Outflow boundary

What goes up must come down.

A simple conceptual model of a thunderstorm includes an updraft and downdraft. Sometimes the downdraft causes an “outflow boundary.”

What is an outflow boundary?

Outflow Boundary

A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin.

New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).

Source Credit: Glossary, NOAA National Weather Service.

Recent outflow boundary near my location

Notice the intense thunderstorm located to the northwest of the blue reticle that marks my location. The large cell produced heavy rainfall, as indicated by the red radar echoes and green polygon that outlines an area where a flash flood warning was issued.

Click on the following image to see a full-screen view of the animated GIF.

05 JUL 2023 | KWLX | Outflow boundary

Now notice the ring of radar echoes that radiate outward from the dissipating thunderstorm cell. That’s an outflow boundary!

If you pour water on a flat surface, then the water will spread out in all directions. Like water, the atmosphere is a fluid (albeit much less dense than water) and behaves similarly.

Also notice another strong thunderstorm that formed along the outlfow boundary (to the southeast of the blue reticle) where the gust front might have interacted with some type of atmospheric boundary that caused uplift and fueled a new thunderstorm cell.

Related Resources

• What Causes a Thunderstorm? [Featuring embedded video (2:38).]
• How Thunderstorms Form
• Life Cycle of a Thunderstorm

Copyright © 2023 Walter Sanford. All rights reserved.

Interactive infrared weather satellite image

The AMS interactive infrared [weather] satellite image resulted from the collaboration between the American Meteorological Society (AMS) education initiatives and the Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison.

Infrared imagery is one of three types of weather satellite imagery. Black, white, and shades of gray are used to represent temperatures from the tops of clouds and the land & water surfaces on Earth. Black is the hottest temperature; white is the coldest. Sometimes this type of weather satellite imagery is color-enhanced for use by broadcast meteorologists.

The first image (shown below) is the non-interactive version of two screenshots from my Apple iPad mini 6. All three images in this blog post are from the same date and time.

08 May 2023 at 19:00 UTC (03:00 pm EDT).

As you move the cursor (red reticle) over the interactive image, the temperature (in degrees Celsius) and location (latitude and longitude) are listed.

The first screenshot shows the cursor (red reticle) over a dark area located somewhere along the Florida peninsula where the temperature is 27°C (78.8°F or 299.2 K). At such a warm temperature, we are almost certainly looking at land rather than water.

Cursor (red reticle) located somewhere along the Florida peninsula.

The the second screenshot shows the cursor (red reticle) over a bright white area located somewhere in/above Alabama where the temperature is -55°C (-67°F or ~218.2 K)! At such a cold temperature, we can be certain we are looking at the tops of very high clouds. This could indicate hazardous weather is occurring at the Earth’s surface.

Cursor (red reticle) located somewhere over Alabama.

Science can and should be fun. Have fun exploring using the AMS interactive infrared [weather] satellite image!

Related Resources

• AMS interactive infrared [weather] satellite image – the American Meteorological Society (AMS) and the Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin-Madison.
• Weather Calculators, including temperature conversion – a page of interactive calculators that still work, rescued from the Wayback Machine Internet Archive.
• UTC/EST/EDT – a table listing equivalent times among Universal Time Coordinated (UTC), Eastern Standard Time (EST), and Eastern Daylight Time (EDT).
• AMS Interactive Infrared Weather Satellite Image – Making the Invisible Visible. Another resource rescued from the Wayback Machine Internet Archive. I created this activity as an extra credit opportunity for my Grade 8 Physical Science students when we were studying energy and energy transformations. In retrospect the activity is probably too long to use “as is,” but it might serve as a data bank of questions that teachers could use to create their own version of the activity. For non-educators, the activity questions can help to guide your thinking about what you can tell from the interactive infrared imagery.

Tech Tips

The interactive infrared [weather] satellite image was tested using my Apple iPad mini 2 and 6, Apple MacBook Air (13″, M1, 2020), and Apple iMac desktop computer (vintage 2009) and is compatible with all of those devices.

Did you notice both the non-interactive and interactive infrared images are GIF files?

The [GIF] format supports up to 8 bits per pixel for each image, allowing a single image to reference its own palette of up to 256 different colors chosen from the 24-bit RGB color space. Source Credit: GIF, by Wikipedia.

In the old days before the AMS interactive infrared [weather] satellite image, we would use a scientific image-processing tool such a NIH Image (now ImageJ) to infer temperature from pixel values (0-255). Labor intensive, but it was fun!

Copyright © 2023 Walter Sanford. All rights reserved.

Thunderstorms, mesocyclones, and tornadoes. Oh my!

As a weather enthusiast, RadarScope is my go-to weather app for tracking the approach and passing of weather systems such as the line of strong thunderstorms that affected the Washington, D.C. metropolitan region on Saturday, 22 April 2023.

Animated GIF created by RadarScope app. (11:44 AM to 11:58 AM)

The following annotated screenshot shows a few basic buttons; their function is described below the graphic.

My location, radar sites, and warnings. (12:01 PM)

1. My location. (See blue reticle, center screen.)
2. Radar sites. (KWLX, located in Sterling, Virginia, is the National Weather Service radar site nearest to my location.
3. Warnings. (Two warnings were in effect when this screenshot was captured.)

A fly-out panel appears when you click on the Warnings button. As you can see, there were two Severe Thunderstorm Warnings in effect at the time of the screenshot. If you click on one of the warnings then RadarScope automatically takes you to a zoomed-in view of the warning area. Click on the button for “My location” to return to your home location.

Warnings. (11:58 AM)

Tornado Warning

Soon afterward, a Tornado Warning was issued for Culpeper- and Madison Counties in Virginia, as indicated by the red polygon. Notice the red polygon is nested inside a yellow polygon that outlines an area where a Severe Thunderstorm Warning was issued.

Tornado Warning (red polygon). (12:30 PM)

Click on the red polygon for more information about the Tornado Warning.

Tornado Warning (information). (12:30 PM)

The next screenshot shows the “Super-Res Reflectivity” radar product, zoomed in on the Tornado Warning area. With a lot of imagination, you can almost see something that looks a little like the classic “hook echo” associated with tornadoes. Almost, but not quite.

Super-Res Reflectivity. (12:30 PM)

Time to switch to the Storm Relative Velocity radar product, shown below. This is where Doppler weather radar really shines. Greens indicate radar echoes moving toward the KWLX radar site; reds indicate radar echoes moving away from the radar site (like brake lights on a car driving away from you).

The following image shows the thunderstorm cell is rotating counterclockwise — this is known as a mesocyclone and is the reason for the Tornado Warning.

Storm Relative Velocity. (12:27 PM)

Within the broader area of counterclockwise circulation there is a tighter area of greens and reds, as shown more clearly in the Super-Res Storm Relative Velocity radar product.

Super-Res Storm Relative Velocity. (12:32 PM)

It’s important to note that the orientation of side-by-side greens and reds typical of rotating thunderstorm cells varies depending upon the location of the storm cell relative to the weather radar site. In the example shown above the greens are on the right and the reds are on the left because the warning area is located to the southwest of KWLX. In contrast, if the warning area were located to the northeast of the radar site, then the reds would be on the right and the greens on the left.

As it turns out, there were’nt any official Tornado Reports for Virginia. Later the same day, a small F0 tornado touched down briefly in Montgomery County, Maryland.

Related Resources

The following resources from the National Weather Service provide excellent background information about Doppler weather radar.

• How radar works
• Composite Reflectivity
• Storm Relative Velocity

More RadarScope-specific resources are available from the creators of the app.

• User’s Guide
• Information Articles
• RadarScope Blog

Copyright © 2023 Walter Sanford. All rights reserved.

Happy summer!

01 June is the first day of summer. What? Perhaps you’re thinking, no that’s incorrect — the first day of summer is on the day of the June Solstice that occurs annually around 21 June. Well, as it turns out both of us are correct.

Atmospheric scientists, including climatologists and meteorologists, define summer as the three-month time period that includes June, July, and August. Astronomers define summer as the three-month time period from the June Solstice to the September Equinox.

Now you know. Again I say, Happy summer!

Related Resource: The Seasons, the Equinox, and the Soltices (by the National Weather Service).

Copyright © 2022 Walter Sanford. All rights reserved.

RadarScope app

RadarScope, one of my favorite apps for Apple iOS devices, is a full-featured app that provides access to nearly the entire suite of Doppler weather radar products generated by the National Weather Service.

As a wildlife photographer I use RadarScope to make go/no-go decisions for photowalking outings. And when I’m already in the field, I use the app to decide whether it’s time to seek shelter from pop-up thunderstorms.

As a weather enthusiast, RadarScope enables me to track the approach and passing of weather systems such as the line of strong thunderstorms that affected the Washington, D.C. metropolitan region on Friday, 27 May 2022.

Composite Reflectivity

The first image shows “Composite Reflectivity.” This is the same type of weather radar imagery that has been used by TV weathercasters for years. In a nutshell, composite reflectivity shows precipitation intensity within range of a weather radar site, in this case KWLX — the NWS Forecast Office located in Sterling, Virginia.

27 MAY 2022 | 10:29 AM EDT | KWLX Sterling

Notice the line of heavy precipitation, indicated by a narrow band of red radar echoes, just to the west of my location in suburban Washington, D.C. (see blue reticle at the center of the screen). Forecast storm tracks (see incremented white lines) indicate individual storm cells are moving generally from southwest to northeast.

To view storm tracks in RadarScope, tap the settings icon in the lower right of the screen, then choose Layers and turn on the Storm Tracks option. The estimated times of arrival can be seen by touching anywhere along the track. Source Credit: RadarScope: How are Storm Tracks Computed? [Editor’s Note: In my experience, this feature works only when I tap the white circle at the origin of each storm track.]

Storm Relative Velocity

“Storm Relative Velocity” shows the wind velocity in a storm minus the forward motion of the storm. Greens show motion toward the weather radar site; reds show motion away from the radar (like car tail lights).

27 MAY 2022 | 11:21 AM EDT | KWLX Sterling

Look closely at the full-size version of the preceding image. Notice the yellow polygon located between Beaverdam and Fredericksburg, Virginia that delineates the boundaries of a severe thunderstorm warning area.

There is a red polygon (located inside the yellow polygon) that represents a tornado warning area. Within the boundaries of the red polygon, notice the juxtaposition of greens and reds — a good indicator of counterclockwise rotation in a storm cell. As it turns out, there were several official reports of a tornado on 27 May 2022 in the same location as indicated by the NWS Doppler weather radar.

It’s important to note that the orientation of side-by-side greens and reds typical of rotating thunderstorm cells varies depending upon the location of the storm cell relative to the weather radar site. In the example shown above the greens are on the right and the reds are on the left because the warning area is located to the southwest of KWLX. In contrast, if the warning area were located to the northeast of the radar site, then the reds would be on the right and the greens on the left.

Related Resources

The following resources from the National Weather Service provide excellent background information about Doppler weather radar.

• How radar works
• Composite Reflectivity
• Storm Relative Velocity

RadarScope features good in-app documentation, as evidenced by the following screen captures.

RadarScope | Help

RadarScope | User’s Guide

RadarScope | User’s Guide – Velocity Products

The same resources (and more) are available online.

• User’s Guide
• Information Articles
• RadarScope Blog

Copyright © 2022 Walter Sanford. All rights reserved.

 

Waiting for the sky to clear

A picturesque dam was visited during a recent photowalk with Michael Powell along a mid-size stream at an undisclosed location in Prince William County, Virginia USA.

13 APR 2021 | Prince William County, VA

As you can see, the sky was completely overcast when the photograph was taken and the weather was cool and breezy. Not ideal conditions for odonate hunting, but hey, sometimes you just have to make lemonade from lemons by stopping to enjoy the beautiful scenery!

Tech Tips

I used Adobe Photoshop Lightroom CC to edit the preceding photo. A graduated filter was used to enhance detail in clouds in the sky without making the shadows too dark in the rest of the image. The area affected by the graduated filter is highlighted in red, as shown in the following screen capture: I decreased the Exposure, affecting the clouds/sky); and increased the Shadows, affecting the tree tops.

The last screen capture shows the global adjustments I made to the entire photo using the Develop module — notice these settings are different from the settings for the selective adjustments I made using the graduated filter, shown above.

Related Resources

Two complementary videos demonstrate how to use the graduated filter in Adobe Lightroom: Matt Kloskowski shows a practical example of how to use a graduated filter to enhance the sky; Julieanne Kost provides an excellent tutorial that explains in detail how the graduated filter works.

• The Coolest Trick for Working With Skies in Adobe Lightroom, by KelbyOne (3:39) featuring Matt Kloskowski
• Lightroom CC – Using the Radial and Graduated Filters, by Adobe Photoshop Lightroom (7:43) featuring Julieanne Kost

Copyright © 2021 Walter Sanford. All rights reserved.

Selys’s Sundragon dragonfly (male)

I discovered a Selys’ Sundragon dragonfly (Helocordulia selysii) during a recent photowalk with Michael Powell at an undisclosed location in Prince William County, Virginia USA. Selys’s Sundragon is a new species for my Life List of odonates and for Prince William County, VA. [Odonata Species (p. 1 of 2) — current as of 14 April 2021 — shows part of the species list for Prince William County before Selys’s was added.]

This individual is a male with a malformed abdomen. Notice his abdomen is twisted so that the terminal appendages aren’t in their usual alignment. The cerci should be on top and the epiproct should be on the bottom; they aren’t where they should be.

13 APR 2021 | Prince William County, VA | Selys’s Sundragon (male)

All male dragonflies have three terminal appendages, collectively called “claspers,” that are used to grab and hold female dragonflies during mating. Male dragonfly terminal appendages don’t look exactly the same for all species of dragonflies, but their function is identical. The misalignment of this Selys’s terminal appendages might be a problem when attempting to form the “wheel position” with females.

The Backstory

Mike Powell and I were men on a mission to photograph Uhler’s Sundragon dragonflies (Helocordulia uhleri). The sky was completely overcast when we arrived at our destination. According to the weather forecast, the sky was supposed to clear around 1:00 pm, and sure enough it did. Soon afterward, we spotted our first Uhler’s of the day and spent some time photographing several individuals.

All of the Uhler’s we saw were female. At some point I said to Mike (paraphrasing) “I need to photograph at least one male before we leave!” I walked a little farther downstream from a place where Mike was shooting macro photos of a very cooperative female Uhler’s. That’s when I spotted the male shown in the preceding photo.

My first impression was the dragonfly seemed to be noticeably smaller than the female Uhler’s we had been photographing. Turns out I was right! According to Dragonflies and Damselflies of the East by Dennis Paulson, Uhler’s are 41-46 mm in total length (4.1-4.6 cm) and Selys’s are 38-41 mm in total length (3.8-4.1 cm). For those of you keeping score at home, that’s only ~1.5″ long — small for many if not most dragonflies!

Related Resource: Selys’s Sundragon dragonfly – a blog post by Michael Powell, my good friend and photowalking buddy.

Copyright © 2021 Walter Sanford. All rights reserved.

Service call for HMP weather station

The following photo gallery shows David M. Lawlor, Natural Resource Manager at Huntley Meadows Park (HMP), Fairfax County, Virginia USA, working to repair the automated weather observation station located in the central wetland area.

According to Dave, the components of the weather station were working properly although data couldn’t be accessed remotely.

The first photo shows Dave getting out a volt meter in order to check battery voltage and power to the weather station data logger.

14 OCT 2020 | Huntley Meadows Park | David M. Lawlor

The next photo shows Dave preparing to connect a laptop computer to the data logger.

14 OCT 2020 | Huntley Meadows Park | David M. Lawlor

Dave testing battery voltage and power to data logger…

14 OCT 2020 | Huntley Meadows Park | David M. Lawlor

14 OCT 2020 | Huntley Meadows Park | David M. Lawlor

14 OCT 2020 | Huntley Meadows Park | David M. Lawlor

The last photo shows Dave using a laptop computer, connected to the data logger, in an attempt to diagnose the connection issue.

14 OCT 2020 | Huntley Meadows Park | David M. Lawlor

The Backstory

During a photowalk with Michael Powell along the boardwalk that goes through the hemi-marsh at Huntley Meadows Park, we ran into Dave Lawlor when he was about to go overboard into the wetlands.

Related Resource

New HMP Weather Station (posted on 10 December 2016) – Real-time weather data was available from the old weather station, installed and maintained by Virginia Tech University, until it went offline after 23 September 2016. We look forward to a time when the new weather station goes online for public access.

In the meantime, real-time weather data is available at a new exhibit located just inside the front doors of the HMP Visitor Center.

Copyright © 2020 Walter Sanford. All rights reserved.

Cobra Clubtail dragonflies (females)

At least 11 Cobra Clubtail dragonflies (Gomphurus vastus) were spotted during a photowalk with Michael Powell in Fairfax County, Virginia USA, including 10 females and one male. This blog post features photos of the first two females that I spotted.

No. 1a

08 JUN 2020 | Fairfax County, VA | Cobra Clubtail (female)

This individual is a female, as indicated by her terminal appendages and rounded hind wings. Notice the injury to her right rear leg.

08 JUN 2020 | Fairfax County, VA | Cobra Clubtail (female)

No. 1b

Inspired by Fred Siskind’s portfolio of dew-covered insects, Mike Powell and I are on a never-ending quest to find and photograph dew-covered odonates. The last photo shows my best effort to date.

As we were photographing female No. 1a, I noticed another individual perched nearby. No. 1a was perched in a sunny spot where most of the morning dew had evaporated; No. 1b was perched in a shady spot where everything was still covered by dew.

08 JUN 2020 | Fairfax County, VA | Cobra Clubtail (female)

Unfortunately, this female was quite skittish so her glamor shoot was one-and-done.

What is dew and how does it form?

Dew forms when the atmosphere is cooled until its temperature reaches the “dew point temperature” and water vapor in the atmosphere (an invisible gas) condenses to become liquid water. (The temperature when this phase change occurs is also known as the “frost point temperature.”)

The dew point temperature varies depending upon the amount of moisture in the air. Typical dew points in the mid-Atlantic states are in the 60s and 70s during the summer months, 40s and 50s during spring and fall, and 20s and 30s during winter.

Check your local weather forecast to see whether the predicted overnight low air temperature will reach the dew point temperature. Sometimes close is good enough, as surfaces that are good radiators of thermal energy can cool a thin layer of air to the dew point.

Related Resources

• Dewpoint (AMETSOC)
• Dew Point vs Relative Humidity (NWS/NOAA)
• Observed Dew Point Temperature (UIUC)

Copyright © 2020 Walter Sanford. All rights reserved.

Viceroy butterfly

A Viceroy butterfly (Limenitis archippus) was spotted during a photowalk at Occoquan Bay National Wildlife Refuge, Prince William County, Virginia USA.

18 SEP 2019 | Occoquan Bay NWR | Viceroy butterfly

Viceroy butterflies look similar to Monarch butterflies (Danaus plexippus).

It can be distinguished from the Monarch by its smaller size and the post-median black line that runs across the veins on the hindwing. Source Credit: Viceroy (butterfly), Wikipedia.

The Backstory

I noticed the Viceroy butterfly as I was searching intensively for Fine-lined Emerald dragonflies (S. filosa). The juxtaposition of complementary colors was too perfect to pass up, so I stopped to shoot a couple of photos. The photo “feels like” a harbinger of fall, despite the persistence of late-summer in the mid-Atlantic region of the United States of America.

Copyright © 2019 Walter Sanford. All rights reserved.