Pollen forecast with COSMO

During the pollen season, MeteoSwiss publishes text forecasts and pollen maps that are updated daily. The maps show the concentrations of alder, birch, grasses and ragweed pollen according to the season (approx. February to end of September). In this way, members of the public can find out the pollen load for anywhere in Switzerland.

Pollen forecasts are an important tool for those suffering from allergies, allowing them to minimise the symptoms of their allergy, plan outdoor activities and take their medication in a more targeted manner. People suffering from allergies can also better coordinate their holidays and holiday destination with the blooming period for the allergenic pollen types.

Pollen forecast with COSMO-ART – how should the maps be interpreted?

The forecast maps depict the pollen concentrations of alder, birch, grasses and ragweed pollen. They show the average pollen concentrations [pollen concentration per cubic metre] over 24 hours and an area of 1.2 square kilometres. The maximum pollen concentration for the day can be significantly higher, particularly near blooming plants. On the other hand, the concentrations in the early hours of the morning tend to be lower. This is not a generally applicable rule, however, as the pollen concentration is largely determined by the weather and the conditions for pollen transport. In general, the map provides information about the emission and/or transport conditions on a particular day. The colours of the pollen categories reflect their relevance to allergy sufferers.

Since 2017, the horizontal resolution of COSMO is 1.1 km, which means that nationwide pollen forecasts and, for the first time, also very local pollen forecasts can be made for Switzerland. For example, alpine valleys where the valley floor is wider than 1 km can be realistically mapped. For narrow valleys such as the valley of Lauterbrunnen, however, the resolution of 1.1 km is still too coarse for a local pollen forecast. Nevertheless, the map is still helpful for narrow valleys as nearby, wider alpine valleys can provide reference points. It should be borne in mind, however, that plants bloom later as the height above sea level increases.

How pollen forecast with COSMO-ART works

The numerical weather forecast model used by MeteoSwiss, COSMO, can model the pollen count with the ART pollen module. The pollen module was developed by the Karlsruhe Institute of Technology (KIT) and MeteoSwiss. The modelling of pollen with the COSMO-ART pollen module comprises an entire process chain: Based on plant distribution and with the help of details on the weather over recent months, the module calculates the blooming start date for each grid point using an accumulated temperature model. This is followed by the calculation of the pollen emissions, which is greatly dependent on the current weather: High temperatures, dry conditions and a little wind generally favour pollen release. All the pollen that is released is then carried away by the wind and either falls to the ground somewhere, or is washed away by the rain.

Quality of the pollen forecasts

Pollen forecasts can only be as good as the weather models. Even state-of-the-art weather models such as COSMO do not, for example, always forecast precipitation absolutely correctly. Precipitation, however, is very important for the pollen load. This is where the expertise of the pollen forecasters comes into play. Their assessment of the plants’ current readiness to bloom and of the weather model is integrated into the published forecast, which allows certain deviations from the model-based map representation.

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