force the air out of a rubber suction cap, such as is used to fasten reading lamps to the head of a bed, the air pressure outside holds the suction cap tightly to the object against which you first pressed it, making it stick there.
We can easily experiment with air pressure because we can get almost entirely rid of it in places and can then watch what happens. A place from which the air is practically all pumped out is called a vacuum. ”

“ 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. ”

“ Consequently, air under a pressure equal to that of two atmospheres will occupy only half the volume it occupied under the pressure of one atmosphere ; under the pressure of three atmospheres, one- third of that volume, &c. By doubling the pressure we double the elasticity. If, however, the temperature be increased, and the air occupy the same space, the pressure will be increased ; but if the pressure is to remain the same, the air must occupy a larger space. ”

“ We know that air, or any gas, confined in a vessel, presses against its sides, and against the surface of any body placed within it. This pressure is due to the impact of the flying molecules; and the constant succession of their strokes is, according to this theory, the sole cause of what is called the pressure of air and other gases. As each molecule strikes the side of the vessel the same number of times, and with an impulse of the same magnitude, the pressure in a vessel of given size must be proportionate to the number of molecules—that is, to the quantity of gas in it ”

“ It leads to the conclusion that, however air changes its state, the internal work done is proportional to the change of temperature. When, in expanding, air does work against an external resistance, either heat must be supplied or the temperature falls.
To fix the conditions, suppose 1 ℔ of air confined behind a piston of 1 sq. ft. area (fig. 76) . Let the initial pressure be p1 and the volume of the air v1, and suppose this to expand to the pressure p2 and volume v2. ”

“ He said, "Certainly; it's quite simple. Take wind. Wind is caused by differences of pressure."
"What is pressure? Who is pressing what?"
"Pressure is what the barometer tells you—not the thermometer; you must keep the thermometer out of this. Suppose it is very hot in London—"
"Don't be ridiculous."
"Well, suppose it is very hot at a place A—"
"I thought we were keeping the thermometer out of this."
"It comes in indirectly. But don't keep interrupting. If it is very hot at the place A, the air at A rises. You see?" ”

“ Composition of air.
It consists essentially of two gases, called oxygen and nitrogen, and also contains a variable quantity of aqueous vapour.
Qualities of air.
In common with matter in every state, the air possesses impenetrability. It can be compressed, but cannot be annihilated. It has weight, inertia, momentum, and elasticity.
In consequence of its weight is its pressure, which acts uniformly on all bodies, and is equal to between 14lbs. and 15lbs. on every square inch of surface at the sea-level.
Early idea of air’s resistance. ”

“ At the area of low pressure the air is ascending, as we have seen, and, owing to the peculiar way it ascends—by circling spirally upward around a region of comparative calm—it creates a partial vacuum, which is more pronounced in the center of the area. At the area of high pressure the air will be condensed by the descending current being arrested by the earth. The descending current—coming, as it does, from the upper and colder regions—accounts for the cool weather that most always prevails at a high-pressure area. ”

“ This air heats by compression because in its downward movement it is continually leaving more and more air above it to exercise pressure upon it. It gains about twenty degrees with each mile of descent, and if there were no other factors to the problem it would be hot air when it reached the surface of the earth instead of cold air. ”

“ 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. ”

“ Now, since vast volumes of air are in this way poured into the space where pressure is low, without increasing that pressure, and, on the other hand, vast volumes flow out of the space where pressure is high, without diminishing that pressure, it necessarily follows that the air poured in is not allowed to accumulate over this space, but must escape into other regions ”

“ Air, like gas, possesses an elastic force,—that is to say, that it constantly endeavors to distend its molecules, and presses without ceasing upon the sides of the vase which contained it, or upon the surrounding air. Now, when you began to work the lever there was still enough air in the globe to balance, through its elastic force, the air outside; and, as the piston receives an almost equal pressure of air from the atmosphere above and from the globe below, it is easily raised and lowered. ”

“ The upward stroke of the piston continues, and the air expands without receiving heat, until its temperature falls to that of the refrigerator, that is, to the temperature that it had at the beginning of the first operation. The fall of pressure is represented by the adiabatic C D, and work represented by the area C D d c is done by the air. ”

“ The air that surrounds us weighs about one-thirteenth of a pound per cubic foot and exerts a pressure, at sea level, of nearly fifteen pounds per square inch. Its temperature varies from 30° below to 100° above the Fahrenheit zero. ”

“ 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. ”

“ Since the pressure decreases with height in a geometrical progression, it would be halved for each three and a half miles of ascent were the temperature constant, but as the temperature also decreases with height, the successive intervals, beginning with three and a half miles, become shorter because the volume of a gas depends on its temperature as well as on the pressure upon it. ”

“ 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. ”

“ Density.—It is evident that a close relation exists between the temperature, pressure and density of the air. With lowering temperature, the volume of air contracts, and air flows in to equalize the loss. The cold air contains a greater number of molecules per given volume and therefore its density is increased; because it contains more matter per given volume its weight, and therefore its pressure is increased. ”

“ He explains the fact that pressure will condense vapor, by supposing that it brings the particles within the distance at which the repulsion arising from fire ceases. In this way, he also explains the fact, that though external pressure does thus condense steam, the mixture of a body of air, by which the pressure is equally increased, will not produce the same effect; and therefore, vapors can exist in the atmosphere. They make no fixed proportion of it; but at the same temperature we have the same pressure arising from them, whether they are in air or not. ”

“ From this example it is obvious that the greater the pressure of the atmosphere, as indicated by the barometer, the greater will be the lift of the airship or balloon.
Temperature is another factor which must be considered while discussing lift. The volume of gas is affected by temperature, as gases expand or contract about 1/500 part for every degree Fahrenheit rise or fall in temperature.
In the case of the 1,000 cubic feet balloon, the air at 30 inches barometric pressure and 60 degrees Fahrenheit weighs 75 lb., and the hydrogen weighs 5 lb. ”

“ The determination of mean pressure over each of the two hemispheres is more important than that of the mean pressure of the earth as a whole. The fact that the pressure over the southern hemisphere is lowest at the time when it is highest over the northern hemisphere, and vice versa, indicates the shifting of an enormous volume and weight of air from one hemisphere to the other, twice a year. ”