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How does the weather affect hay fever?

The weather plays a significant role in the production, distribution and dispersion of pollen.  Rain, wind, temperature and sunshine all affect how much pollen will be produced and how much it will be dispersed.

Any rainfall causes a marked decrease in pollen concentration in the air, but the time and amount that it rains during the day is very important. Early, heavy and prolonged rain is likely to keep counts low all day whereas rain in the afternoon will have less of an impact.

Temperature plays an important part in the release of pollen. For grass, a maximum temperature between 18 - 28 °C could give a high count if it’s a dry day with low humidity and a gentle breeze. Trees respond best when the temperature range is between 13 - 15 °C. However, if the temperature rises above 28 °C then all pollen levels decrease. If several warm days occur in a row, then the supply of pollen can run out altogether.

When it comes to wind then it all gets a bit Goldilocks. Too light and the pollen won’t be dispersed, but too strong and it gets blown further afield and thins out. Different types of pollen will need different wind speeds for ideal dispersion but Birch pollen, for example, needs moderate to high winds.

The amount of daylight, or the “photoperiod”, is also crucial to pollen production because of photosynthesis. If there is a particularly cloudy spell of weather then plants and trees will produce less pollen because they are getting less light.

As well as the weather at the time of pollen dispersal, there are seasonal weather patterns that change things too. We know that on a daily basis any rainfall decreases pollen, but over a period of months, intermittent wet days tend to produce a more severe season overall. On the other hand, hot, dry summers mean that grass stops producing pollen, which gives low counts.

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