Monday, August 21, 2023

Episode 662 (8-21-23): At the Dew Point

Click to listen to episode (4:20).

Sections below are the following:

Transcript of Audio
Audio Notes and Acknowledgments
Extra Information
Related Water Radio Episodes
For Virginia Teachers (Relevant SOLs, etc.)

Unless otherwise noted, all Web addresses mentioned were functional as of 8-18-23.


From the Cumberland Gap to the Atlantic Ocean, this is Virginia Water Radio for the weeks of August 21 and August 28, 2023.

MUSIC – ~15 sec – instrumental.

That’s part of “The Foggy Dew,” an Irish song performed here by Timothy Seaman of Williamsburg, Virginia, with Ann Robinson on Celtic Harp.  In the song’s traditional lyrics, fog and dew set the scene for the Easter Uprising in 1916, during the Irish Revolution for independence from Great Britain.  This Water Radio episode’s focus isn’t on those important historical events, but rather on a number related to fog and dew, and to whether the weather on a summer day feels comfortable or close.  That number is the dew point temperature, or simply the dew point.

The National Weather Service gives the following descriptions of “general comfort levels” at various dew points:
“less than or equal to 55 degrees Fahrenheit: dry and comfortable;
“between 55 and 65: becoming ‘sticky’ with muggy evenings;
“[at 65 or more]: ...becoming oppressive.”

Note, however, that perceived comfort levels at different dew points depend on the climate conditons to which a person is acclimated.

Let’s explore some of the science of the dew point.

Any parcel of air can hold a given amount of water vapor, depending on the air parcel’s temperature; air with higher temperature can hold more water vapor.  The term relative humidity refers to how much water vapor an air parcel actually holds, compared to its potential maximum.  The dew point, then, is the temperature at which an air parcel reaches a relative humidity of 100 percent.  Cooling air below its dew point results in water vapor condensing into fog, dew, or some other kind of precipitation; if temperatures are below freezing, the dew point is then considered the frost point.

Now, here’s the key concept for how humid the air feels: a higher dew point indicates that an air parcel is holding more moisture at any given temperature or relative humidity.  With more moisture in the air, the human body has more trouble evaporating sweat, the process that removes heat and cools the body.  With sweat not evaporating as readily, it feels hotter and more humid; the term “heat index” refers to how hot people feel in combinations of temperature and humidity.

Humidity, sweat and evaporation, comfort or mugginess: there’s a lot to learn from the daily dew point.

Thanks to Timothy Seaman for permission to use this episode’s music, and we close with about 35 more seconds of “The Foggy Dew.”

MUSIC – ~34 sec – instrumental.


Virginia Water Radio is produced by the Virginia Water Resources Research Center, part of Virginia Tech’s College of Natural Resources and Environment.  For more Virginia water sounds, music, or information, visit us online at, or call the Water Center at (540) 231-5624.  Thanks to Stewart Scales for his banjo version of “Cripple Creek” to open and close this episode.  In Blacksburg, I’m Alan Raflo, thanking you for listening, and wishing you health, wisdom, and good water.


“The Foggy Dew,” a 2023 single release, is copyright by Timothy Seaman, used with permission.  It features Ann Robinson on Celtic Harp.  More information about Timothy Seaman is available online at  “The Foggy Dew” is a traditional Irish song, whose lyrics talk about the 1916 Easter Uprising, part of the Irish Revolution and War of Independence (1919-1921) against Great Britain.  The song describes scenes of battle on Easter morning taking place amidst “the foggy dew.”  Information about the song is available online at

Virginia Water Radio thanks David Carroll, of the Virginia Tech Department of Geography, and Kevin McGuire and Stephen Schoenholtz, of the Virginia Water Resources Research Center and Virginia Tech Department of Forest Resources and Conservation, for their help with this episode.

Click here if you’d like to hear the full version (1 min./11 sec.) of the “Cripple Creek” arrangement/performance by Stewart Scales that opens and closes this episode.  More information about Mr. Scales and the group New Standard, with which Mr. Scales plays, is available online at


The following two photos of dew and were taken by Virginia Water Radio in Blacksburg, Va., on August 12, 2023, around 8 a.m. EDT, when the dew point and the actual temperature were the same.


The following information is quoted from the National Weather Service, accessed at the Web sites noted on August 21, 2023.

Dew Point Information

From “Dew Point vs. Humidity,” online at

“The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%.  At this point the air cannot hold more water in the gas form.  If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.

“The higher the dew point rises, the greater the amount of moisture in the air.  This directly affects how ‘comfortable’ it will feel outside.  Many times, relative humidity can be misleading.  For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%.  It would feel much more ‘humid’ on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity.  This is because of the higher dew point.

“So if you want a real judge of just how ‘dry’ or ‘humid’ it will feel outside, look at the dew point instead of the [relative humidity].  The higher the dew point, the muggier it will feel.”

“General comfort levels using dew point that can be expected during the summer months:
*less than or equal to 55: dry and comfortable;
*between 55 and 65: becoming ‘sticky’ with muggy evenings;
*greater than or equal to 65: lots of moisture in the air, becoming oppressive.”

Heat Index Information

From “What is the heat index?” online at

It's not the heat, it's the humidity.’  That's a partly valid phrase you may have heard in the summer, but it's actually both.  The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature.  This has important considerations for the human body's comfort.  When the body gets too hot, it begins to perspire or sweat to cool itself off.  If the perspiration is not able to evaporate, the body cannot regulate its temperature.  Evaporation is a cooling process.  When perspiration is evaporated off the body, it effectively reduces the body's temperature.  When the atmospheric moisture content (i.e. relative humidity) is high, the rate of evaporation from the body decreases.  In other words, the human body feels warmer in humid conditions.  The opposite is true when the relative humidity decreases because the rate of perspiration increases.  The body actually feels cooler in arid conditions.  There is direct relationship between the air temperature and relative humidity and the heat index, meaning as the air temperature and relative humidity increase (decrease), the heat index increases (decreases).”


Encyclopedia Britannica, “Average relative humidity,” online at

Angela Fritz, “Capital Weather Gang: Stop telling us to use ‘percent humidity.’  It’s terrible and here’s why,” Washington Post, July 12, 2017.

Anne Helmenstine, “What is Dew Point?”  Science Notes, June 5, 2023, online at

Mesonet, “National Dewpoint Temperature (map),” online at 

Olivia Munson, “What’s the dew point?  Understanding the difference between humidity and the dew point,” USA Today, December 15, 2022.

National Weather Service:
“Dew Point vs. Humidity,” online at (this is the reference for the “general comfort levels” at various dew points mentioned in this episode’s audio): and
“What is the heat index?” online at

University of Illinois, “Dew Point Reports and Contours,” online at

Weather Spark, “Climate and Average Weather Year Round in Dublin, Ireland,” online at

Ashley Williams/AccuWeather, “Why does humidity make hot days feel more miserable?  Undated.  Accessed August 10, 2023.


All Water Radio episodes are listed by category at the Index link above (  See particularly the subject category “Weather/Climate/Natural Disasters.”

Following is a link a previous episode on fog.

Episode 512, 2-17-20.


Following are some Virginia Standards of Learning (SOLs) that may be supported by this episode’s audio/transcript, sources, or other information included in this post.

2020 Music SOLs

SOLs at various grade levels that call for “examining the relationship of music to the other fine arts and other fields of knowledge.”

2018 Science SOLs

Grades K-4: Living Systems and Processes
4.3 – Organisms, including humans, interact with one another and with the nonliving components in the ecosystem.

Grades K-5: Earth and Space Systems
1.7 – There are weather and seasonal changes; including that changes in temperature, light, and precipitation affect plants and animals, including humans.
2.7 – Weather patterns and seasonal changes affect plants, animals, and their surroundings.
4.4 – Weather conditions and climate have effects on ecosystems and can be predicted.

Grade 6
6.6 – Water has unique physical properties and has a role in the natural and human-made environment.
6.7 – Air has properties and the Earth’s atmosphere has structure and is dynamic.

Earth Science
ES.11 – The atmosphere is a complex, dynamic system subject to long-and short-term variations.
ES.12 – The Earth’s weather and climate result from the interaction of the sun’s energy with the atmosphere, oceans, and the land.

Following are links to Water Radio episodes (various topics) designed especially for certain K-12 grade levels.

Episode 250, 1-26-15 – on boiling, for kindergarten through 3rd grade.
Episode 255, 3-2-15 – on density, for 5th and 6th grade.
Episode 282, 9-21-15 – on living vs. non-living, for kindergarten.
Episode 309, 3-28-16 – on temperature regulation in animals, for kindergarten through 12th grade.
Episode 333, 9-12-16 – on dissolved gases, especially dissolved oxygen in aquatic habitats, for 5th grade.
Episode 404, 1-22-18 – on ice on ponds and lakes, for 4th through 8th grade.
Episode 407, 2-12-18 – on snow chemistry and physics, for high school.
Episode 483, 7-29-19 – on buoyancy and drag, for middle school and high school.
Episode 524, 5-11-20 – on sounds by water-related animals, for elementary school through high school.
Episode 531, 6-29-20 – on various ways that animals get water, for 3rd and 4th grade.
Episode 539, 8-24-20 – on basic numbers and facts about Virginia’s water resources, for 4th and 6th grade.
Episode 606, 12-6-21 – on freezing and ice, for kindergarten through 3rd grade.