SUMMARY: To measure the rate at which spore clouds were depleted over the sea, spores were collected with isokinetic suction impactors mounted in aircraft of the Meteorological Research Flight, Farnborough. Remote from sources able to replenish spore clouds, preferential deposition from the lower layers caused ‘erosion’ of the base of vertical profiles of spore concentration leaving maxima at heights between 500 and 1500 m. To determine vertical spore distributions throughout the largest possible distance downwind of the English coast, a saw-tooth flight plan of alternating ascent and descent was used. Of four flights, two encountered favourable weather, rain interfered with sampling on one and unexpected winds across the track converted another into an unintended but interesting cross-wind section of spore distribution. Pollens and Cladosporium spores were counted as examples of large and small spores liberated typically by day, and a composite group of spores liberated in damp air was chosen as an indicator of spores liberated mostly at night. In two flights in fine weather maximum spore concentrations occurred hundreds of miles off-shore. Diagrams showing height, distance from the coast and lines of equal spore concentration (‘isospores’) demonstrated discrete clouds of each marker spore type. Pollen and Cladosporium clouds were centred at approximately the same distances from the coast but with the pollen about 500 m. lower, probably because the pollen grains sedimented faster. Maximum concentrations of the damp-air group sometimes coincided and sometimes alternated with the day-liberated groups. Known periodicities of these spores over land, surface air trajectories and previous weather, suggested that the spore clouds which the aircraft overtook over the North Sea, were the residue of those produced from the British Isles on previous days or nights. Interpretation of the results was limited by meteorological uncertainties, the geographical complexity of probable source areas, and perhaps most by changes in the number of spores crossing the coast at different times of day, which prevented accurate measurement of rates of spore deposition. Spores of many species were recognized over the North Sea. The plant pathogens included established distant migrants such as uredospores of , which apparently originated east of the Baltic. The viability of the spores was not tested, but it seems safe to assume that distant transport is both frequent and extensive and probably important in temperate latitudes in summer.


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