Here is a quiz for you. How many times a year have you gotten to your car on a cold morning only to notice that your car tires look deflated? Even better, many modern cars have fancy digital displays that tell you exactly how much pressure has been lost. The temperature in parts of Georgia dropped into the forties overnight this week. It is not surprising that when I headed out to drop my son off at basketball tryouts that my tire pressure lights were illuminated. While a mild annoyance, this is a very common thing, and something that I suspect that you have experienced as well. Many people are curious about why this happens. Here’s an explanation that tries to make complex physics and meteorology accessible to the layperson.
14 October 2019, Bavaria, Nuremberg: A fitter in a tyre centre mounts a winter tyre on a car. Photo: ... [+]
In order to really address this question, I have to introduce a few scientific concepts so stay with me. The Ideal Gas Law or the Equation of State is written as PV = nRT. The termsP, V, and T represent pressure, volume and temperature respectively. The term n is a representation of how many molecules of gas are present in the volume (for example, the tire). This law, according to Lumen Learning’s website, was “originally deduced from experimental measurements of Charles’ law (that volume occupied by a gas is proportional to temperature at a fixed pressure) and from Boyle’s law (that for a fixed temperature, the product PV is a constant).”
The University of California - Santa Barbra Science Line website says, “Now, while the air in our tires do not behave as "ideal gasses," they do still exhibit a proportionality between pressure in temperature.” This would suggest that as the temperature (T) decreases so does the pressure.University of Georgia physics professor Craig Wiegert added to the discussion. Wiegert, an associate professor and associate department head said,
At low temperatures, the tire might visually "look" a bit flatter, but this is primarily due to lower pressure. The force that the tire exerts on the road needs to match the force that the road exerts on the tire in order to support the car's weight. The area of contact between tire and road will increase as the pressure decreases, because force = pressure * contact area, and thus the bottom of the tire flattens a bit more. Incidentally, I wonder if there's a misconception that tires "lose air" at low temperatures, i.e., that somehow the cold causes the air to leave. We say that phrase colloquially, but really the mass of the air isn't decreasing.
Dr. Fred Bortz is a physicist and author of science books for young readers. He and I actually co-authored a weather book for kids called Dr. Fred’s Weather Watch. Bortz says the properties of the rubber may also depend on the pressure and temperature.
One lesson from this little explainer is that you probably shouldn’t check your automobile tire pressure after driving the car. As you drive, the frictional effects of the road will cause heating. This can create an “over-inflation” bias. Proper tire inflation, however, is very important. According to TireSafety.com:
When you maintain proper tire pressure, you ensure that you’ll get more life of your tires. On the other hand, improper tire inflation may result in rapid or irregular wear, which can cause significant internal tire damage, and can lead to sudden tire failure and result in serious injury.
Tire pressure should be checked monthly, and many modern cars make that very easy with automatic pressure sensors. TireSafety.com goes on to say that for every 10 degrees F in temperature change, tire pressure changes by roughly 1 PSI (Pounds Per Square Inch).
You probably would be surprised at how these basic science principles come into play in meteorology. These are some of the first lessons that I teach in an introductory weather and climate class. In order to understand fronts or why temperature decreases with altitude, the Ideal Gas Law is relevant. Hmmmm....Maybe I will write an entire series of articles on the role of the Ideal Gas Law in weather....Stay tuned.
By the way, does anybody have 4 quarters for the air machine at the gas station (smile)?
The equation of state
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