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Why is dew point so important?
First, what is it?
Dew point is a measure of the absolute amount of water in the air. As air is circulated into and around a building, its absolute moisture content—and therefore its dew point—does not change unless it is humidified or dehumidified. In other words, unless the mechanical systems add or remove water from the air, the outdoor dew point and the indoor dew point are the same. Dew point is the temperature at which water will begin to condense out of the air expressed in degrees F or C. When the dew point is higher, the actual weight of water per unit volume of air is higher.
Why it’s important
To manage the storage environment effectively, you need to develop an understanding of what temperature, relative humidity, and dew point are, and how they relate to each other. This is the key to good analysis and decision-making about your storage environments. In many cases, the preservation quality of a storage environment depends more on achieving low summertime dew points than on any other single factor.
Some key facts:
- At constant temperature, RH and dew point increase and decrease together.
- At constant RH, temperature and dew point increase and decrease together.
- At constant dew point, temperature and RH go in opposite directions.
- The higher the temperature, the lower the RH.
- The lower the temperature, the higher the RH.
As the air cools, the RH increases until it reaches 100%; this is where condensation occurs. Cooling air below the dew point is the normal means by which mechanical systems achieve dehumidification in the summer. As long as condensation doesn’t occur and no water is added to the air, temperature and RH will vary, but dew point will stay the same.
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