PV=nRT

As you know, I recently got new tires for my bike.  What was bugging me was that even though my tires felt well inflated inside, a few blocks from home I got the familiar "slogging though sand" feel of flat tires.

So, let's talk science.

First off, here is the Ideal Gas Law defined.

Now for simple comparisons, the constants can be ignored, so you are left with PV/T; meaning, the ratio of  the product of the pressure and the volume to the temperature is constant.

So let's look at my bicycle tires.  The volume (the space inside the inner tube) is constant.  So when I take it outside and the temperature of the air inside drops, then the pressure has to drop as well.

Then I started to think about the differences during the year.  So in the winter, the tires are experiencing a drop in temperature of around 40 degrees (yes, I know that's Fahrenheit, so maybe 20 degrees Celsius).  In the summer to get the same effect, I'd have to be riding in 110 degree heat or more.  Fortunately it never got that hot in Walnut Creek this year.  At least not while I was riding.