The catches of tree pollen at Lerwick in 1970–72 are related to Lamb's catalogue of daily weather types. This is done by examination of weather maps on particular days, and statistically by finding the effectiveness of certain weather types for pollen transport, and by examining the source areas of trajectories. It is found that in April-July, of all days when the surface track is from likely source areas, 50–80% bring measurable amounts of tree pollen to Shetland, depending on the weather type.
The results are applied to statistics of the frequencies of the different weather types. It is estimated that the number of days per year of airborne tree pollen at Lerwick is 15 ±3, and that there has been no secular trend in this figure over the past hundred years.