I worry a little about humidity since my grain is already dry and do not want to dry it any more, so have controller set to only run when humidity is above 80%.  When the temp is below 30 there is very little moisture in the air and is very difficult to add moisture.  

There is some worry when running fan when there is precipitation falling when temp is below freezing as it might be possible to cover the screen with ice.  I have seen ice build up on the screen from freezing fog.