The summer months are coming and that means the return of warm temperatures and humid conditions. During the warm season, one meteorological term that you will often hear is “dewpoint temperature” especially on days when humid conditions are expected. However, this term is often misunderstood and many do not know its importance to forecasting.

The dewpoint temperature (also known as saturation temperature) is the temperature at which air must cool to for condensation (formation of water droplets) to occur.

Given it’s name, the dewpoint temperature is often only thought of in regards to the formation of dew on summer mornings. The dewpoint temperature can be utilized for many other meteorological variables.

For example, the dewpoint temperature can be used to forecast the formation of clouds. Clouds form when warm air rises and eventually cools with increasing height. When the temperature of the air reaches the saturation temperature, then a cloud will form. Below is an example of this process courtesy of the National Weather Service. Notice that the cloud only forms when the temperature and dewpoint temperature are the same at 4,000 feet above the ground.

Example of rising air cooling with height and a cloud forming when the air temperature = dewpoint temperature. Image courtesy of the National Weather Service.

The dewpoint temperature also has an effect on the height of clouds. If the dewpoint temperature is high, then clouds are likely to be lower and vice versa.

Blame dewpoint temperature for humid conditions

The difference between dewpoint temperature and relative humidity is one of the most misunderstood topics among the general public. When conditions are humid, a common phrase you might hear is: “It feels so humid today! The relative humidity is really high!”

While the relative humidity can be high during humid conditions, this is certainly not always the case. The dewpoint temperature is a much better measure of how “humid” the air feels compared to relative humidity.

Here are a couple of examples:

Example #1:

Temperature: 32°F

Dewpoint temperature: 32°F

Relative Humidity: 100%

Example #2:

Temperature: 90°F

Dewpoint temperature: 70°F

Relative Humidity: 52%

When the temperature and dewpoint temperature are the same, the relative humidity is 100%. Example 1 features a day where the air temperature and dewpoint temperature are at 32°F. This results in a relative humidity of 100%. Does it feel humid to you when it is snowing outside? Absolutely not.

Example 2 is a day with a 90°F air temperature and a 65°F dewpoint temperature. This results in a relative humidity around 52%, but the air will feel much more humid in example 2 compared to example 1. In fact, the heat index in the second example would be around 96°F!

Now, the relative humidity is not completely useless when it comes to assessing humid conditions. If hot temperature are expected with high relative humidity values, then you can infer that the dewpoint temperature will be high. The heat index (Feels Like Temperature) increases with warmer temperatures and higher relative humidity values.

Heat index chart from the National Weather Service.

Thus, this summer, when warm and humid conditions are in the forecast make sure you bring plenty of water and stay cool!