Nov. 19, 2017
So why have the pollen counts been so LOW for the spring of 2017 in Canberra? This is a question that I often hear this year and it's a good one.
So why have the pollen counts been so LOW for the spring of 2017 in Canberra? This is a question that I often hear this year and it's a good one. The first thing to point out is that the pollen counts that are reported by CanberraPollen are only related to Grass pollen, and this has been remarkably low (consistently <20 grains/m3 and often <10grains/m3) during the current spring monitoring season that began on 1st September 2017. In contrast, the levels of other pollen types that are know to cause allergic responses such as the Plane, Birch, Oak, Elm and Ash trees, all widely planted throughout Canberra’s urban landscape, have exhibited high pollen levels through the season. In order to understand the drivers of pollen abundance in our atmosphere we need to take a closer look at where the pollen is coming from and what’s been happening with the weather over the last couple of months.
Where does the grass pollen come from on HIGH pollen days?
The majority of grass pollen in the air of Canberra is derived from the extensive pastures that are found beyond our northern, western and eastern suburbs. An analysis of the grass pollen count versus wind direction across the last 3 years (Figure 1) shows that wind direction and speed play critical roles in determining HIGH pollen days in Canberra. Strong winds between the Westerly and Northerly wind sector bring high levels of grass pollen to Canberra during the Spring season, whereas, the southerly and easterly winds appear to bring less grass pollen, possibly due to the dominance of native forest covered mountain ranges in these directions.
Figure 1. Windrose analysis of daily grass pollen concentrations versus maximum wind gust speed and direction (using the r package “polarplot” with assistance from Dr Larissa Schneider).
Why have we not had any HIGH grass pollen days so far this year?
In order to answer this question we need to take a look at what’s been happening with the weather over the last couple of months and how this might be different from previous years. The winter months in Canberra where considered to be relatively dry by the Bureau of Meteorology, which appears to have reduced the productivity of grassland growth into the spring season. The October Vegetation Index confirms that in October 2017 the ACT and region are experiencing below average vegetation growth this spring (Figure 2). This can be compared to the same time for October 2014, when grass pollen counts reached some of the highest levels recorded, and there was above average growth.
Figure 2. Vegetation Index Anomaly for October 2014 and 2017 shows the above average and below average vegetation growth, respectively, around the northern and eastern agricultural pasture lands around the ACT.
Finally by comparing the wind conditions and rainfall over the 6 weeks of 1st October to the 11th November for the current and previous years we can see that some key climate conditions may be causing the LOW grass pollen counts for 2017. Figure 3 compares previous low grass pollen years (2007) and high grass pollen years (2014) with the current season (2017) and shows that key climate indicators are likely to be driving the low grass pollen numbers. These include:
Figure 3. Climate influences on grass pollen counts from 2017 (1st Oct – 11th Nov) compared to previous pollen monitoring years (2007-2010, 2014-2016). Climate data derived from BOM tables (1st October to 11th November for each year) and the SOI Index for Oct-Nov.
It appears that we really are experiencing a LOW grass pollen season this year and that the climatic conditions can explain these patterns. But what about those people who are still suffering from hay fever symptoms this spring. Well the explanation may lie with the type of pollen or other allergenic particles that you are allergic to. The high levels of tree pollen plus days of high allergenic spore (Alternaria) counts that have been recorded this season are likely to provide the explanation.