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Introduction to Dew Point and Relative Humidity
Relative humidity is a percentage of the moisture contained in the atmosphere, relative to the temperature.
The dew point is an absolute value and although dependent on temperature, it reflects the amount of moisture in the air more realistically.
We shall examine relative humidity and low dewpoint in the following section, and explain the difference between humidity and dew point.
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Meaning of Relative Humidity & Low Dew Point
The difference between dew point and humidity are defined below;
Relative humidity (RH) is the amount of moisture the atmosphere contains at a particular temperature against the maximum amount of moisture it could hold at the same temperature, and it is expressed as a percentage.
Dew point is the exact point where dew forms from the moisture in the air at a stable barometric pressure and is expressed in ᴼF or ᴼC. A low dew point indicates a dry atmosphere.
The main thing to remember when quoting relative humidity and low dew point is that RH does not define the amount of moisture in the air; it only indicates how full the air is with moisture.
The dew point is a much better indication of the moisture content as it is given in ᴼF or ᴼC. This means if we record a dew point of 65ᴼF, we know it is going to be quite humid or “sticky", whereas a low dew point of 30ᴼF means a dry atmosphere.
Low dew points are normally found in hot arid places like desserts, or polar regions; high dew points are found in equatorial tropical regions.
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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
- Sling psychrometer.
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.
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We have written about relative humidity and low dewpoint, examining the difference between humidity and dew point. Here we also looked into the various methods used to measure their values comprising of manual instruments and graphs. The values can also be achieved through calculations.
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1.Relative Humidity and Dew Point: http://www.ace.mmu.ac.uk/eae/weather/older/Dew_Point.html
2. Dew point online calculator: http://www.ringbell.co.uk/info/humid.htm