When I lived in Denver, our data center would sometimes have HVAC trouble because cottonwood tree seeds would clog up the fins of the roof-top heat exchangers.

When I lived in Denver, our data center would sometimes have HVAC trouble because cottonwood tree seeds would clog up the fins of the roof-top heat exchangers. They look like white fluff and some years would cover lawns looking like a light dusting of snow. The worst year, the fins were covered (on one side) with a solid white felt that clogged the fins enough to cause the HVAC to blow circuit breakers and shut down our data center (which was mostly a DR site for other branches of the corporation).