Winter Driving, Visibility and Defogging the Windscreen/ Windshield

The most basic guideline in instances of poor visibility through the windshield is “If you cannot see you should not be driving”. We often share reports of poor visibility from smoke, mist and fog - expecting drivers to heed the calls for safety in these weather and road conditions. In most cases, there is little a driver can do but wait and delay his travel.

But what about those conditions where we are able to do something? A good example is where the windshield/windscreen is fogged from the inside! In this section, we would like to take a closer look at defogging a windscreen!

It is especially in winter [and very humid days] that we are in a rush, get into our car and then find our windshield all fogged up from the inside. This is both annoying and a hazard to our visibility when driving!

Why does this happen and what are we to do?

Fog on the inside of the Windscreen / Why and How does it Happen?

Windshields fog up because of differences in temperature and moisture in the air [humidity]. Window fog is caused by moist, humid air inside the vehicle coming in contact with the cold windshield before it’s warmed up. Your windshield will be colder than the air inside the car as it is in contact with cooler outside air.

There are several factors contributing to this humid air:

• Driver or passenger breathing/inhaling and exhaling
• A steaming mug of coffee
• The rain and snow on your clothes

The humid/moist air comes into contact with the cold surface of the windshield glass, releases moisture and condenses into what we know as fog. The key to whether we can expect to find fog is the temperature at which the maximum amount of water vapour can be held in the air at a given pressure before condensing into a liquid. This is known as the dew point.

It is also important to consider that the air trapped in the car from the previous day will be more humid than the outside humidity level, therefore holding more moisture.

How to Defog the Windshield / Windscreen - Do’s and Don’ts

What not to do:

• Do not drive distracted, constantly fiddling with knobs on the dashboard.
• Avoid wiping the window with hands unless it’s an emergency - it will leave streaks all over the windshield.
• Rubbing the windshield with your sleeve to remove some of the condensed water vapour will not do much - more vapour will condense from your next breath and fog it right up again.
• Do not drive with the fan set at "recirculate".  This is simply re-using air inside the car - the relative humidity of the air in the car will keep rising

What to do:

Defogging the windshield is all about regulating the temperature and humidity inside the vehicle correctly. The bigger the difference between the air and window temperatures, the lower the relative humidity that will still fog the windows up.

Before the drive - Some basic maintenance

Ensure that the intake vents at the base of your windshield, on the outside, are clean of ice and snow. These vents pull fresh outside air into the vehicle.

Vehicle owners are familiar with cleaning the outside of the windscreen, yet mostly neglect the inside. To fight against continual fog and condensation it is important to clean the inside of the windscreen as well.

Dirt particles give the condensation more to hold onto; hence why a dirty window fogs up more than a clean window.

Test your windows for cleanliness by simply dragging a finger over the front windshield; if you leave behind finger tracks then your windows have become magnets for fog and dirt. Find a glass chemical cleaner and wipe in circles until all signs of dirt and streaks are gone.

Using the window

A fast but possibly uncomfortable way to defog the windscreen while the engine is warming up is to lower the window and letting cold, dry air from outside flow into your vehicle. Lower your windows about two centimetres to pull in the dry air. This will lower the dew point inside the car and cause the fog to dissipate.

Blasting the heat

Your windshield will be much colder than the air inside the vehicle, especially with cold rain or snow hitting it while driving. You need to direct the warm air from the vents against the windows to compensate for this.

This will raise the temperature of the windshield -but will only work if the vehicle is sufficiently warmed up. Use the defrost/defog vent and crank it to high. Heating the air will dry the windscreen a little through evaporation, but may also add more moisture in the air.

Using the Air-conditioning

Most drivers are aware of the benefits of air-conditioning as a way to cool things down but do not know that their air-conditioning is also capable of producing hot, dry air. Air-conditioners cool your car on a hot day by removing water vapour from the air inside the car, which reduces the temperature of the air. Operating your air-conditioner will immediately start removing water vapour from your car on a cold morning too. The warm, moist breath will disperse into the air inside the car rather than condensing on the windscreen.

Your air-conditioning is also a dehumidifier. So even while using warm air to defrost the windshield, make sure the air-conditioning is turned on so your car will dehumidify the air before it warms it. Cool air from the air conditioner will dry the air, removing the moisture. The cool dry air will flow over the windscreen sucking further moisture off the windshield.

“The best way to operate the air conditioner is to start it on cold and then as the air in the car dries out, you can slowly increase the temperature. You'll know when you've gone too far if the glass starts to fog up again," advises Dr Caecilia Ewenz, a lecturer in meteorology and environmental science.

Conclusion

Always pay close attention to vehicle maintenance and keeping the vehicle and windscreen in the best possible condition. Your windscreen/ windshield is important not only for maintaining the structural integrity of the vehicle but also to provide the best visibility for the driver,  enabling the driver to better observe road and traffic conditions.

For more information also view:

Safe driving in winter / Driving on Snow and Ice

Safe driving in bad weather conditions

Road Safety and your Windscreen

Windscreen Wiper Blades and Safe Driving