Every year, it seems, weathermen claim this will be the worst allergy season ever. While that is highly unlikely, seasonal allergies, which affect some 30% of Americans, do vary widely from year to year. One expert, Estelle Levetin, a professor of biology at the University of Tulsa, explains how a long winter can be a boon for people with hay fever and why last summer’s weather is critical to this spring’s pollen.