Grass pollen counts in the West Netherlands as a function of the weather conditions

Eight hundred and ten four-hourly mean grass pollen counts (observed in Leiden, west-Netherland) during the period 1 June - 15 July 1979-1981, are correlated with different meteorological parameters such as trajectory direction, windspeed at 10 m height, the effective cloudiness Ne, air temperature, dewpoint temperature, actual and preceding weathertype, inversion strength and the height of the mixing layer.

Both the grass pollen release process and the diurnal variation of the grass pollen concentration near the ground are strongly influenced by the weather.
Pollen release is low or negligible on dull or wet days or when the grasses are wet due to rain, dew or fog, but high on sunny, dry and warm days. On these days processes involving mixing and removal from the boundary layer cause an afternoon and also a late evening or night-time pollen peak.
Profiles in the diurnal variation can be explained by using a simple mathematical model, based on the release mechanisms and meteorological processes. Some of these profiles, especially during evening and night-time periods, are influenced by release processes in east-England the preceding afternoon and morning.

The existing hayfever forecasting system can be improved by using the results of this study.