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Pollen forecast with COSMO

During the pollen season, MeteoSwiss publishes text forecasts and pollen count maps that are updated daily. The maps show the concentrations of hazel, alder, birch, grasses and ragweed pollen according to the season (approx. January to end of September). This enables the public to obtain information on pollen levels for every location in Switzerland.


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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?

According to the season, the forecast maps depict the pollen concentrations of hazel, alder, birch, grasses and ragweed pollen. They show the average pollen concentrations [pollen per m3] over an area of 1.2 km2. The daily maximum can be significantly higher, particularly near blooming plants. The pollen concentration is largely determined by the abundance of flowering plants, the weather and the conditions for pollen transport. The colours of the pollen categories reflect their relevance to allergy sufferers.

The resolution of COSMO on the horizontal plane is 1.1 km, which means that nationwide pollen forecasts as well as very localised 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 Lauterbrunnental (Fr: Val d’Hérens, It: Val Verzasca), 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 wider alpine valleys nearby can provide reference points. It should be borne in mind, however, that the higher the height above sea level, the later the plants flower.

The COSMO pollen module calculates the pollen concentrations of hazel, alder, birche, grasses and ragweed. With these five species, the most important species are covered throughout the entire pollen season, as they bloom one after another over the course of the year. Depending on the progress of the blooming seasons in a particular year, there may be brief overlaps.

MeteoSwiss is now expanding this system and investing in development work concerning the calculation of the pollen count, for example to improve the quality of the pollen counts for the existing species or to introduce additional pollen species that are important for allergy sufferers. This is also why hazel pollen has been calculated since 2023.

How COSMO pollen forecasting 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 from pollen calendars with COSMO comprises an entire process chain: Based on plant distribution and with the help of details on the weather over recent months, the model 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.
Since 2023, the official pollen measurement network of MeteoSwiss consists of automatic measuring devices that can measure pollen in real time. This makes it possible to correct the COSMO pollen forecast at regular intervals with the real-time data and thus improve it.

Quality of the pollen forecasts by COSMO

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 count. 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.