Why Does Heat Expand Windshield Cracks?

Glass expands when it is heated, so parking in direct sunlight will cause your windscreen to expand imperceptibly. While this is not good for existing chips or cracks – more on that later – it isn’t going to result in you returning to your car and finding a crack spread out over your windscreen.

As the glass cools, it naturally shrinks again, and this shouldn’t be a problem either, if the cooling process is gradual. However, our natural response to getting into a stiflingly hot car is to turn the air con on full-blast and set to the lowest temperature possible. The blast of arctic air feels great on your skin, but it can result in your windscreen cooling and contracting too quickly, increasing the likelihood of a crack forming.