It always happens on a winter day when you're already running late to work. You race out to the car, hop inside and notice that your windshield is covered in ice. While you're scrambling to scrape off the ice, you may wonder why it forms. The answer is more complicated than you may think.
You will usually find ice on windshields on very cold mornings, even if there is no rain or snow in sight. This ice has actually formed from water vapor. When the temperature on the ground dips below freezing levels, water vapor in the air becomes "supercooled." After going through this phase, it turns into frost, and then possibly into ice. Ice forms on windshields even before it forms on plants or the ground because windshields are made of glass, which cools more quickly than many other materials.
Most people who live in areas that have colder climates are familiar with waking up to ice on the outside of their windshields, but some people may find that even when that ice is cleared off, there is ice on the inside of their windshields. This is caused by moisture inside of the car, either from a very small heater leak that produces moisture or from wet floor mats or other surfaces inside of the car. This moisture evaporates, turns to water vapor, becomes supercooled and then forms ice on the inside of the car windows. One way to try to avoid this is by making sure to knock any snow off your shoes before entering your car and to keep any liquids that are in your car in a sealed container.
When your outside thermometer reads 38 or even 40 degrees, you probably don't expect to see ice on your windshield. Interestingly enough, however, it can form even when the temperature does not seem to be below freezing. This happens because the windshield is often much cooler than the air around it. In addition, if your windshield has small pieces of dirt on it, they can attract water droplets and cause them to cool more than they would otherwise. In addition, you might see the ice appear on your windshield only after you turn on the windshield wipers to get the dew off. The dew droplets may have already supercooled but not yet frozen, and when the wipers spread them across the windshield, they turn to ice.