Dew Point

The ratio between the actual amount of water vapor in the air and the maximum the air could hold at the current temperature is called the relative humidity (R.H.). It is expressed as a percentage (%).

At 50% R.H., the air contains half the amount of water vapor it could potentially hold at that temperature. When the air cools, the relative humidity increases. In short: colder air can hold less moisture. So if the amount of water vapor stays the same but the temperature drops, the relative humidity rises.

If the air continues to cool and the R.H. reaches 100%, condensation starts. The temperature at which this first occurs is called the dew point (in °C). (This assumes constant air/vapor pressure, which has limited influence on the dew point.)

You can observe this in nature: in the early morning, dew appears on the grass. As the sun rises and the grass warms up... the dew disappears. Or more accurately, it transforms back into vapor.

But the dew point can also pose risks. If the dew point is around 0 °C, the dew can freeze, forming frost.

This can lead to hazardous road conditions. Authorities may issue warnings when the air temperature drops to the point where any moisture — dew, rain, mist — can freeze on the road surface.

So... don’t forget to keep an eye on the dew point — not just the weather forecast.