![]() ![]() The way we actually calculate the dew point is to much for this answer so if you wish to know please ask it separately. In this case that point would be about half way between the red line (100% RH) and the green line (50% RH). You determine this by drawing a straight horizontal line from the dew point (red line) until you get to the temperature. So using the chart above, if the dew point is 10 degrees and the temperature is 15 degrees the relative humidity would be about 75%. We have to measure the dew point and from that we can determine the relative humidity. The reason I say that relative humidity is dependent on dew point is relative humidity is not measurable. The equilibrium relative humidity (ERH) of a food product is defined as relative humidity of the air surrounding the food that is in equilibrium with its environment. It is the temperature that the air would have to drop to in order to reach 100% relative humidity with a constant absolute humidity. Relative humidity of air is defined as the ratio of the vapor pressure of air to its saturation vapor pressure. So if the absolute humidity is 10 grams of water vapor per kilogram of air and the air temperature is 25 degrees, you have a relative humidity of 50% or in other words the air is holding half the amount of water vapor it can hold.ĭew point is a measurement with a constant absolute humidity. For this instrument, cold water was added to water in a vessel until dew formed on the vessel, and the temperature of the vessel, the dew point, provided a direct index of humidity. A dew-point hygrometer was invented in 1751. ![]() The relative humidity is the percentage of the absolute humidity of the air compared to the total amount of water vapor that the air can hold at that temperature. The clear meaning of dew point suggests a means of measuring humidity. Now the actual amount of water vapor in the air (grams of water/kilogram of air) is referred to as the absolute humidity (also as the mixing ratio). The chart above shows you that as temperature goes up the amount of water vapor that the air can hold goes up. The amount of water vapor that air can hold is dependent on the temperature of the air. ![]()
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