Dear Bree,

I don’t really get this “polar vortex” thing, but here’s what I can tell you—it’s been ridiculously cold!  Unfortunately, I have to park my car on the street, and let me just tell you, I had some problems on a few below-zero mornings recently!  What the heck is happening?  First, my car door was frozen shut.  Then, I got in the car (It started! A miracle!) and it said I had low tire pressure! At this point, it seemed like I should take a sick day and go back to bed, so I did, but later I took the car in to have the tires checked and there was nothing wrong with them that a little extra air couldn’t fix.  The guys at the shop said the tires were low because of the cold.  Can you explain this, please?

Out in the Cold

Dear Out in the Cold,

Sounds like your tires were suffering from what I like to call the “Mylar Conundrum.”  Let me explain—once upon a time I had to go pick up some balloons for my son’s birthday party, and since his birthday is in November, it was pretty cold out.  I went into the store, grabbed my balloons and high-tailed it back to my car.  As I was trying to find my keys to get in the car, I noticed that my balloons weren’t floating quite as a high.  In fact, they were looking a little deflated!  I was annoyed, to say the least, to have gotten a bunch of dud balloons, so I marched right back into the store where they explained to me that because the air outside was so cold, the balloons appeared to be deflating.  They also said that when the temperature warmed back up (when I took them home), they would expand again like normal.  I was dubious, but they were right.

The pressure inside those balloons is all about the force exerted on the walls of the balloon by the helium molecules.  As the temperature rises, those molecules move faster and create more kinetic energy, thus “filling up” the balloon.  But as the temperature falls, the molecules slow down and lose energy, which is why the balloons appeared deflated.  This is called Gay-Lussac’s Law.  Another piece of the puzzle: Because helium molecules are so small, the can pass through the “pores” of the balloon.  This is called effusion.

These same principles apply to tires–when the temperature drops, tire pressure drops too. It takes a larger temperature change for tires to lose pressure (about 1 psi/10 degrees F) than my helium balloons, but it happens, especially with the kind of temperatures we’ve seen the last few weeks.  Effusion can happen with tires as well, but because air and nitrogen molecules are much larger than helium molecules, this process occurs much more slowly.  It’s these two little scientific facts that made your TPMS kick into motion.  Likely, you lost tire pressure through one or both of these principles.

All that was needed was a little extra air and you were good to go.  Your TPMS kicks in to let you know when your tire pressure drops because tires with low pressure aren’t safe on the road, so be happy—you’re TPMS system works!

RNR offers nitrogen fill for tires which helps keep tire pressure more stable.  Nitrogen molecules are less likely than oxygen molecules to migrate through the “pores” in your tires.  There’s also an issue with water vapor in tires that can cause pressure changes.  Nitrogen-filled tires are filled with dry nitrogen—no water vapor—unlike the air we breathe every day or regularly fill our tires with.  For more info on nitrogen fill, check out this past blog, or just come into any RNR location and we’ll tell you all about it.

Hope the polar vortex goes home and stays there!

Bree