Most of us have seen some kind of fogged window. A fogged window is not exactly the same as fog in the air. Fog can happen for a multitude of reasons, but before we know why fog happens, we need to know what it is.


What is Fog?

Fog is a mass of thick moisture or water droplets. Sometimes fog is airborne, especially in near bodies of water. Other times it appears on surfaces. If you’ve wondered why fog often appears on windows, it’s mainly due to the temperature difference between the two sides of the thin glass surface.


Cars and Fogging

Cold weather brings with it lots of frost and fog. The reason for this is the temperature difference between the inside and outside of your car. The heat from the inside of the car warms up the frost around the window and under the hood of the car. The moisture from the frost then sticks to the window as fog, trapped in a limbo of temperature changes. Likewise, the inside of the car window fogs when the moisture of the warm air comes into contact with the cold glass of the window.


Outside Conditions

There are times when fog is already present outside. In areas with large bodies of water, wind currents, as well as temperature changes throughout the day can create fog. Wind traveling and cooling as it moves up mountains results in perpetual fog for seasons, or even year-round. These conditions can’t be prevented or avoided by drivers and are persistent.


Excessive Moisture

Excessive moisture on the outside or inside of a car can lead to window fogging. While some causes like rain and snow can fall into the outside conditions category, their effect is two-fold, landing them a spot here. Moisture left by outside conditions can accumulate in your car, staying there for long periods until redistributed back into the air. A common place this happens is the air conditioning, which can lead to sudden fogging of windows. Sweat, heavy breathing, pets, damp shoes and clothing as well as food and beverages in a closed space can also lead to moisture collecting over time.