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The Dew Point Temperature
13/03/05 Please Email Us if you have any comments or queries on this article. 
Firstly, here are a couple of definitions gleaned from the web:
The temperature at which water vapour begins to condense. http://www.campbellsci.com/glossary
The temperature to which air must be cooled at a constant pressure to become saturated.
http://www.weather.com/glossary/d.html
What is Dew and how does it get there?
When you get up in the morning and find the grass, for example, coated in water droplets, then this may very well be dew rather than rain droplets.
As well as oxygen, nitrogen etc., air also contains water vapour; water in the form of a gas. If the temperature gets low enough during the night, then some of this vapour will condense out, forming water droplets that we know as dew. This is similar to when water vapour in your bathroom condenses out on a cold window or wall to form droplets of water (and often enough encourages the growth of mould!)
The Dew Point Temperature
At lower temperatures, the air can hold less water vapour than at higher temperatures. So, as the temperature decreases, there will come a point where the air can no longer "hold" all the moisture in it - we say the air is saturated with water vapour. And this point is called the dew point temperature . As the temperature gets lower, the vapour simply starts to condense out as water droplets; i.e. clouds, dew , fog or frost etc.
The temperature at which water vapour begins to condense, depends on how much water vapour there is in the first place. If there is very little moisture in the air - low humidity - then the temperature will have to be very low before condensation occurs. Conversely, if there is a lot of moisture in the air - high humidity - then condensation will occur at a much higher temperature.
We can see now that the dew point gives some sort of measure of how much moisture is in the air - the higher the dew point temperature, the higher the amount of water vapour in the air.
When the air is saturated (i.e., cannot hold any more water vapour), then it must be at the dew point temperature, because the definition of dew point temperature is the temperature at which water vapour begins to condense. In other words, the actual temperature is equal to the dew point temperature when humidity is 100%.
Notice that the dew point can never be higher than the air temperature. This is because if the temperature decreases, some of the moisture in the air will condense out and the dew point will get lower as there is now less moisture in the air!
Note. A decrease in air pressure will also cause water vapour to condense out.
How would it work in practice?
Suppose the temperature is 40 ° F. Suppose the humidity is fairly low so that the dew point is, say 20 ° F. Then what this is telling us is that with this amount of water vapour in the air, the temperature would have to drop to 20 ° F before the water vapour in the air started to condense out.
You can see todays dewpoint temperatures (in Celsius) for the UK here. From this map you can see what the temperature has to fall to before water vapour starts to consense out of the air.
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