Fog is essentially a dense cloud of water droplets, which is the exact same as a cloud, except it occurs close to the ground. When night conditions are cold, the ground releases the heat it absorbed from the sun during the daytime. This clash of hot and cold close the ground results in the condensation of water, such as the dew you find covering grass and plants in the early morning. Water droplets are only 0.01 mm in diameter. As such, they are not sufficient to soak anything which happens through a fog. However, the high concentration of water vapor becomes partially visible, appearing as a soft obscuring white. This presented a danger when traveling on foot or horse many years ago, now it proves a different problem.
Fog can form in thin layers on the outside of your car's windows, drastically reducing visibility and increasing the danger of collisions. Such fog is formed as a result of temperature differences between the air within the car and the air without. Air conditioners serve to cool the air within the car as much as 20 degrees F below the temperature outside the car. Glass is always a little cool to begin with, as it doesn't transfer heat well. The air conditioning chills the glass of your windows even further. Warmer outside air comes into contact with the exterior of the cold windows and causes the water vapor naturally in the air to condense on it, forming a thin layer of fog which is very difficult to see through.
There are many ways to prevent or reverse window fogging. If your windows should fog up while driving, you can roll them down in an attempt to let the inside temperature equalize with the outside temperature. You could also just turn off the air conditioner. Another method is to turn on your windshield wipers and the window cleaner spray. The difference in surface tensions between the cleaner and the fog will cause it to dissolve. The wipers will then flick the liquid off the windshield of the car. As a preventative measure, wipe the exterior of your windows with a half-and-half solution of vinegar and water before driving. The vinegar helps to keep the moisture in the air from condensing on the windows.
