How is relative humidity related to the dew point?
1 Answer
Relative humidity is dependent on dew point.
Explanation:
The amount of water vapor that air can hold is dependent on the temperature of the air.
The chart above shows you that as temperature goes up the amount of water vapor that the air can hold goes up. 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 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.
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.
Dew point is a measurement with a constant absolute humidity. It is the temperature that the air would have to drop to in order to reach 100% relative humidity with a constant absolute humidity.
The reason I say that relative humidity is dependent on dew point is relative humidity is not measurable. We have to measure the dew point and from that we can determine the relative humidity.
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%. You determine this by drawing a straight horizontal line from the dew point (red line) until you get to the temperature. In this case that point would be about half way between the red line (100% RH) and the green line (50% RH).
The way we actually calculate the dew point is to much for this answer so if you wish to know please ask it separately.
