It’s a freezing morning. Your car is covered in frost, you’re running late, and your breath is visible inside the cabin. Your instinct is to crank the heater and the defroster to “High” immediately.
Stop right there.
While you might be warming up your hands, you could be putting your windshield under immense structural stress. This phenomenon is called thermal shock, and it is one of the leading causes of windshield cracks during the winter months.
What is Thermal Shock? Glass expands when it is hot and contracts when it is cold. When your car has been sitting in freezing temperatures overnight, the glass is contracted and rigid.
If you blast hot air directly onto that cold glass, the inner layer of the windshield expands rapidly while the outer layer remains freezing cold and contracted. This difference in temperature creates tension between the layers of the glass.
- The result: If the tension exceeds the strength of the glass, it cracks.
The “Existing Chip” Danger Factor Thermal shock is risky enough on a pristine windshield, but it is devastating if you have an existing rock chip.
Note: A small chip compromises the structural integrity of the glass. When thermal shock occurs, that tiny, manageable chip acts as a weak point that can instantly “run” into a 12-inch crack across your line of sight.
How to Warm Up Your Car Safely You don’t have to freeze to save your windshield. Just follow these rules:
- Start Low and Slow: Turn your heater on a low setting first. Allow the air to warm up gradually as the engine warms up.
- Don’t Direct Air at the Glass Immediately: Use the floor vents initially to warm the ambient air in the cabin before switching to the defroster vents.
- Clear Snow from the Outside: Remove snow and ice from the outside of the glass before heating the inside. This reduces the temperature difference between the two sides.
Did Winter Win This Round? If the cold weather turned a small chip into a massive crack, don’t panic. You need to replace it quickly to ensure your car remains safe and legal to drive.
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