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Relative Humidity
Colloquially when we talk about ‘humidity’ we probably mean Relative Humidity (RH). This is a comparison of how much water vapour is in the air compared to how much could theoretically be held by the air. |
So, for example, at 100% RH, the air is saturated and no more vapour can be held by the air, at that temperature. I added ‘at that temperature’ as humidity and temperature are inexorably linked. As temperature rises, the air can hold more vapour and conversely, as it falls, it can hold less, as seen in this graph. |
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Thus, on a cold morning when we breath out, we see our breath because that air we’ve exhaled is warm (perhaps 30℃) and full of water (perhaps 90% RH). As our breath cools, it can no longer hold as much moisture and tiny water droplets are seen as mist or steam. |
Dew Point
Dew Point tells us at what temperature condensation would start to form. But it is actually a measure of the quantity of water vapor in the air, but it is independent of temperature. In a domestic environment it will tell us the temperature that ones windows will have moisture, and will tend to be within ‘normal’ temperature ranges from -30 ℃ to +30℃ (always less than the ambient). In industry there may be a very wide range of gases that are dryer or wetter and might range from -100 ℃ to +100 ℃. Some years ago, when selling a Dew point transmitter to a customer, I could make no sense of his compressed air having a -60 ℃ range! At least I know better now! |
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