When moist air meets a surface having a temperature lower than its dewpoint, oversaturated humidity in the thin layer of air touching the surface turns into liquid water (dew or condensate). This rather simple phenomenon is often overlooked and misunderstood. A typical misunderstanding is that condensation happens only when the greenhouse air is saturated or nearly saturated (i.e., 100% relative humidity). This is not true, and condensation can happen inside a
greenhouse whose air humidity is not yet saturated. In fact, condensation does not happen even when the air humidity is 100% if the surfaces are all higher temperature than the air (i.e., dewpoint).
Warmer air can hold more moisture. Saturation level of humidity doubles every 10 degrees Celsius (18 degrees F). shown as lower saturation levels in Figure 1. When air cools down, the capacity to hold moisture is lowered. When air temperature decreases further to the level where the air can no longer hold the humidity, excessive humidity becomes small droplets of liquid water or dew (or fog when floating in the air). This process, turning humidity (vapor) into liquid water, is called condensation, and the temperature at which condensation occurs is called dewpoint. Air at the dewpoint temperature has 100% relative humidity when the greenhouse has a localized low-temperature area (e.g., near a cold surface). Dew forms on such a cold surface due to condensation.
How to find dewpoint temperatures. There are a number of web-based resources available for dewpoint calculators based on air temperature and relative humidity. A psychrometric chart is a useful tool if you wish to better understand the properties of moist air when heating and cooling are considered. Digital psychrometric charts are also available, and the author has used one available from Let’sGrow.
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