Lunar tides are larger than solar tides11/9/2023 ![]() ![]() Detection of such a tiny signal masked by much larger pressure variations associated with weather phenomena required the development of special statistical techniques and the accumulation of a long series of regular observations. Theory predicts stronger lunar pressure oscillations in the tropics but their amplitude rarely exceeds 100 microbars or 0.01 percent of the average surface pressure. The dominant lunar tide in the atmosphere is therefore semidiurnal (half-daily). One occurs approximately when the moon is directly overhead, the other half-a-day later. It is clear then that Laplace's theory predicts two pressure maxima per lunar day corresponding to the two ocean bulges. This is the usual atmospheric surface pressure that we hear about in weather forecasts. Just as our weight puts pressure on the ground beneath our feet, the weight of the atmosphere above us exerts pressure on the planet's surface and everything located on it (recall that pressure is defined as force per unit surface). But Laplace's theory is perfectly applicable to the atmosphere if ocean depth in the tidal equation is replaced by a quantity called equivalent depth, characterizing the extent of the atmosphere above the surface. Real ocean tides are of course complicated by the waters uneven depth and the presence of land. (The moon also revolves around Earth in the same direction as Earth's rotation but at a much slower rate.) For an observer stationed on the surface and revolving with it, the bulges would appear as a giant wave, which follows the apparent motion of the moon to the west and has two crests per lunar day. As the planet rotates from west to east the two bulges tend to stay on the Earth-moon line. On the opposite side of Earth (point B), its attractive force is weakest, which allows the ocean to bulge outward again, in this case away from the moon. Īt the point on the ocean's surface closest to the moon (point A in the illustration), the lunar gravitational attractive force is strongest and it pulls the ocean toward itself. Laplace's equation describes the motions of an ocean of uniform depth covering a spherical Earth. Roughly a century later it was also used to predict the existence of atmospheric tides when Laplace developed a quantitative theory based on a tidal equation now bearing his name. Newton's theory of gravity provided the first correct explanation of ocean tides and their long known correlation with the phases of the moon. The short answer is yes, and at various times this question of lunar tides in the atmosphere occupied such famous scientists as Isaac Newton and Pierre-Simon Laplace, among others. Rashid Akmaev, a research scientist at the University of Colorado, explains. ![]()
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