the colder it is outside the cooler the air is. if it is in the 40's the heater is ok, but when it;s in the 20's it just blows semi warm air.
i have a 1999 acura cl and it does not blow hot air like it should. on 1999 Acura CL
Most heater problems are from one of two causes, either the heater core is getting clogged or not getting water running through it properly. The other issue could be from a defective thermostat. one not opening or closing correctly can lead to under performance when heat is needed.