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What is Dew Point?

What is Dew Point?
By Kari Strenfel March 22, 2018 19592 Views No comments

What is Dew Point?

The dew point is a measure of moisture in the air, or more specifically, the temperature at which the air becomes fully saturated with water. The dew point formula relies on environmental data including the relative humidity and ambient air temperature.


The dew point temperature will always be lower than or equal to the air temperature. For example, let's say your weather station records an outdoor temperature of 50 degrees Fahrenheit (10 degrees Celsius) and a humidity reading of 80%. The temperature at which these two values intersect is the dew point. In this example, dew will begin to form at 44 degrees Fahrenheit (7 degrees Celsius).



Why Do I Care?

When the air temperature equals that of the dew point (a relative humidity of 100%), clouds and fog begin to form. If the air temperature continues to get cooler, the water vapor will condense and drop out of the atmosphere as precipitation or dew. During warmer seasons, the dew point temperature can be a good indicator of how humid the outside air feels, as well as how likely it is to rain or storm.

In this image from the National Weather Service, you can see the surface temperature 92 degrees Fahrenheit (33 degrees Celsius) is much warmer than the air where clouds form 60 degrees Fahrenheit (15 degrees Celsius). This is because air temperature typically decreases with height. You also see the dew point is always 60 degrees Fahrenheit (15 degrees Celsius), showing the amount of water the air can hold at that time. So the upper air temperature is cool enough to match the dew point temperature, allowing for condensation to occur and clouds to form. With this example you see the daytime temperature at the surface is 92 degrees Fahrenheit (33 degrees Celsius), well let's image the overnight low drops to 59 degrees Fahrenheit (15 degrees Celsius). Since the dew point temperature is still 60 degrees Fahrenheit (15 degrees Celsius), you would see dew form on the grass when you wake up in the morning. It would again dry out as the surface temperature increases during the day. When temperatures are below 32 degrees Fahrenheit (0 degrees Celsius), frost forms instead of dew so we call it the frost point.



You can view your Dew Point temperature calculated from your AcuRite outdoor weather sensors on all AcuRite displays and the My AcuRite dashboard. Some AcuRite displays calculate Dew Point seasonally, when temperatures are between 40 to 79 degrees Fahrenheit (4 to 26 degrees Celsius), and show Wind Chill in colder weather and Heat Index in warmer weather.

  • Dew Point Calculator by the National Weather Service – This online dew point calculator allows you to enter a temperature, relative humidity and pressure to calculate the dew point.
  • Dew Point Calculator for Collectible Items by Rochester Institute of Technology – This online dew point calculator helps to determine the preservation quality of environmental conditions as it relates to collectible items. The calculator indicates an environment that promotes natural aging, mechanical damage, mold risk or metal corrosion.