National Weather Service United States Department of Commerce

Week 3: What is Dewpoint?

The dewpoint is a measure of how much moisture the atmosphere contains.  More specifically, it is the temperature to which a parcel of air with constant atmospheric pressure and moisture content must be cooled in order for that parcel to become saturated.  To understand this concept, it is important to first realize that warm air has a greater capacity for holding water vapor than cool air.  The diagram below illustrates this by showing three samples of air with the exact same water vapor content (represented by the three drops), but different temperatures.  The warmest sample at 15C still has the ability to hold quite a bit more water, while the same sample at 5C has reached its maximum capactiy and is now completely saturated.  In this example, an air parcel with a temperature of 15C must be cooled to 5C in order for it to become saturated.  This is its dewpoint temperature.

When saturation occurs, water vapor (a gas) condenses into water droplets (liquid).  This often happens early in the morning when the surface temperature cools to its dewpoint, creating dew or frost on the grass.  This process also happens when a parcel of air is lifted, gradually cooling it to its dewpoint temperature and forming a cloud.



Since the dewpoint temperature describes the point at which saturation and condensation occur, it is obviously a very important parameter for meteorologists to keep track of.  The dewpoint can be measured with a device called a sling psychrometer, which consists of two to sample the ambient air temperature and the other to measure the wet-bulb temperature.  A small wet cloth is placed over the bulb of one thermometer, then the entire device is repeatedly twirled in the air until the moisture from the cloth has evaporated.  The corresponding reading that is achieved is known as the wet-bulb temperature and can be used in conjunction with the actual air temperature and the pressure to calculate dewpoint.  Below is a picture of a sling psychrometer.

Example of a sling psychrometer
Image courtesy NWS Riverton, WY


Dewpoints are highest during the spring and summer months when readings routinely climb into the 60s and 70s, resulting in very humid conditions.  In fact, summertime dewpoints are often higher here in central Illinois than along the Gulf Coast due to evapotranspiration from corn and soybean crops.  This process has been shown to add a staggering 3,000 to 4,000 gallons of water to the atmosphere per acre of corn per day during the peak growing season!  Occasionally the dewpoint can top the 80-degree mark, although this occurrence is fairly rare in Illinois.  The highest dewpoint temperature ever recorded at Chicago O'Hare was 83 degrees on July 30, 1999 in the midst of a prolonged heatwave.


Week 1: What is CAPE?


Week 2: What is Wind Shear?