Definition - What does Humidity mean?
Humidity is the amount of water vapor present in the atmosphere or a confined space. A higher level of humidity indicates a greater level of water vapor. A high humidity level results in a higher dew point temperature. This means that condensation will occur even at high temperatures if the humidity level is high enough.
Humidity can have a profound effect on corrosion in many ways. For example, it can make coating a material with a waterborne coating difficult. A humid environment prevents a waterborne coating from drying rapidly. Blushing (water spotting) can occur on epoxy coatings as well.
Another way that humidity relates to corrosion is that it increases the likelihood of condensation. When condensation forms on an unprotected material the liquid water can serve as an electrolytic solution that enables the corrosive process.
Corrosionpedia explains Humidity
A common way to measure humidity is through relative humidity. Relative humidity is measured by taking the current amount of water vapor in the air and describing it as a percentage of the maximum amount of water vapor the air could hold at a given temperature. One hundred percent relative humidity indicates complete water vapor saturation of the air.
An instrument known as a psychrometer is used to measure the humidity level.