In areas where the temperature drops close to freezing or lower during the night, the windshield of a car is very often covered with a layer of ice in the morning. Knowing if the windshield has ice allows a notification to be sent to the driver advising them to get to the car a little bit earlier to manually de-ice the windshield or, in a more automated car, the windshield to be automatically de-iced in the morning before the driver gets to the car to eliminate the wait. Since the formation of ice is dependent on very local conditions (see details below), a local sensor is necessary.

The formation of ice can be estimated well with an RH/T sensor. This can be either an existing sensor in an anti-fogging module or a dedicated sensor.

Two conditions need to be fulfilled in order for ice to form on the windshield (or on any other body). First, the surface temperature needs to be below freezing. Second, the relative humidity of the surrounding air needs to be close to 100% at the temperature of the surface. Both conditions can be measured with an RH/T sensor.

In the case of the windshield, the temperature is often not the same as the temperature of the surrounding air. Obviously, during a sunny day, the windshield will be warmer due to the absorbed heat radiation. See figure 1:

The opposite situation can also occur. When there is very little incoming radiation, as is usually the case during a cloudless night, the temperature of the windshield falls below the temperature of the surrounding air. This can lead to surface temperatures below freezing although the ambient temperature is well above this level.

Since a car has always some natural ventilation (leakage and overpressure vent) and the rate of change in absolute humidity is rather slow in the atmosphere, a sensor on the inside of the windshield is feasible for this application. Either a dedicated sensor or the RH/T sensor of the anti-fogging module.

Here is a sample measurement from a car parked outside. The formation of ice can be seen in the time-lapse video. Various sensors were placed in the car, on the car and around the car to monitor the conditions.

Ice starts to form the moment the surface temperature drops close to the dew point of the ambient air so that deposition can occur.

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