Tires lose air pressure when it is cold especially in the winter because the cold air compresses and shrinks, when the tire warms up after driving for awhile or when the air temperature raises the air pressure increases with the temps.
as temps fall air molecules inside your tire compress,and therefore loose pressure.Have you ever tried to dribble a basketball that has been left out in the cold,same thing applies here.try this blow up a balloon inside your home,then take it out into the cold,and watch how the balloon reacts to the cold.You should check you tire pressure at least once a week to assure propper inflation.Under/over inflation can result in poor gas mileage,sub par handling,and premature tire wear.This can be a major saftey issue!!!!,and cost you lots of $$$,if not your life!!!
My 98 Cad with factory alum wheels slowly loses pressure on two wheels in very cold weather. Im told there are hairline cracks in the wheel hubs that open slightly when cold. Stranger things have happened.