Spring and tree pollens peak at different times as the year progresses. But they are both spreading northward across the U.S. as May begins. Tree pollen is about to get worse in cities like New York, Chicago and Seattle, while Omaha, Denver and D.C. should be nearing peak tree pollen right now. As temperatures warm, cities including Atlanta, Dallas, Raleigh and Tucson are seeing the worst grass pollen now. Weed pollen will be coming soon, too. Here’s how weather can affect pollen.