Archive for the ‘weather’ Category

Happy summer!

June 1, 2022

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

May 27, 2022

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.

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.

Copyright © 2022 Walter Sanford. All rights reserved.


Waiting for the sky to clear

May 11, 2021

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.

Copyright © 2021 Walter Sanford. All rights reserved.

Selys’s Sundragon dragonfly (male)

April 16, 2021

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

October 23, 2020

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)

June 12, 2020

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

Copyright © 2020 Walter Sanford. All rights reserved.

Viceroy butterfly

September 23, 2019

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

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.

Hey Mike, don’t move!

August 30, 2019

There’s a butterfly on your hat. A Red-spotted Purple butterfly (Limenitis arthemis astyanax).

This comical butterfly-man union was observed during a photowalk with Michael Powell at Painted Turtle PondOccoquan Bay National Wildlife Refuge, Prince William County, Virginia USA.

The weather was extremely hot and humid. (Notice the Cumulus congestus clouds building in the background.) Both Mike and I were soaked with sweat as soon as we started our photowalk earlier the same day at another site. The butterfly was feeding upon mineral salts on Mike’s “Duck Dynasty” hat.

Copyright © 2019 Walter Sanford. All rights reserved.

Arrowhead Spiketail dragonfly (female)

May 27, 2019

An Arrowhead Spiketail dragonfly (Cordulegaster obliqua) was spotted in a sunny clearing along a small forest stream at an undisclosed location in Prince William County, Virginia USA.

21 MAY 2019 | PNC. William County, VA | Arrowhead Spiketail (female)

This individual is a female, as indicated by her rounded hind wings, terminal appendages, and prominent ovipositor at the tip of her abdomen.

[Females in the Family Cordulegastridae (Spiketails) feature a] …pointed and spikelike (thus the group name) ovipositor, really a “pseudo-ovipositor” formed from the prolonged subgenital plate. Source Credit: Paulson, Dennis (2011-12-19). Dragonflies and Damselflies of the East (Princeton Field Guides) (Kindle Locations 7005-7006). Princeton University Press. Kindle Edition.

21 MAY 2019 | PNC. William County, VA | Arrowhead Spiketail (female)

Although the pattern of arrowhead-shaped markings visible on the dorsal side of her abdomen is unmistakeable, notice that the thorax features two stripes. The latter field mark can be used to differentiate spiketails from cruisers that have one stripe on their thorax.

21 MAY 2019 | PNC. William County, VA | Arrowhead Spiketail (female)

Mike Powell and I have photowalked together so many times that we are comfortable working cooperatively to shoot a subject. I wanted to get a shot of the face of the Arrowhead Spiketail but was concerned it would spook the dragonfly if I were to get “up in her grill.” So I waited until Mike had taken all of the photographs he wanted before approaching the dragonlfy for her “beauty shot.” As it turns out, the model was extraordinarily tolerant and didn’t fly away until sometime after Mike and I moved on to the next site.

21 MAY 2019 | PNC. William County, VA | Arrowhead Spiketail (female)

Notice the interrupted fern (Osmunda claytoniana) that appears in the background of every photo of the Arrowhead Spiketail.

Location, location, location.

Some species of odonates are habitat generalists, meaning they can be found almost anywhere there is water.

Habitat-specific odonates can be found only in specific habitats — for these species, finding them is all about location, location, location. Arrowhead Spiketail dragonflies are habitat-specialists.

Habitat: Small swift streams and soft-bottomed muddy seeps in forest, also streams reduced to series of small pools during drier weather. As in some other spiketails, skunk cabbage often present. Source Credit: Paulson, Dennis (2011-12-19). Dragonflies and Damselflies of the East (Princeton Field Guides) (Kindle Locations 7081-7082). Princeton University Press. Kindle Edition.

Skunk cabbage (Symplocarpus foetidus) and interrupted fern (Osmunda claytoniana) were observed growing in a seep located between a dirt trail and the small stream where the Arrowhead Spiketail was spotted — the perfect place for several species of habitat-specific odonates!

The last photo shows the clearing along a small forest stream where several species of dragonflies were spotted, including the Arrowhead Spiketail featured in this blog post. The stream is no more than a few feet wide and only a few inches deep in most places.

The small stream where several species of dragonflies were spotted.

The backstory

Telephoto lenses can cause a type of distortion called “foreshortening,” as seen in the preceding photo. Mike Powell and I were standing at the edge of the stream bank trying to decide whether we wanted to go down the short, steep slope to explore the clearing when we spotted a large UFO, that is, an “Unidentified Flying Odonate.” Mike and I took “record shots” of the dragonfly; looking at the LCD of our cameras, we identified the UFO as a Gray Petaltail dragonfly (Tachopteryx thoreyi).

Mike and I had seen a Gray Petaltail at another location (near the beginning of our photowalk), but we were unable to photograph it. So down the bank we went! As it turns out, there were at least two Grays in the meadow: a female; and a male. As Mike was photographing one of the Gray Petaltails he noticed another “large dragonfly.” As we slowly moved closer to the new unknown dragonfly, I quickly realized Mike had spotted an Arrowhead Clubtail. Great catch, Mike!

Please see Female Arrowhead Spiketail dragonfly for Mike’s take on our shared experience.


Arrowhead Spiketail is classified as an uncommon species of odonate. The following map shows all official records for Arrowhead Clubtail (C. obliqua) in the United States of America.

DSA Distribution Viewer | Arrowhead Spiketail

Source Credit: Abbott, J.C. 2006-2019. OdonataCentral: An online resource for the distribution and identification of Odonata. Available at (Accessed: May 27, 2019).

Key: blue dots = Dot Map Project; green dots = Accepted records; yellow dots = Pending records.

Our spotting of Arrowhead Spiketail is a new DSA record for Prince William County, Virginia.

Adult flight period

According to records for the Commonwealth of Virginia maintained by Dr. Steve Roble, Staff Zoologist at the Virginia Department of Conservation and Recreation, Division of Natural Heritage, the adult flight period for C. obliqua is 11 May to 17 July. The species is classified as uncommon. Its habitat is “small streams.”

Bear in mind, Dr. Roble’s records are for the entire state, therefore the adult flight period for C. obliqua seems to be longer than it is in reality. The adult flight period for a single site is probably shorter — more likely around one month. For example, according to records for Northern Virginia maintained by Kevin Munroe, former manager of Huntley Meadows Park, the adult flight period for Arrowhead is 28 May to 27 June (peaks in June).

Copyright © 2019 Walter Sanford. All rights reserved.


Common Baskettail (teneral female)

April 29, 2018

A Common Baskettail dragonfly (Epitheca cynosura) was spotted near Painted Turtle Pond during a photowalk at Occoquan Bay National Wildlife Refuge, Prince William County, Virginia USA.

This individual is a teneral female, as indicated by her tenuous wings and relatively short, straight terminal appendages.

Spring 2018 has been slow to spring in the mid-Atlantic USA, as evidenced by the fact that the first Common Baskettails were spotted at Painted Turtle Pond beginning in mid-April 2017.

Related Resources

Copyright © 2018 Walter Sanford. All rights reserved.

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