Why windscreen chips spread in the heat and the storms
A chip spreads because the glass around it is already under stress, and sudden changes in temperature, vibration and storms add more stress until the chip runs into a crack.
Glass under tension
A windscreen is two layers of glass with a plastic layer between them, bonded into the car. A chip is a weak point in the outer layer. Anything that makes the glass expand, contract or flex pushes on that weak point, and the easiest way for the glass to release that stress is for the chip to grow a leg.
Heat and cold at the same time
On a hot day the outside of the glass bakes in the sun while the air conditioning cools the inside. The two sides want to move by different amounts, and the chip sits right where that difference concentrates. The same thing happens the other way round with a cold car wash on a hot windscreen, or a demister on a cold morning.
Summer in South East Queensland brings both: strong sun and long drives with the air conditioning on.
Storms, rain and water in the break
Storm season brings hail and sudden temperature drops when the rain hits hot glass. Water also gets into an open chip. Dirty water left in the break makes a clean repair harder, which is one more reason not to leave it.
Vibration and potholes
Every pothole and rough road flexes the body and the glass with it. A chip can sit still for weeks and then run on one bump.
What to do
Keep the car in the shade when you can, avoid blasting cold air straight on the glass on a hot day, and get the chip fixed while it is still a chip. You can get a fixed price in three taps.
Habits that help until it is fixed
Park in the shade or under cover when you can, especially in the middle of the day. When you get into a hot car, let the air conditioning come on gradually rather than pointing cold air straight at the windscreen. Avoid the car wash until the chip is fixed, because cold water on hot glass is exactly the kind of sudden change that makes a chip run.
Close the door gently, slow down for potholes and speed bumps, and keep some distance from trucks on gravel roads, so you do not collect a second chip while you wait.
Signs a chip is about to run
Look at the chip every few days. If a leg has grown since last time, if you can see a fine line starting from the edge of the bullseye, or if the chip has gone cloudy because water got in, it is time to fix it. A chip that has started to run is still often repairable if the line is short, but the chance is running out.
Why sooner is cheaper
A chip repair is a small fixed price. A crack across the windscreen usually means a new windscreen, an insurance excess or the full cost, and days of organising it. The difference between the two is often just a few weeks of waiting.
What actually happens inside the glass
A windscreen is made to hold together when it breaks: the plastic layer in the middle keeps the pieces in place. A stone breaks the outer layer only, leaving a small cone of broken glass with air in it. The legs of a star are fine cracks running out from that cone. Each time the glass is stressed, the end of a leg is the weakest place, and that is where it grows. Filling the break with resin takes the air out and bonds the broken faces together again, which is why a repaired chip is far less likely to run.
Questions
Can I stop a chip spreading with tape?
Clear tape over the chip keeps dirt and water out until it is repaired. It does not stop it spreading.
Is a chip more likely to spread in summer?
Big temperature differences across the glass add stress, and summer brings more of them: hot sun outside and air conditioning inside.
How fast can a chip turn into a crack?
There is no set time. Some sit for months, others run in a day after a hot drive or a pothole.