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  1. George
    21 January 2017 @ 2:44 am

    Clusters are more prevalent in northern latitudes, where the day/night difference through the course of the year is more pronounced: hence the change of length of daylight is greater. The day/night variation is in fact a year-long sine wave, with the steepest slope at the spring and autumn equinoxes. Springtime a couple of weeks after the equinox is my danger zone.

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