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Many factors affect tire pressure including ambient temperature changes and tire damage. Tire pressure increases or drops apx 1 psi for every 10 degrees of ambient temperature change. Tires can lose up to 1.5 psi each month just from the air that naturally escapes the tire and rim.
See the articles, "How Important is Tire Pressure?" and "What is the Proper Pressure for My Tires?" for more information.