Just like we feel ourselves becoming a little lighter when the weather is nice, so does pollen. When the air is dry, pollen particles have no trouble moving with the breeze. However, The Weather Channel reports that when it begins to rain, the air is so dense with water vapor and raindrops that pollen is quite literally rinsed from the air as it becomes bogged down with moisture. For a short time, at least, this can cause pollen counts to drop and improve symptoms of allergic rhinitis in people with pollen allergies. On the other hand, because rain contains nitrates, which act as food for flowering plants, a good spring shower can often generate more pollen in the future.
What's more, rain and high levels of humidity can also be a catalyst for mold growth. For that reason, people with mold allergies may notice their allergies become much worse a few days after heavy rain once mold has had time to grow. After all, nearly 20% of Americans have a mold allergy, according to the Cleveland Clinic.