People have allergies only when the pollen to which they are allergic is in the air. Each plant pollinates more or less the same time from year to year. Exactly when a plant starts to pollinate depends on geographical location, not the weather. In mild areas, trees pollinate in the spring, grass in the middle of the warm season, and weeds in the fall leading up to the first frost. In warm places, pollination can occur any time of year.
The National Allergy Bureau (NAB) records pollen and mold levels from certified stations throughout the United States. Track your geographic area by going to NAB at pollen.aaaai.org.