From statistical data, it is found that very marked differences in visibility at a given relative humidity may occur at a station if the surface wind direction is taken into account. At Leeuwarden, the visibility is much lower for wind from land than for wind from the sea at the same relative humidity. It is shown that this is due to the condensation nuclei which may be more numerous and more hygroscopic in continental air than in maritime air; this result differs somewhat from that published by Junge. Comparison of our results with those of Appleman concerning Frankfurt shows good agreement of Frankfurt and Leeuwarden for maritime air, which was not to be expected.

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