Summary Since the season of summer hay fever may last only 4 weeks and rainfall may produce days with virtually no airborne pollen, the timing of clinical trials is important. To facilitate this a method of forecasting the first possible 100% symptom day, and the severity of the season in terms of light, moderate or severe, is presented. The forecast is based on temperature in April and May, and formulae tested by reference to records of grass pollen concentrations over Central London during 1961–70.

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Meteorological factors afTecting the grass pollen content of the air-streams over London during the period 1961-70 are examined. Seasonal variations in pollen catch are eliminated by translating pollen concentrations into pollen units, and the elVcct of rain, temperature, sunshine, convection, wind direction and strength examined independently by the adjustment of the other parameters to standard values. Rainfall of 2 mm or more on the day of observation reduces the pollen count. Wind direction and pollen catch are correlated with the areas of grassland in the 'home counties'.It is shown that airborne grass pollen concentrations are positively correlated with day maximum temperature and hours of bright sunshine when the pattern of pollen release is taken into account. Strong wind and convection decrease pollen concentrations since the pollen cloud is distributed through a deeper layer of air.

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