Airlock sucking air in?

So earlier today I commented in this thread about some strange yeast I've been using.
https://www.homebrewtalk.com/showthread.php?t=48206&page=2

This evening I noticed something odd in my S-airlock -- it appears to be sucking air in. I haven't seen it bubble inwards, but the "out" chamber (for lack of better description) is very low, almost to the bottom turn, and the "in" chamber is very high, as if the air pressure inside the primary is lower than the ambient air pressure (which, according to weather.com, is going down). Anyone ever seen this before?

BTW, the yeast appear to be multiplying and settling out. There wasn't much of a krausen, but with a light Kolsch like beer, I didn't expect a very big one. I'm not panicking, but this is kinda weird.

Perhaps your wort was warmer than the ambient air when you attached the airlock. As the wort cooled the air above it cooled too and contracted. Just a guess....

Fingers said:

Perhaps your wort was warmer than the ambient air when you attached the airlock. As the wort cooled the air above it cooled too and contracted. Just a guess....

Click to expand...

That seems likely. I'll move it to another part of the house. The weather in Indiana hasn't been good to us lately. Thanks!

If the beer isn't activly fermenting, i.e. producing positve presure, then atmospheric pressure changes can cause that to happen as well.