Above average rainfall in early November quells an angry Godzilla hay fever season

Predictions of a Godzilla hay fever season in October of this year appeared to be well on track to come true. That is until a period of unusually high rainfall for early November subdued the angry beast. Daily pollen counts for grass in Canberra over the October-November period showed that the early season pollen levels where well above average, however, just after the beginning of November grass pollen levels began to dip below the long term average. So much so that people in Canberra have encountered a total of only 1500 grass pollen grains/m3 across the season this year compared to 2200 grass pollen grains/m3 in 2014. Why did this happen and what can we learn from these observations?

The Bureau of Meteorology reported that the long term average rainfall for Canberra in November (64mm) had fallen entirely within the first two weeks of November. The occurrence of rainfall during the day can have a significant impact on the concentration of hay fever causing pollen by effectively “washing out” these small particles floating in the atmosphere.

The occurrence of high rainfall during the first two weeks of November also coincided with the peak time for grass pollen production in the ACT. This resulted in a significant reduction in the total grass pollen atmospheric load for the season. However, this did not eliminate all allergenic particles as there has been a significant rise in the fungal spore Alternaria during these wetter periods. One of the key lessons from this year is that predicting pollen levels is a tricky thing and will remain highly problematic until we can accumulate multiple years of real data to analyse. 

Top graph shows the cumulative pollen load for Canberra illustrated as the minimum, average and maximum for years since recording began in 2007 (Oct-Dec recording period). The pollen load for 2015 up until the end of November is depicted in red. The bottom graph shows the long term average and standard deviation for an entire year based on historic records of grass pollen in Canberra. High rainfall period (total = 64mm) recorded in first 2 weeks of November 2015.

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