Where do you store your racket?
All weather conditions can affect your racket, or more so your strings.
Rackets can withstand extreme heat, but very high temperatures can cause strings to stretch and lose tension.
This has hit home to me more since I moved to Doha, where the temperature is currently averaging around 40'c.
Most racket bags now come equipped with foil linings and this offers a certain amount of protection, but generally when the string stretches it never really returns to its original state.
The opposite is the case in cold weather, the strings can shrink which makes the tension seem tighter. The other worry in cold weather is that cold strings are more brittle and therefore can break quicker.
The best advice is don't store your racket in the boot of your car for long periods. If you do have to take your rackets to work with you because of an early evening match, then bring them into your place of work with you (if possible).
Having them in the boot for travelling to and from the court won't make any difference, it is only long periods that will affect the strings.