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All sailors are aware of the effects of the sun and moon on sea levels and tidal streams. Some may have heard about atmospheric tides and wondered whether these have any effect on our weather. Now, read on....

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Both the sun and the moon exert a gravitational pull on the atmosphere. The moon has, by far, the smaller observed effect. Careful analysis of pressure data shows a lunar tide of about 0.09 hPa near the equator, decreasing to the north and south. This is about 0.01% of the total pressure.

The solar "tide" is really a combination of a small gravitational effect and a much larger heating effect. Near the equator the effects can be seen clearly on barographs as an oscillation of about 2 hPa either side of the average pressure. Maxima occur at about 1000 and 2200 hours local time with minima at 0400 and 1600. In mid-latitudes the variation is about 0.7 mb and is usually hidden by the much larger effects of weather systems.

The effects of the solar tide upon winds and weather systems are very small amounting to small fractions of a knot in wind speed. At great heights in the stratosphere winds due to solar effects can be very large but still have negligible effects on our weather. Similarly, tidal effects from other planets are extremely and immeasurably small.

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