To understand further why you should address high levels of humidity in the crawl space, let’s first look at what humidity is and how a rise in humidity happens. Humidity is what the National Weather Service says we “feel” outside and is formally referred to as absolute humidity. Absolute humidity is the measure (expressed in grams) of the amount of water vapor or moisture present in the air. When more water is present in the air, the absolute humidity is higher. Absolute humidity does not change with the temperature.
Cold air cannot hold as much water as warm air can. So, the higher the temperature, the higher the holding capacity is. When the temperature falls, with no change in the amount of water in the air, the relative humidity rises. As temperature rises, with no change in the amount of water in the air, relative humidity falls.
This article, in conjunction with the average meteorologist, refers to relative humidity—the most common measure of moisture content in the air (noted as a percentage). Relative humidity (RH) is the proportion of water vapor in the air relative to the temperature. National Geographic Encyclopedia explains that a relative humidity of 70% means the air is at 70% of its water-holding capacity. The moisture-holding capacity of the air depends on its temperature.