WA Pollen Counts versus Rainfall
The spring pollen season appears to have started rather late this year, at least in western Washington. From a personal perspective, I usually start exhibiting symptoms in about the middle of March, apparently due to an allergy to birch and alder pollen, but this year it happened noticeably later. Through comparing notes with friends and co-workers, I expect many others had similar experiences. Recall that March was abnormally wet across most of the state. Could the abundant rainfall have served to decrease or postpone the occurrence of high pollen concentrations?
The relationship between pollen counts and the weather, specifically precipitation, is examined here for the period of early March through late April 2012. The source of the pollen data is www.pollen.com; overall counts are available for various locations and the values posted for Seattle are considered here. The posted data does not include information on the relative contributions of the different species making up the total pollen count, and the counts are in unknown units. In general, the website lacks documentation of how the data is collected and processed. That constitutes a red flag in terms of using the data for any serious science, but we will forge ahead anyway. Time series of these counts at 2-day intervals, and of 2-day rainfall totals at SeaTac airport (SEA), are plotted in Figure 1 for the period of 9 March through 30 April 2012. These time series reveal that at least in terms of the day-to-day fluctuations, that there is a marked negative correspondence between pollen counts and recent (previous 2 days) rainfall. Consistently higher pollen counts began occurring towards the end of March into the first half of April when it rained much less (with the exception of a few days at the very end of March). A similar pair of time series was examined for Spokane (not shown). The pollen counts in Spokane exhibited smaller short-term fluctuations than in Seattle. Nevertheless, the wet period of 4-6 April for Spokane was also accompanied by a dip in pollen counts. The pollen time series suggest that indeed the allergy season was probably postponed this year, but that can only really be established through a comparison with past years. Such a comparison is also necessary to indicate the effects of temperature; presumably the anomalous cold during March 2012 in Washington state served to delay flower development for various species and ultimately the development of high pollen concentrations. An attempt was made to find historical pollen data, without success. Perhaps some readers of this newsletter happen to know of sources of that kind of data, and if so, the OWSC would appreciate finding out about them.
The conventional wisdom is that pollen counts are lower when it is wet because the rain “cleans” the air, i.e., scavenges minute particles including grains of pollen. In many situations, however, this may be a secondary rather than primary mechanism controlling pollen concentrations. The weather may dictate pollen concentrations more through its impacts on pollen release and dispersal. The release of pollen tends to be enhanced during periods of low humidity and high temperature, and dispersal increases with wind speeds. Yet another reason to celebrate the rainy weather we often get around here, especially during the last couple of springs.