Your tyres support the weight of your vehicle, right? Well they don't! It's the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tyres are to provide all of the handling, traction and durability of which they are capable.
However, you can't set tyre pressure...and then forget about it! Tyre pressure has to be checked periodically to assure that changes in ambient temperatures or a small tread puncture have not caused it to change.
The tyre pressure is normally recommended in your vehicle's owner's manual or tyre information sticker normally located on the door frame or inside the fuel flap cover. Once located this would show the vehicle's recommended cold tyre inflation pressure. This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it.
Since air is a gas, it expands when heated and contracts when cooled. In most parts of the UK this makes the winter months the most critical times to check inflation pressures...days are getting shorter...ambient temperatures are getting colder...and your tyres' inflation pressure is going down!
The rule of thumb is for every 10° Fahrenheit change in temperature, your tyre's pressure will change by about 1 psi (up with higher temperatures and down with lower).
When we do get a summer in the UK difference between average summer and winter temperatures is about -40° Fahrenheit...which results in a potential loss of about 5 psi as winter's temperatures set in. And a 5 psi loss is enough to sacrifice handling, traction, and durability!
Additionally, the difference between cold night time temperatures and hot daytime temperatures in most parts of the country is about 15° Fahrenheit. This means that after setting tyre pressures first thing in the morning, the vehicle's tyre pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle's tyre pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.
And finally, if the vehicle is parked in the sun, the sun's heat will artificially and temporarily increase tyre pressures.
It's important to remember that your vehicle's recommended tyre pressure is its cold tyre inflation pressure. It should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun's radiant heat affects it.