Auto A/C Terms - Glossary
Humidity

Humidity can seriously effect the pressure gauge readings on your auto A/C system. The reason that humidity effects the pressures is that technically, high humidity alone can add up to five times the actual heat load to the system.

Humidity is the moisture that the air holds. When the level of humidity increases, it can make you feel very uncomfortable. When it comes to your auto A/C system, that humidity adds heat load, typically causing high side pressure gauge readings to rise.

As a general rule, the high pressure gauge reading of an auto A/C system will be between 2 and 2 and one half times the ambient air temperature in degrees Fahrenheit (degrees F). That means on an average summer day with ambient air temperatures of about 80 degrees F, your high side reading would be between 160 psi and 200 psi.

On days with high humidity, the pressures on the high side of your A/C system can rise considerably. In fact, according to industry standard pressure temperature charts that factor in humidity, on a day with ambient air temperatures of 80 degrees F and a relative humidity of 80 per cent, it would not by considered abnormal to have high pressures up to 300 psi or possibly higher. That is because humidity adds heat load to an A/C system and because refrigerant has a pressure temperature relationship, higher heat is expressed in higher pressures.