Why do I break out in bad hives when it rains or when its humid?

Partial explanation. Although inhalation of mold spores can rarely cause hives in people who are allergic to them , mold counts are usually elevated after the rain especially when it is followed by hot, windy, and dry days. When the weather is humid, certain molds (ascospores) may release spores but when it is raining all of the spores are washed down. Mold allergy is more likely if you have hay fever or asthma.