If you store your bike indoors during the winter it is important to be sure your psi is at a certain value inside so that when you take the bike outdoors into the cold the tire pressure is where you intend it to be for your ride. A FatTire at 8 psi indoors will become 4 psi at -20.
Use this handy pressure calculator to know what psi to pump your tire to indoors for it to be at the value you wish for your riding once outdoors.
Temperature = Celcius
Pressure = PSI