Tag Archives: August

Shelf clouds, mammatus, gust fronts, oh my!

 

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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.

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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.

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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.

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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…

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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.

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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?

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

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

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Pileus capping cumulus in MD

 

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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.

CumulusGrowth_JamesStaileyJr_BentonAR_17Aug2014_annotated

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.

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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!

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“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!

Pyrocumulonimbus cloud from Canada wildfire

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

Pyrocumulonimbus in Wood Buffalo National Park, Canada (Photo by Mike Smith at 1400 MDT on 5 Aug 2014)

For those of us in the U.S., we’ve been hearing a lot about the wildfires burning in the west, from California up through eastern Washington. But areas of Canada have also seen a devastating fire season. In fact, it has been one of their worst in recorded history with 354 fires so far this year in the Northwest Territories, burning over 2.8 million hectares (Source: Northwest Territories Environment and Natural Resources, Forest Management Division).

Mike Smith is a meteorologist with the Yukon Wildland Fire Management and has been offering on-site support for wildfires. He took these pictures on August 5 of a fire that started in the northwest portion of Wood Buffalo National Park and grew rapidly during August 3-5. To get ourselves oriented, here is a map showing where this park is located.

Location of Wood Buffalo National Park

Location of Wood Buffalo National Park

This particular fire, given the identifier 14WB-025, is one of many in this park on that day. This map from the Forest Fire Management Program shows the location of this fire in the northwest corner of the park, near Buffalo Lake; one noted as being managed.

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To put this fire in context of other fires in the greater region, this map, provided by the Canadian Wildland Fire Information System, shows fire “hotspots.” These are satellite pixels that indicate burning vegetation and may indicate one fire or several hot spots within a larger fire. You can see the high concentration of these hot spots in the Northwest Territories on the day the picture was taken, and this may even be underestimating hotspots if there was cloud cover involved.

Fire "hotspots" derived from satellite infrared data. Courtesy of the Canadian Wildfire Information System.

Fire “hotspots” derived from satellite infrared data. Courtesy of the Canadian Wildfire Information System.

Speaking of clouds, which is the purpose of this post, the heat from the fire rises rapidly into the atmosphere and cools, leading to condensation of water vapor to form cloud droplets. This heat typically produces even more intense upward motion than just the sun heating the ground on a warm summer day, and the ash/smoke in the air gives the clouds a darker appearance and can serve as nuclei for water droplets and ice crystals to form on. These storms can become so intense that they can produce rain and lightning; therefore technically being called “pyrocumulonimbus” clouds.

Luckily in this case, rainfall from this and other storms moving through helped to stall the growth as fire crews were protecting structures in a small community near the fire. Mike and the fire crews have been working hard to protect lives and property and deserve a huge thanks.

As if this pyrocumulonimbus cloud wasn’t impressive enough from the ground, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite passed over this location around that same time (20:20 UTC or 14:20 MDT), offering an incredible perspective of this cloud from above. Wow!

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Aqua/MODIS, 5 Aug 2014, 20:20 UTC

Views of storms from near and afar

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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.

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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.

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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:

12340anvil

 

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.

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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.

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

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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.