Tire Pressure (from the web.)

Time and outside temperature affect the pressure within your tires. It is normal for a tire to lose about 1 pound per square inch (psi) per month. Outside temperatures affect your tire pressure far more profoundly, however. A tire's pressure can change by 1 psi for every 10 degrees Fahrenheit of temperature change. As temperature goes, so goes pressure.

For example, if a tire is found to have 38 psi on an 80-degree mid-summer day, it could lose enough air to have an inflation pressure of 26 psi on a 20-degree day six months later. This represents a loss of 6 psi over six months and an additional loss of 6 psi due to the 60-degree temperature reduction. At 26 psi, your tire is severely under inflated and dangerous!

There is nothing wrong with your tire if it behaves like this, of course. What is being illustrated here is that you must check your tire pressures on a regular basis (about once a week is reasonable) and to be particularly aware of it on cold days. Remember the tires on your 4 wheel vehicles too.