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Reason for lunar clarity is a bit cloudy

Posted on: Friday, 13 June 2003, 06:00 CDT

There is an old belief with a widespread credence that "The full moon eats clouds", or "The moon grows fat on clouds". The implication is that the night sky is less cloudy when the moon is full than at other times of the lunar cycle.

One reason why the full moon might seem to have gobbled up the clouds might be the undisputed fact that, on average, there is less cloud at night than during the day. Many clouds that develop on sunny afternoons result from solar heating of the ground; as temperatures fall with the approach of darkness, and this stimulus is removed, the clouds dissipate and clearer skies prevail throughout the night.

But this phenomenon is common to all stages of the lunar cycle. It does not explain why full moon skies should seem more cloud-free than those, say, one week earlier or later. Is there, then, some possibility that it might be true? Do clouds melt in the moonshine, so to speak, like snow melting in the sun? Three researchers called Lauroesch, Edinder and Lauroesch published the results of a study on this issue in the International Journal of Climatology in 1996. LEL, as we shall call them, took the weather records for Rochester, New York, for a 44-year period from 1950 to 1993, and examined the night- time full moon cloudiness statistically to see if it did indeed differ from that at any other time of month.

First they had to decide how the night of the full moon might be defined. This is not as simple as it seems, since full moon is an instantaneous astronomical event which may well occur in the middle of the day. They decided that the appropriate night was that on which the moon was full sometime after 12 noon local time, and before 12 noon the following day. The associated cloud data was taken as the average of the amounts of cloud reported at each of the hourly observations during the hours of darkness, and this full- moon cloudiness was then compared to the average cloud reported on a "control night" for each month - taken as the first day of the month in question. For obvious reasons, dates when the full moon occurred on the control date were eliminated from consideration.

LEL reported their results as follows: "With a confidence of 95 per cent the null hypothesis should be accepted; the difference between the mean cloud cover at the full moon and that on a control date has not been shown to be statistically significant." Translated into English, this might read: "Rubbish! It is just as cloudy when the moon is full as when it's not." Their conclusion is that the impression of frequent clear skies on nights with a full moon arises from people noticing a full moon on clear nights. The absence of cloud is made visible, and is therefore more likely to attract attention, when there is a bright moon rather than no moon.

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