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Storms rolling through the Mid-Atlantic

After a relatively cool summer, hot humid conditions have prevailed in the Mid-Atlantic and Northeast of the U.S. in the past week. Today, in particular, featured hot, humid conditions, providing fuel for storms. A system passed over the region, providing the necessary lift to get these storms going. The result was a long line of storms moving through Maryland, producing heavy rain, lightning, and damaging winds. Multiple people shared their view of the storm, both during and after, which is displayed on this map showing their corresponding location for each picture.

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Overlay of pictures on map of MD. Times vary.

The two pictures in western MD were taken following the passage of the storm. The radar image below roughly corresponds to times of these pictures. You can see a north-south oriented line of reds, indicating heavy rain, that had just passed through.


Radar image from 4:46 EDT, showing line of storms passing through Washington County, MD

Let’s take a closer look at the pictures from this time. While the overall sky remained covered in cloud, a key feature in both of these photos are the lower level clouds hanging beneath. These are referred to as fractus and form in the moist environment that remains after the rain has passed.


Photo courtesy of Nancy Kirkpatrick in Cascade, MD (2 Sept 2014, after storm passed) showing fractus clouds.


Photo after the storm passed showing lower residual clouds in Smithsburg, MD (Debbie Rowe, 2 Sept 2014)


Another view from Debbie Rowe of the back edge of the storm.

This line of storm continued its path across Maryland throughout the evening. The radar image below shows that hours later, it maintained a linear structure on the other side of the state. Also notice storms developing over northern VA at this time; a focus for the next picture.


Radar image from 6:43 EDT

This new cluster of storms, forming in a similar environment as the earlier round, later went on to also organize into a linear structure. In this later radar image, you can see this structure, the reds indicating heavy rain, and a line of lighter blues out ahead. This latter feature is what we call an outflow boundary, and indicates strong, cold winds moving out ahead of the storm, lifting up warm, moist air ahead of it along with any dust, insects, etc. in its path. (Check out our previous post to learn more about this outflow.)

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Radar image from 7:27 EDT. White arrow shows location of outflow boundary.

Notice at this time, the leading edge of this storm is moving over Washington, D.C. Jen Horneman was driving just outside the city at this time and snapped this photo out the window (the third picture in the map above). A close-up view reveals a much lower cloud base as the warm moist air being lifted along the outflow cools and condenses to form this menacing, low cloud. Strong winds were expected with this feature, followed by heavy rain.


Photo courtesy of Jen Horneman near Arlington, VA

After these storms moved through, in addition to the heavy rain indicated by the oranges and reds in the radar image, damaging winds were associated with these storms. Many reports indicated large trees down throughout the state.


Severe weather reports from the Storm Prediction Center. Blues indicate severe winds.

Were you affected by these storms? Feel free to share with us any pictures or stories you may have!


Shelf clouds, mammatus, gust fronts, oh my!



Shelf cloud courtesy of Kevin Sheely (Warrenville, Illinois, 8/25/2014)

What an ominous sight heading your way! Kevin Sheely captured this incredible shot of a shelf cloud near Warrenville, Illinois. On this day, numerous storms moved into and formed within Illinois.

To understand what creates this shelf-like appearance, we first need to understand that the rain that is falling from the core of the storm cools the atmosphere. This cold air is denser than the surrounding air so it sinks to the ground and then spreads outward from where the rain is falling. This cold air spreading outward is referred to as “outflow” and the leading edge of this rapidly outward moving air is called an outflow boundary or gust front. This boundary (like other weather fronts) separates air of different temperatures/densities: in this case, the cold air from the storm and the warm air surrounding it.


Same picture as above from Kevin Sheely with the cold outflow shown as blue arrow spreading outward and the warm moist air being lifted out ahead shown in red.

The warm, moist air ahead of it is less dense so it is lifted up and over the spreading cold air. This air is lifted and cooled, to the point where it condenses and forms the shelf cloud extending out ahead of the main storm along this outflow.


Diagram showing shelf cloud formation along gust front

Notice in that diagram that the warm air out ahead is being lifted into relatively stable air above. This leads to the layered characteristics of the shelf cloud as it extends outward instead of continuing to grow upward like the parent storm.

This rapidly expanding air is often responsible for strong, potentially damaging winds at the surface. As these storms, with their shelf clouds, passed through Illinois on the 25th, downed trees were left in their path. These storm reports from the Storm Prediction Center show a cluster of blues in northern IL, indicating strong winds.


So while the shelf clouds themselves aren’t dangerous, if you see them coming, you can expect strong, cool gusty winds shortly after, followed by heavy rain and possibly even hail.

Luckily these outflow boundaries can also be detected by radar to help warn of this impending gustiness. As the cold air lifts up the warm air ahead of it, it’s also lifting up dust, insects, birds, etc. These can be detected by radar, but the power return is much less than the heavy rain falling. So while the heavy rain in these storms appear orange and red on these radar images, you can see the “fine lines” of blue ahead of these storms indicating the location of the gust front. Notice in these series of radar images that storms produce these boundaries, which then go on to produce new storms that then produce their own gust fronts. And so the cycle continued on this day…


Series of radar images from the Chicago National Weather Service radar (KLOT) showing numerous storms and their outflow boundaries.


We’ve learned that these shelf clouds are seen at the leading edge of the storm, indicating cold gusty winds to come, but what about behind the storm? Well, storms can only reach a certain height in the atmosphere, at which level they spread out horizontally. This is often seen as an anvil. The air below the anvil is typically drier and therefore sometimes mammatus clouds can form underneath. These bulbous beauties were seen on this day in other parts of Illinois by Bill Morris.


Mammatus over Grundy County, IL courtesy of Bill Morris (8/25/2014)

And that’s not all! Moving ahead to the next day (8/26/2014), another round of storms moved through Illinois, allowing for another opportunity to photograph shelf clouds. This picture was submitted to us by Melissa Godbee. Even though this is looking at the storm from the side compared to the head-on view shown by Kevin, can you still see the resemblance?


Shelf cloud (Melissa Godbee, Illinois, 8/26/2014)

Have you seen these ominous, yet beautiful, clouds where you live?

Pileus capping cumulus in MD



Pileus atop cumulus congestus, Richard Barnhill (eastern MD, 12 Aug 2014)

This beautiful picture was taken by Richard Barnhill in eastern MD on 12 August 2014. The sun is highlighting the tops of growing cumulus congestus clouds, which are capped by another cloud, called a pileus cloud. Pileus is Latin for “cap” and resembles lenticular clouds that are also highlighted in this atlas. Lenticular clouds form when moist stable air encounters a mountain barrier, whereas these pileus clouds form when moist stable air is disrupted by the growing cumulus cloud below.

Strong updrafts occur within these growing cumulus clouds, defined by their well-defined edges and tufted appearance. If the air above is moist and relatively stable, but is forced upward by this strong upward motion from the cloud below, it can cool to its dewpoint, leading to condensation and the formation of this pileus cloud. This happens quite rapidly and the pileus cloud does not last long as typically the cumulus cloud beneath continues to grow through it.

Here is a schematic we created to try to simply explain this process:

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Schematic showing formation of pileus (courtesy of the Community Cloud Atlas admins)

The following series of images shows another example of a pileus cloud, where the cumulus below quickly produced this cap cloud and then grow through it to form a mature cumulonimbus.


Example of cumulus growing through the pileus cloud

Here is yet another great example of a pileus cloud, sent to us from North Carolina back in May. Notice how clearly it sits atop the cumulus congestus cloud, resembling a cap cloud hugging the top of a mountain barrier.


Pileus atop growing cumulus, Blake Smith (North Carolina, 18 May 2014)

The term “pileus” isn’t unique to clouds, however. Another example of this “cap” feature is given for the tops of mushrooms, such as shown in this diagram below. It’s great to see the commonalities in nature!


“Pileus” term applied to mushrooms


Given that pileus are quite the fleeting phenomena, we are curious: have you seen a pileus? We would love to see your examples!

Views of storms from near and afar


Richard Barnhill, View from Washington, D.C. on August 3, 2014

Ah, thunderstorms. A typical sight in the summer months where warm moist air rises in unstable environments to produce heavy rain, lightning, and beautiful views. On Sunday, moist onshore flow from the Atlantic impacted the eastern seaboard of the U.S. while a series of disturbances in the atmosphere provided the lift to create scattered storms.

Up close, these storms present an ominous sky, with dark clouds moving overhead. Storms were in the vicinity of D.C. this day, and you can see their threatening undersides near the top of this photo. But looking out beyond these nearby storms, you can see others in the distance. To the far right, there’s the characteristic puffy tops of growing cumulus clouds. To the left in the far distance, a more mature storm can be seen.


So how far away was that storm in the distance? Well, Richard Barnhill not only captured this photo, but also a screen shot of the current radar at this time. You can see where this storm was relatively to D.C., nearly 75 nautical miles (140 kilometers) away.



Looking back at the picture of this distant storm, notice how the top of it seems to spread out horizontally like an anvil. Need a refresher on what we mean by an anvil? Well, here you go:



As the storm grows in the atmosphere, it is limited by how high it can reach by how the temperature of the surrounding environment changes with height. In the troposphere, where most of our weather occurs, temperature tends to decrease as you go higher in the atmosphere. Certain conditions can lead to levels where temperature begins to increase with height, called an inversion, which serves as a sort of “lid” for these storms. The most common lid in the atmosphere is at the top of the troposphere, where above that temperature begins to increase with height in the stratosphere. So a storm can tap into the energy available in the troposphere as long as it remains warmer than its environment and can continue to rise until this equilibrium level. Here’s a handy diagram that can help show what we mean by this.



The take-home message from this image is that once the cumulonimbus reaches that stable point, the ice crystals in the upper-levels of the storm will begin to spread out horizontally in the stronger upper-level winds, creating the anvil. This type of cumulonimbus, that has the characteristic anvil, is called the “incus” variety. Incus is another name for “anvil” and is not only used to describe these clouds, but is also the name for a part of our ear.


See the “incus” in the middle part of the ear


Zoomed in on the parts of the “incus” of our ear

For those who regularly follow our page, notice another familiar term that’s used to describe clouds?

So the distant anvil of Richard’s picture is an indicator of a more mature storm that has reached its maximum vertical growth. The clouds overhead were not posing an immediate threat, but shortly after, more intense storms rolled through the region.






A honeycomb sky

Cirrocumulus lacunosus in the early morning of 1 Aug 2014 (Jerry Tangren, East Wenatchee, WA)

Cirrocumulus lacunosus in the early morning of 1 Aug 2014 (Jerry Tangren, East Wenatchee, WA)

Cirrocumulus clouds occur high in the sky, typically short-lived as the ice crystals that comprise them are carried away in the strong upper-level winds. Even more fleeting is the variety of these clouds referred to as lacunosus. This word is Latin for “full of holes” and is commonly referred to as appearing like a honeycomb.

The American Meteorology Society’s glossary defines this cloud variety as follows:

cloud variety characterized more by the appearance of the spaces between the cloud elements than by the elements themselves. The gaps are generally rounded and often have fringed edges. The overall appearance is that of a honeycomb or net, the negative of that of clouds composed of separate rounded elements. This variety is a modification mainly of the genera cirrocumulus and altocumulus and may apply to the species stratiformiscastellanus, or floccus
How do these form? The holes indicate areas of sinking air, while the fringed edges indicate localized areas of compensating rising motion. This can happen when a layer of colder air moves over warmer air. The cold air is more dense, creating those pockets of sinking motion. This process is relatively quick so it’s rare to see this pattern persist for very long.

A smile in the sky

Circumzenithal arc over Ridgecrest, CA on 30 July 2014. Photo courtesy of Marian Murdoch.

Circumzenithal arc over Ridgecrest, CA on 30 July 2014. Photo courtesy of Marian Murdoch.

What appears to be a curved rainbow in the sky is actually what’s called a circumzenithal arc and is quite different from the more familiar rainbows that grace the sky after a rainy day. The thin, wispy cirrus clouds in this picture occur at high levels in the atmosphere where temperatures are way below freezing. At these cold temperatures, these clouds are therefore made up of ice crystals. Ice crystals can come in many shapes and sizes, as seen in this diagram.


What controls the type of ice crystal that forms? Well, that depends on the temperature and moisture content of the environment it forms and grows in. For the circumzenithal arc to form, the ice crystals need to be plate-like. These plates have horizontal faces and shorter vertical side faces. Sunlight enters these faces and is bent within the crystal; this is called refraction. A simple diagram will help to visualize this concept:


The separation of the lines in this picture represents the sunlight being separated into the colors of the visible spectrum (ROYGBIV), such as what you see when light passes through a prism. For the circumzenithal arc to form, the sun’s rays enter the uppermost horizontal face and exists through one of the vertical side faces. For the light to enter these crystals at these angles (nearly parallel), the sun has to be lower than about 32 degrees above the horizon.

So why is it called a circumzenithal arc? Well, the word “zenith” refers to directly overhead. If this arc were a complete circle, the center would be directly above you. The arc is seen directly above the sun.

How does it differ from a circumhorizon arc, which is typically seen below the sun? Circumhorizon arcs also form as light is refracted through plate-like ice crystals, but the sun’s rays first enter the vertical side face of the crystals and exit out of the bottommost horizontal face. For this to happen, the sun has to be at a much higher angle in the sky (~58 degrees above the horizon).

Incredible Seattle sunset

Last night, Seattleites were treated to a spectacular sunset. I, Angela, was attending the Seattle Sounders game and admit that I was quite distracted by the clouds, as were many others at the stadium. Here was one of the views I had from my seat:
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Angela Rowe
Location: Seattle, Washington
Date: 14 July 2014

What causes these vibrant colors? First, let’s take a look at the visible spectrum. The sun’s rays are made up of a range of wavelengths and therefore colors. The cool colors (blues) are the shorter wavelengths, while the reds are the longer wavelengths.

This simple diagram from the National Weather Service online education course (JetStream) shows what happens to the sun’s rays as the sun sets in the sky. At lower angles (B and C), the sun’s rays have to pass through more of the atmosphere, therefore more of the shorter wavelengths are scattered by the air. This leaves the longer wavelengths (the reds and oranges), which can be reflected off of clouds in the mid- and upper-levels of the atmosphere.

While there are many colorful sunsets (and sunrises) in Washington and beyond, the conditions last night were particularly conducive for a colorful show due to the mid-level clouds present over much of the area. Earlier in the evening, water vapor satellite imagery showed an upper-level low pressure system sitting off the coast, spinning counter-clockwise and pumping moisture into the area, seen as the pink colors in this image.

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This moisture was primarily concentrated in the mid-levels, as could be seen as a thickening altostratus layer covered the sun over the city. Note there are some darker clouds underneath as moisture continued to move in from the southwest.
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Showers and thunderstorms were prominent to the south, especially over northern Oregon, but the lower levels over Seattle and much of NW Washington remained dry. Every day, twice a day, the National Weather Service launches instruments on balloons called radiosondes that measure the temperature, moisture, pressure, and winds as the balloon ascends into the sky. Here is an example of what this balloon data looked like last evening around this time, with the red line indicating temperature and the blue dewpoint. The dewpoint is the temperature at which the air would have to be for saturation to occur. The higher the number, the more moisture in the air. Where the dewpoint and temperature are far apart indicates dry conditions (low humidity), while areas where they are nearly the same are where you might expect clouds. So, if you look at this example, you’ll see that most of the atmosphere near Seattle was very dry, with the exception of right near the surface and in between pressure levels of 400 and 350 mb. This corresponds to heights above the surface between roughly 7.5 and 9 km, where many of the clouds were observed on this particular evening.

So as I entered the stadium around this time, I quickly became distracted by the mammatus clouds that were forming overhead. The dry air below the moist levels helped create these brief, but beautiful bulbous clouds and it became increasingly difficult to pay attention to the game.

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In order to keep track of what was going on around the stadium, I kept an eye on the radar. Some of the showers from the south looked like they were making their way toward Seattle, as can be seen in this radar image during the game.

At the stadium itself though, some light drizzle could be seen, but overall the rain was evaporating before reaching the ground. This is referred to as virga and can be seen in these pictures that I took from the stadium around this time.
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So as you might have guessed, I was quite distracted by the sky throughout the game. The Sounders ended up beating Portland in an exciting 2-0 game, but I bet a large portion of the 64,000+ fans at the stadium were also paying attention to the sky. In fact, social media sites exploded with pictures of this fiery sunset, from the stadium and throughout other areas near the Puget Sound (http://blogs.seattletimes.com/today/2014/07/sunday-nights-seattle-sunset-captured-on-twitter/).

Later on at night, lightning flashed in the distance as storms persisted over the southern Olympic Peninsula. The low was moving onshore, bringing with it cold air aloft that created an unstable environment, in addition to the moisture available, to maintain these storms. Living in a place like this, where there aren’t a lot of thunderstorms, I’ll certainly take what I can get.

This morning, I was curious to see how much rain did end up reaching the ground across the area last night. The CoCoRaHS network (www.cocorahs.org) is a volunteer organization where people report rain at their house. This map shows the reports for the 24-hours ending this morning, with most locations near Seattle receiving less than one-hundredth of an inch!
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And finally, for your viewing pleasure, here’s a link to time lapse video of the clouds and sunset as viewed looking west from the University of Washington’s Atmospheric Science building rooftop. Can you see the different cloud layers before the colorful finish to the day? Click here to see this time-lapse video!