What happenes to tire pressure in hot weather?

pressure increases with temperature, per the universal gas law

This is true, but a better way of putting it is that temperature is the measure of the average kinetic energy of a substance (in this case particles of a gas) and so as temperature increases, the kinetic energy increases because the two things are directly proportional. As the energy increases, there are more collisions between the gas particles and the inside of the tire, making the pressure increase, which is also why you should check tire pressure before a trip, not during it.