In the springtime, warm, dry weather prompts trees and flowers to bloom. But with those blooms, comes the pollen.
On days when the breeze blows, it can make it even worse, as it'll carry the pollen over larger distances and help to keep the particles airborne. On the other hand, a rainy day will help you breathe better, as the rain tends to wash the pollen out of the sky. Higher humidity also means your nasal passages carry more moisture, helping to block out the pollen from getting beyond your nose.
Then again, the rain helps even more flowers to bloom once the sun comes out again, thus helping the pollen cycle continue.
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