What is Dew Point? Measuring Temperature and Condensation

Regardless of the air's temperature it is able to hold water vapor. When the water vapor that is in the air reaches its maximum, saturation results. Saturation is also referred to as 100% relative humidity. Once 100% relative humidity is reached, its dew point temperature is reached.

What is Dew Point Temperature?

The temperature at which air temperature must become completely cooled in order for water vapor saturation to occur is known as dew point temperature. Once the air has reached its dew point temperature, water vapor saturation will occur. Once this occurs, condensation will start to form.

What is Condensation?

Dew Point

Condensation is the drops of water you see in the grass on chilly summer mornings. Condensation can also be seen on the outside of a cold glass on a warm day. It resembles small drop of water. Dew forms when the air temperature has reached 100% relative humidity. The lowest temperature of the day occurs just before sunrise so this is when dew is most likely to form because the dew point temperature will have been reached.

Differences in Temperature Affect Dew Point

When the air is warmer it can hold more water vapor. As the air temperature and humidity level rise, so does the dew point. On a warm day, at or above twenty degrees Celsius or seventy degrees Fahrenheit, the dew point can get quite high. Colder temperatures present a lower dew point. As the temperature decreases and the dry air sets in, a lower dew point occurs. Because of this, frost will occur once the dew point sinks below freezing. Dew point temperature will never be higher than the actual air temperature. If both the dew point temperature and the air temperature are equal, the air becomes saturated and is holding its maximum amount of water vapor.


Once air becomes saturated at its dew point, condensation results. This condensation can lead to the formation of fog or clouds. In some cases, the fog or clouds are near the ground. When this occurs, the air near the ground as reached condensation because it has reached its dew point.


National Weather Service. (2011). Dew Point Temperature. Retrieved on April 20, 2011 from the National Weather Service: https://www.weather.gov/forecasts/graphical/definitions/defineTd.html

University of Illinois. (2010). Observed Dew Point Temperature. Retrieved on April 20, 2011 from the University of Illinois: https://ww2010.atmos.uiuc.edu/%28Gh%29/guides/maps/sfcobs/dwp.rxml

Image Credits

Water Drops: sxc.hu – johnnyberg