Methods of Measuring Relative Humidity and Dew Point
There are several methods of obtaining the relative humidity and low dew point in the atmosphere.
1. Relative Humidity
This instrument consists of a set of two identical thermometers except that one is a wet bulb (wrapped in wet muslin) and the other a dry bulb. Both are mounted on a wooden board that is as light as possible being just large enough to hold the thermometers.
A handle is attached to the board by a leather strap, as shown below, and this is used to whirl the apparatus around in the atmosphere.
The temperatures of both thermometers are recorded and a psychrometer chart consulted; the relative humidity read off where the recorded temperatures converge.
A typical chart is shown below; the dots indicate a wet and dry temperature, showing how the relative humidity is arrived at.
2. Find the Dew Point.
There are several methods to measure the dew point. Some of them use calculations or on-line calculators that require a bit of input; other methods are listed below:
- Chilled mirror hygrometer
- Graph of dewpoint versus air temperature
We will examine the dew point graph based on the findings of Magnus-Tetens approximation.
This graph consists of two horizontal temperature gridlines and two vertical dew point scales in ᴼF and ᴼC joined by gridlines.
Running diagonally through the gridlines are % humidity values and when the atmospheric temperature is plotted and intersects the %humidity value line, the dew point can be read in ᴼF or ᴼC. This sounds a bit complicated but refer to the graph below for clarification.