Defogger

A defogger or defroster is a device for clearing condensation from a window, and is also useful for melting frost, ice, and snow that has collected on the window surface. They commonly exist in automobiles, in two basic varieties.

Forced air

Vents can be used to blow warm air over the window surface. This is typically used on the front windshield or windscreen, which is right above the dashboard. Issued around the year 1965. The dashboard has special vents that redirect air upwards from the top and to the sides, across the glass instead of directly to the driver and other occupants. Many if not most cars can direct air in both directions if desired.

Heating mode is typically used in winter, not only to melt anything on the outside, but also to defog on the inside. In this case, the warmth of the air lowers the relative humidity by increasing the moisture capacity of the interior air, causing evaporation.

Cooling can be used in the summer, when the humidity is very high, such as during or after rain. In this case, refrigeration causes condensation on the refrigerant pipe coils, actually removing water from the air. This also lowers the relative humidity and promotes drying by evaporation. Some cars force this mode when set to defrost even if the driver has not selected the cooling button.

Electrical

Another type, commonly used on a vehicle's rear window, consists of an array of heating elements attached in very narrow (about one millimetre) strips to the window interior (known simply as a "heated rear window" in the UK). These strips connect at both ends to wider vertical bars at the sides of the window, to which electrical contacts are applied to connect it to the supply wires. The electrically conductive lines are composed of a silver-ceramic material which when fired on glass becomes bonded to the glass and is highly resistant to abrasion. There is no danger of electric shock, because of the low voltage, and no danger of burns as the elements are only heated slightly.

The thin layer of rust-colored conductive material is easily damaged by scratches, but can be repaired easily with a conductive paint.

Some Ford cars also have very thin wires embedded in the front windscreen glass to give rapid demisting before the engine is warm. Ford calls this system Quickclear in Europe and Instaclear in North America, while General Motors branded their own version as Electriclear.

Usage

In cold weather, it is often helpful to activate the defroster before using an ice scraper or the windshield wipers to clear the car. It is also important for safety to clear the windows, and the side and rear-view mirrors for proper visibility.

Rear-window defoggers may operate on a timer, turning off after a predetermined amount of time, usually 10 to 15 minutes. They may also be linked to a warning lamp on the instrument panel.