Heat pump working at 28 degrees ambient

I know my Carrier will make heat down into the upper twenties-although the exact temp does vary I think it depends on the wind/humidity maybe-anyhow eventually it gives up and flips over to the furnace(usually it will cycle once or twice-and sound it-and then the furnace kicks on) I've tried to make it a point to shut it down when the temp is below 32 or 33 because I don't want to strain it.

I'm impressed that the unit is producing heat that warm.