A study of the diurnal variations of airborne birch pollen in Sweden showed that the effect of rainfall on pollen counts is more complex than was previously thought. Most of the pollen counting sites in Sweden showed the same pattern of diurnal variations as described by other observers, i. e. lower concentrations during the early morning and a maximum in the middle of the day or early in the evening. Especially in one pollen counting site, viz, in #xD6;stersund, near the geographical centre of Sweden, we noted many occasions when the concentration of birch pollen increased concurrently with the beginning of heavy showers of rain. A few hours later the number of pollen grains decreased, when the pollen was washed out of the air. We noted great differences in the occurrence of such pollen peaks between different parts of the country. The phenomenon was more frequent in #xD6;stersund than in Stockholm, maybe due to the differing topography. Heavy rains seem to be more influential if large amounts of pollen have previously been brought high up into the air, e.g. by warm air or by long-distance transport of pollen, and this should be considered when making pollen forecasts.
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