If you have allergies, especially seasonal allergies, you may have watched weather forecasts on TV or the Internet that predict how severe allergies will be in your area on a given day. But how do the forecasters know? Is there an actual relationship between the weather and allergies, and if so, what is it?

Forecasts of mold growth, which can increase indoor allergies, are about as reliable as any weather forecast. They are based on predictions and/or current measurements of rainfall and humidity, both of which encourage the growth of mold since it prefers a damp environment. If there is a long spell of rain (with associated high humidity), mold spores may increase dramatically along with a corresponding increase in visits to family doctors and allergists. Indoor allergies can occur at any time of the year, and may be chronic depending on the climate.

Seasonal allergies vary during the year because they are caused by pollen in the air. On The Weather Channel, seasonal allergy forecasts are based on actual pollen counts. According to the American Academy of Allergy, Asthma, and Immunology, volunteers, (e.g. National Allergy Bureau) or companies (e.g. Multidata) supply The Weather Channel and other news outlets with the latest figures for different types of pollen, namely tree, grass, and weed. This type of forecast is not related to the weather itself – or is it?

Since pollen comes from plants, which need sunlight, warm temperatures, and rain to grow, these three weather factors might influence future pollen counts. As reported on Medline, results from scientific studies in Spain, Australia, Taiwan, and the USA (among other countries) suggest that this theory is true. In one study, rainfall and temperatures in the months preceding tree flowering best predicted the levels of pollen during pollen season. Grass pollen was shown to be more prevalent in warmer years in Australia; this study also gave credence to the saying “a good rain will wash the pollen from the air” (however, rain will also help growing plants, resulting in more pollen later!) Other studies indicate that weather conditions during the previous year’s pollen season will affect the amount and timing for the next year, particularly with tree pollen.

Some allergy forecasters include these weather variables along with pollen counts, allowing them to forecast pollen levels in advance. Data concerning rainfall and temperature are programmed into computer models, resulting in general, seasonal predictions as well as more reliable 4-day forecasts.

At present, pollen prediction is still in its infancy. You’re better off handling your seasonal allergies on a day by day basis (at least, no more than 4 days). But remember – if your area has a warm and wet winter, you may want to stock up on tissues!