Most drivers assume winter is the season to worry about a cracked windshield. Cold nights, frost, and the defroster get all the attention. But every summer, Surrey and Vancouver auto glass shops see the same pattern: chips and cracks that sat stable all spring suddenly spread the first week of a heat wave.
If you’ve got a small crack and you’re waiting for a cooler day to deal with it — stop. Heat is not the safe season. In some ways, it’s worse.
This guide explains exactly why hot weather spreads windshield cracks, what specifically puts Lower Mainland drivers at risk during summer, and what to do if you’re stuck with a crack during a heat wave and can’t get in for a repair right away.
Why Heat Spreads Windshield Cracks — The Physics of It
A windshield crack is a break in glass that’s under tension. Glass expands when it heats up and contracts when it cools. That’s true of the whole windshield — but a crack means the glass isn’t a single uniform sheet anymore. It’s two (or more) pieces held together by the laminate layer in between.
When the sun heats one part of the windshield faster than another, or heats the surface faster than the air can cool it, the glass on either side of the crack expands at slightly different rates. That mismatch concentrates stress exactly at the tip of the crack — the weakest point in the glass — and the crack grows to relieve it.
This is the same underlying mechanism that makes cold weather dangerous (uneven temperature change across the glass), just running in the opposite direction. Hot weather isn’t a safer season for a damaged windshield. It’s a different set of triggers for the same problem.
Five Summer-Specific Risks for BC Drivers
1. Parking in Direct Sun
A car parked in direct sun in Surrey or Vancouver on a summer afternoon can see interior and dashboard-area temperatures climb well past 60°C, even when the outside air is a comfortable 25–28°C. The windshield itself absorbs a huge amount of that heat directly, since it’s facing the sun for hours. A crack sitting in a windshield that goes from a cool 15°C parking garage to over 50°C of direct glass-surface heat in an afternoon is under real stress — and it doesn’t need much more to finish spreading.
2. Blasting the AC on a Hot Windshield
This is the summer version of the “cranking the defroster” mistake that spreads cracks in winter. You get into a car that’s been baking in a parking lot, the windshield is hot to the touch, and you immediately max out the air conditioning aimed at the vents near the glass. The rapid cooling of the interior-facing side of the glass, while the outer surface is still hot from the sun, creates the same kind of thermal shock that a cold defroster blast causes in winter — just in reverse. If there’s a crack, that sudden differential is often the trigger.
3. Hot Pavement and Radiant Heat
Asphalt parking lots in Surrey can hit surface temperatures well above the surrounding air temperature on a sunny July or August day. A vehicle parked with a low front end close to hot pavement, or idling in stop-and-go traffic on Highway 1 or King George Boulevard during a heat wave, takes on radiant heat from below in addition to direct sun from above. It’s a double source of thermal stress that’s easy to overlook.
4. Wildfire Smoke Season
BC’s wildfire season overlaps directly with peak summer heat, and it brings a secondary risk that’s specific to this region: airborne ash and fine particulate. Ash settling on a hot windshield doesn’t cause cracks to spread on its own, but it does two things that matter if you already have damage — it can work into an existing chip or crack alongside moisture from morning dew, and it turns automatic wiper use (to clear haze and ash) into another source of repeated vibration and abrasion right at the damage site. If smoke is in the forecast and you’ve got a chip, that’s one more reason not to wait.
5. Long Summer Road Trips on Hot Highways
A road trip along the Coquihalla or Sea-to-Sky in peak summer heat combines several of the above at once: hours of direct sun exposure, hot pavement, highway vibration, and — if the AC is running hard on a hot windshield — thermal shock. A crack that seemed stable for a local commute can behave very differently over four or five hours of highway driving in July heat.
What To Do If You Have a Crack During a Heat Wave
If you can’t get in for a repair immediately and a hot stretch of weather is coming, a few adjustments reduce the risk meaningfully:
- Park in shade or a garage whenever you can. Reducing the direct heat load on the glass is the single biggest thing you control.
- Use a windshield sunshade. A reflective shade in the windshield when parked cuts down how hot the glass gets, which reduces the temperature swing when you start the car again.
- Let the car vent before blasting the AC. Crack a window or run the fan on low for the first minute or two before aiming max AC directly at the windshield. Let the glass cool gradually rather than shocking it.
- Avoid automatic high-pressure car washes. Same advice as cold weather — pressurized water can drive moisture or debris into a crack, and the temperature change from a cold wash on a hot windshield adds its own thermal stress.
- Don’t delay because “it’s not winter.” The repairable window for a chip or short crack still closes the same way in summer as it does in winter — it just gets there through a different set of triggers.
None of these steps repair the damage. They only reduce how much stress the glass is under while you arrange a professional repair.
Repair Timelines Don’t Change With the Season — Only the Risk Does
Whether a crack is spreading because of a cold snap in December or a heat wave in July, the underlying advice is the same: a chip smaller than a loonie with no long crack running from it is usually still repairable, and getting it fixed quickly under ICBC comprehensive coverage typically costs nothing. Once a crack has grown past a repairable length or reached the edge of the glass, full replacement becomes the only safe option.
Summer heat doesn’t give you more time to decide. It just changes what’s putting the crack at risk.
Key Takeaways
- Heat and cold cause the same underlying problem — uneven temperature change across the glass — just through different triggers.
- Direct sun exposure and hot AC blasts are the summer equivalent of winter’s defroster shock, and both concentrate stress at the site of existing damage.
- BC’s wildfire season adds a unique summer risk: ash and increased wiper use around existing chips and cracks.
- Long summer highway drives combine multiple stress factors at once — sun, hot pavement, vibration, and AC — making a stable-looking crack less predictable over a long trip.
- The repair window doesn’t reopen because it’s warm out. A repairable chip today may not be repairable next week, regardless of season.
FAQs
Can a windshield crack get worse in hot weather?
Yes. Heat causes glass to expand, and if a crack has already broken the windshield’s structural continuity, uneven heating — from direct sun, hot pavement, or a sudden AC blast — concentrates stress at the crack and can cause it to spread.
Is it bad to blast the AC on a cracked windshield?
If the windshield has been sitting in direct sun and is hot to the touch, aiming maximum air conditioning directly at it can create a rapid temperature difference between the inner and outer glass surfaces. On a windshield with an existing crack, this is a common trigger for the crack to spread. Letting the car vent for a minute first reduces the risk.
Does parking in the sun damage a windshield crack?
Extended direct sun exposure heats the glass significantly, especially in a closed vehicle. For an undamaged windshield this isn’t a problem, but for one with an existing chip or crack, the heat buildup — combined with cooling overnight or from AC — adds repeated thermal stress that can accelerate spreading.
Should I worry about my windshield during wildfire season?
Wildfire ash itself doesn’t directly cause cracks to spread, but it can settle into existing chip damage along with moisture, and it increases wiper use to clear haze from the glass — both of which add stress at the site of existing damage. If you have a chip or crack heading into wildfire season, it’s worth getting it repaired before the smoke arrives.
Is a windshield crack more likely to spread in summer or winter?
Neither season is inherently safer. Winter’s main triggers are cold-to-warm thermal shock (defroster, freeze-thaw moisture) and rain. Summer’s main triggers are sun exposure, hot-to-cold AC shock, and radiant heat from pavement. A crack is at risk year-round — the smart move is repairing it promptly regardless of the season.