Study of air tides: phases of the moon versus barometric pressure

Every object in the world is constantly subject to gravitational forces by other objects specially large bodies such as planets and moons.

On the earth, the earth gravity is the largest gravitational force. However, objects on the earth are subject to gravitational forces of moon and sun as well.

You weight less at the times that moon is above your head than other times that moon is on the other side of the earth.

One of the major affects of moon’s gravity on the earth is known as tide. Tide is a shift of the earth’s surface water toward the moon. So if the moon is above, water level will rise and when moon is on the other side of the earth, water level will fall.

A similar thing happens to the air layer, covering the earth. Air layer is always shifted toward the direction of the moon. That will cause higher air pressure in some areas (where moon is above) and lower air pressure on the opposite side.

In this project we will study the effects of air tides on air pressure.

New studies have shown that not only are there tides in the oceans but also in the earth and the air. Only recently, with very highly sensitive instruments, have scientists been able to see a tidal range of about twenty two inches. Compared to the oceans tides, that number is very small, but still a change is occurring every day. Also, this crustal tide is due to the same factor as the ocean tides, the sun and the moon.

We can neither see, nor feel it, because like a ship on the sea, we move with the crust’s flood and ebb. How did scientists then figure out the earth was moving, when even the instruments on it would be moving as well? As I said, highly sensitive instruments were used, such as the horizontal pendulum, and the gravimeter, they are used to measure the amount of gravitational pull from the moon.

There is a set standard for these measurements, and after every possible precaution was taken to make sure there were no errors, and the tests were done, there showed a smaller gravitational pull from the moon. The only feasible explanation was that the earth itself had moved, and the instruments with it, altering the gravitational pull from the moon. Proving that the air does, in fact, have tides!

Like the oceans, the air has tow high tides, and two low, the average range being between eight and twelve inches. Also, the flooding and ebbing of the sea causes a downward and upward tilt of the land. When the ocean’s tides are rising millions of gallons of water move into coastal regions, adding pressure and pushing the land down. When the ocean tides ebb, it releases the pressure, and the crust tilts upward.

This wave of motion does not happen only at end coastal regions though, it extends far inland. A man named Pierre Simon de Laplace, discovered tides in the air. It took him eight years of reading his barometer every day four times a day to come to the conclusion that the air also flows similarly to the ocean tides. Puzzlingly, though, these tides did not follow the moon, as did the ocean, and earth tides, no these tides followed the clock.

High tides were at 10:00 am and pm, and low tides were at 4:00 am and pm. Unlike ocean tides, that flow an hour later each day, these times remained constant, disproving the theory that the tides are following the sun and moon. Scientists now believe that they followed the heat of the sun, and that was what caused the pressure to rise and fall. There is some tidal movement like that of the oceans, in the air, but to a very small extent. Further studies are being done in the outer atmosphere, we have yet to know.

While developing an explanation for sea tides, Isaac Newton pointed out that there should also be gravitational tides in Earth’s atmosphere. He realized that air tides would be barely discernable in England, because tidal effects are maximized at the equator and decrease rapidly toward the poles.

Air tides at the mid-latitudes are indeed at the edge of detectability. At around 10 microbars, or 0.01 millibars, they were only discovered after scientists carefully analyzed records kept over a long period of time. These tiny variations are almost nonexistent when compared to a standard atmospheric pressure of 1013 millibars.

But atmospheric tides are about 100 times stronger at the equator. This still makes them small, but far easier to detect. Seafaring people armed with barometers always saw air pressure in the tropics go up and down with an amplitude of about 1 millibar.

Like sea tides, atmospheric tides have 12-hour cycles, but that’s where the similarity ends. Observers soon discovered that air tides came in intervals of 12 solar hours. This made the tides they were measuring different than the sea tides, which came in twelve-hour intervals related to the moon’s position.

In the 1870s, the British physicist Lord Kelvin raised an obvious point. If these tides were in synch with the Sun, they must be forced by heating. But if this were the case, why 12-hour tides instead of 24? He surmised it had everything to do with how heat is shunted throughout the Earth’s atmosphere.

Armed with his observations, Kelvin made some pretty insightful hunches about how the atmosphere layered itself. Many of his guesses were wrong, but the questions posed by Kelvin were the first time anyone ever suggested using air tides as a tool to plumb the atmosphere. This new line of thinking launched an age of upper-atmospheric science.

It turns out that the main force for a 12-hour thermal tide is heating in the ozone layer. Our atmosphere allows 12-hour oscillations to propagate from the ozone layer to the surface, but it traps 24-hour oscillations.

Thermal tides could be the cause for more dramatic effects on Venus, a planet that orbits closer to the Sun’s heat and has a completely different atmosphere than Earth’s. Venus’ atmosphere circulates 50 times faster than the planet spins.

This is taken from a long document. Read the rest here: scienceprojects.org

Header image: Phys.org

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