because there is something called the "adiabatic process" that describes the relationship between air pressure and temperature.

ever notice how your pump gets hot after you pump up your tires (tyres) ?

In daily practice what this means is, consider the air temperature when setting your tire pressure. If you start off on a ride and it's a summer day with temps likely to reach 85 or 90 (lets just say) when you pump up your tires at the ride start allow for some increase in pressure as the temps rise thru the day.

yet another good reason for not running the max tire pressure shown on the tire sidewall.

Riders using rim brakes may also see a pressure increase when doing braking on long downhills..

/markp