First a few facts
Those are the facts, now the explanation.
At an R.H. of 50%, for example, the air contains half as much water vapor as it could contain at that temperature. As the air cools, the R.H. rises. Summarizing: at lower temperatures the air can contain less water vapor. So if the quantity of water vapor remains the same and the temperature drops, the percentage of RH increases.
Suppose the air cools further and further. Then the R.H. become 100% at some point. When the air cools even further, water vapor condenses. The temperature at which the first condensation occurs is the Dew point (expressed in °C). (All this at the same air/vapor pressure, which has a small influence on the dew point temperature).
This can all be seen in nature. Early in the morning the dew on the grass ....
The sun rises, the temperature will rise, warms the grass ... and the dew disappears.
Or actually it changes shape. Back to water vapor.....
And now something completely different. At least in terms of dew point.
Dew point can also become a concern. Nature was responsable for the disappearing dew as a result of the grass warming sun. In nature it can also get colder and colder as we all know.
When the dew point reaches around 0°C, that is when the previously described dew can freeze and become hoarfrost. This can of course pose a danger on traffic. It is also a warning that the air temperature has reached such a low level that sleet can form with any type of rain. It can get slippery anyway.
...so remember to monitor the dewpoint, not just the weather forecast....
Here you will find two calculators you can use to test calculate Dew point or Relative Humidity
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