Once gas contained inside a pressurized tube escape (inevitably it will escape, balloons shrink too) it cant re pressurize. Entropy dictates that the pressure inside a balloon or tire is trying to stabilize to atmospheric pressure. Only the tire or balloon wall is preventing that from happening. But they all slowly leak regardless which is why you have to re inflate anyway. Cold weather exacerbates this process. Cold temperatures drop pressure which reduces the effectiveness of a good seal on your wheel. Low pressure tires lose air faster than high pressure tires because the shape of the tire with pressure produces a better seal. Once the seal is compromised by temperature change it losses more pressure.