Condensation can affect how well you can look through the windscreen. It can be a nuisance that you might want to prevent or remove. Humidity is very relative and understanding this will help to handle, remove or prevent it.
What is condensation
Condensation is a side-effect of large difference in relative humidity. This occurs when the air isn’t capable anymore to hold the water vapour it absorbed.
For example: air with a temperature of 10 degrees Celsius can contain a certain amount of water vapour. If it contains 100% if what it can carry, the air is saturated and the relative humidity is 100%. Just for this example, we say that air with a temperature of 10 degrees Celsius can hold a maximum of 10 gram water vapour per cubic meter.
When air gets warmer, it can absorb more moisture. So hot air can contain more water vapour then cold air. In this example we’ll say that air with a temperature of 15 degrees Celsius can obtain a maximum of 15 grams of water vapour per cubic meter.
The air inside the car’s interior is 15 degrees, and is saturated. It holds 15 grams of water vapour per cubic meter. This saturated air comes in contact with the window, which is actually colder than the air. The glass of the windscreen will cool down quicker and more than the air in the car.
The thin layer of air that comes into contact with the windscreen cools down, but that means it holds more water vapour than it can carry. The colder air can only hold 10 gram, which is 5 gram less than it holds at the moment. This excess of water vapour needs to go somewhere. The vapour solidifies and shows as condensation on the glass.
To prevent this, the glass would have to be the same temperature (or higher) then the air inside the car. If that was the case, the air wouldn’t need to get rid of the excess water vapour and no condensation will occur.
Relative humidity and condensation
Humidity is a relative term, this means that the humidity is different in certain circumstances. Humidity is expressed in percentage of saturation.
For example: let’s say that air of 10 degrees clesius can contain a maximum of 10 grams of water vapour per cubic meter. Then 50% humidity with 10 degrees Celsius would mean that the air contains 5 grams of water vapour per cubic meter. In essence: half of what it can hold with that temperature.
Air with a temperature of 10 degrees Celsius with 50% humidity will contain a very different amount of water vapour then air with a temperature of 20 degrees and also 50% humidity. If the temperature rises, but the amount of gram water vapour per cubic meter stays the same, the relative humidity will drop. Simply said: 10 degrees Celsius with 50% humidity holds the same amount of water vapour as 20 degrees Celsius with 30% humidity.
However, pressure also plays a big role in this, which is purposely left out to make the examples easier to understand. If the pressure is raised, the relative humidity will also rise. Air under a large amount of pressure can hold very little water vapour.