Can you prevent a windscreen from frosting over? What about using vinegar or a de-icing spray? Photo: Maren Winter
For those living in a cold climate without undercover parking, a frozen windscreen may be a familiar - and frustrating - sight.
So, how can you fix it and avoid both damage and delays when heading to work or school?
How to defrost a car windscreen (and what to avoid)
Nathan Kilah is a senior lecturer in chemistry at the University of Tasmania in lutruwita. His "preferred method" for windscreen defrosting "is using regular tap water".
"All you need to do is get the temperature of the windscreen above freezing and that will be enough to melt the ice."
That could be achieved by using a water bottle to pour water over the windscreen or making use of a nearby hose.
Kilah warns against using hot or boiling water.
Rapid temperature changes can crack the windscreen's laminated glass, which is "two layers of glass separated by a layer of plastic".
He also suggests what he calls the "credit card method", which involves using a plastic card to scratch away the layer of frost. You can also buy a purpose-built scraper.
"[It] works pretty well" if you don't have a hose or tap nearby to use regular tap water, Dr Kilah says.
Can you prevent a windscreen from frosting over?
As far as prevention, Kilah says he has "seen people use things like a towel" when they can't park their car under cover.
"When you have a frost, the temperature of the air drops sufficiently - and the temperature of the surface - that ice crystals start to form, so below zero degrees [Celsius]," he says.
Putting a blanket or towel over the windscreen will see ice crystals form there, "but you'll just be able to peel it off".
Do you have a tried-and-tested way of dealing with an icy windscreen? Share your tips, email iwitness@rnz.co.nz
What about using vinegar or a de-icing spray?
Kilah says some people "take advantage of a property called freezing point depression" to tackle a frosty windscreen.
It works by adding a chemical to "lower the melting point of the ice, so it melts away".
"There are ones for windscreens that involve various alcohols, things like propylene glycol."
Along with de-icer sprays, a solution of vinegar and water is a method of windscreen defrosting that is another example of this.
However, Kilah is not a fan.
"I'm not in favour of adding a range of different chemicals".
He says they can be corrosive, and you may end up causing problems elsewhere.
How to defog a windscreen quickly
Fog on the windscreen inside your car can also be a bit of a pain if you're in a rush.
Kilah says it happens when moisture in the air meets the cold surface of the window, that's below the "dew point".
"That's cold enough to condense the vapour from the air on that surface."
To defog a windscreen from the inside, Kilah says you can either heat up the surface to evaporate the water or cool down the inside of the car, so it equilibrates.
Some people opt for the latter and blast the air conditioner, but his "preferred method" is to "turn on the heater and warm up the surface so that fog won't form".
What if your car doesn't have an air conditioner or heater?
He says cars without an aircon or a heater "might benefit from an anti-fog wipe or spray".
"These products leave a thin film of material on the glass that somewhat prevents the formation of water droplets. But the products can require frequent reapplication or need precleaning for them to work well."
Kilah says another option is "opening the windows to lower the temperature of the air and to ventilate some of the humidity."
"But it won't be very comfortable," he warns.
- ABC