If your allergies flare up on some days and not others, it could be related to mold spores in the air. The weather outside determines which spores are airborne at any moment. 

The two main groups run on opposite schedules

The first is the dry-weather group. This group waits for warm, dry and breezy conditions. Their spores are built to endure harsher conditions and are able to be lifted up into the air with the lightest of breezes. 

This group peaks in the afternoon, when humidity is often at its lowest and the wind can be strongest. This group's numbers drop during peak humidity, which usually occurs right before sunrise.

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The next group is the exact opposite. This damp-weather group loves moisture and can’t launch into the air without it. A droplet of water condenses at the base of the spore and grows until it merges with a film of moisture on the spore’s surface. The sudden shift in balance flings the spore into the air.

Because it depends on humidity, these counts peak before dawn, the exact opposite of the dry group.

What can rain do?

Rain cuts both ways. The first drops striking a leaf vibrate the surface, launching spores into the air. This also can occur with the damp-weather group. 

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But should rain continue, it can also collect spores out of the air, which can help reduce mold spores that cause irritation.

It isn’t necessarily about abundance

Just because there is a high concentration of certain mold spores in the air, it doesn’t mean that you are reacting to the most common spores. The most common mold in outdoor air can outnumber a rarer one.

Yet allergists set the risk threshold for that rarer mold about 10 times lower. That means certain spores may be less abundant, but more people may be allergic to them.