Category Archives: Cumulus

Vog in paradise

Vog? What is that? Well, fog is tiny suspended droplets in the air, while vog are suspended particulates from volcanoes. This volcanic air pollution casts a hazy scene near the Hawaiian Islands as sulfur dioxide from the volcanoes mix with oxygen and water vapor in the atmosphere to form tiny sulfate particles. These particles can reflect the sunlight, making the extent of vog detectable by satellites, such as in this example from 2008.


Image from MODIS on 2 December 2008 showing the milky haze around the Hawaiian Islands indicating vog. Image courtesy of NASA’s Earth Observatory:

While the vog in this example was an extreme case, from the ground, these particulates can create a hazy view. This picture was taken by Stephen Green on a plane near the Kona airport on the Big Island of Hawaii, showing an example of the haze.


Stephen Green (Imaginscape Photography), Kona, Hawaii, 1 Apr 2015

Notice how the haze is trapped in a shallow layer near the ground. This happens because of what’s called an inversion, where the temperature increases with height instead of typical decreasing. This “trade inversion” provides a cap to the vertical growth of clouds, which is why the cumulus clouds in this photo remain shallow in this layer. This stable scenario forms when winds are weak, so the vog persists in this shallow layer of stagnant air near the surface. Balloons launched twice a day from locations around the U.S., including Hawaii, carry instruments into the atmosphere that measure the vertical profile of temperature, moisture, pressure, and winds. An example of these measurements from Hilo, on the day this photo was taken (1 April 2015), shows the existence of this temperature inversion, with dry air above it and moist, relatively calm conditions below.


A study by Guanxia et al. [Guangxia Cao, Thomas W. Giambelluca, Duane E. Stevens, and Thomas A. Schroeder, 2007: Inversion Variability in the Hawaiian Trade Wind Regime. J. Climate20, 1145–1160.] used these observations from Hilo and from another location on the island to determine how often this trade wind inversion occurred. They found that the inversion occurs approximately 82% of the time at each station. The following figure from their paper also shows the height and strength (determined by temperature) of the inversion varies based on time of the year.


Annual cycles of the (a) inversion base height and (b) inversion strength at Hilo and Līhu‘e, Hawai‘i, based on data from 1979–2003. (Figure 6 from Guangxia et al. 2007)

Here is another picture from Stephen of an obstructed view of the sky due to vog. In this example, haze from the Pu’u O’o eruption limited the view of lenticular clouds near Mauna Kea on the big island on 8 February 2015.


Stephen Green (Imaginscape Photography), Hawaii, Feb 2015

These inversions aren’t present all of the time as weather systems can move through and eliminate the stable layer, provide moisture, and remove the vog particulates. In these cases, the view on the big island is clearly stunning.


Stephen Green (Imaginscape Photography), Hawaii, March 2015

The upside to this inversion is that the vog and clouds are trapped in the lower part of the atmosphere, leaving a crystal clear view of the sky above. The Mauna Kea observatory is truly a sight to behold and we’ve had the fortune to gaze at the stars from that location on one of these clear nights.

Check out more of Stephen’s pictures on his Facebook page:


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:

Screen Shot 2014-08-28 at 4.48.35 PM

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.






Evaporating rain in Nevada

Just because rain is produced in a cloud, doesn’t mean the precipitation will reach the ground. This is referred to as “virga” and this evaporation cools the air (the same reason you feel chilly when getting out of the shower), leading to strong downward motion because cold air is denser. This can result in periods of gusty winds and if the storm also produces lightning on a hot day, could create a serious fire threat in this dry environment.

This morning, in Elko, NV, a ballon was launched from the local National Weather Service office (as is done both morning and evening every day) that carries instruments to measure temperature, moisture, pressure, and winds in the atmosphere. The diagram below shows what this data looks like, with the red line showing temperature decreasing as you go up in the atmosphere and dewpoint temperature (a measure of the amount of moisture in the air) shown in green. The difference between the temperature and dewpoint is the humidity so the farther apart they are, the drier the atmosphere. This morning, you can see that there were some nearly saturated conditions in the mid-levels (where the green and red are closest) while below, near the surface, the lines are far apart indicating very dry conditions.


Brian Boyd sent us the following picture early this afternoon from near Elko, showing the cumulus clouds that were forming. The temperature at this time, according to Brian, was 95 degrees (Fahrenheit) with a relatively humidity of 9%; consistent with the dry atmosphere that was observed in the balloon data (called a sounding) earlier that morning. The heat from the sun was enough to create lift as the warm surface air rose and cooled to produce clouds at the level where the temperature dropped to the dewpoint.

Brian Boyd
Location: Elko, NV
Date: 2 July 2014

Nearly an hour later, from essentially the same view, Brian sent us another photo that showed that these clouds had increased in coverage, appearing darker and, as Brian told us, produced lightning.

Brian Boyd
Location: Elko, NV
Date: 2 July 2014

If we take a look at the visible satellite image from around this time (2245 UTC), you can see these clouds scattered across much of northeastern Nevada.


Now, if we take a look at the Elko radar data from this same time (but over a smaller area), you can see backscatter power returned from the raindrops in these growing cumulonimbus clouds near Elko (the blues, greens, and yellows).


Despite seeing what looks like precipitation on radar, if you look back at Brian’s second photo, and based on his observations, there wasn’t actually rain hitting the ground. It was evaporating in that very dry air below. That, combined with the lightning he observed, posed a serious fire threat for this region. Add to that the gusty winds due to the evaporative cooling, and you can see why this could be a concern.

Speaking of gusty winds and virga, we saw a video recorded yesterday south of Elko from the National Weather Service office in Reno, NV, a place also characterized by dry, hot low-levels recently and evaporating rainfall. In this video, you’ll also see the clouds building and, towards the end, you’ll see the “microburst” as strong winds hit the surface resulting from strong downward motion due to the evaporative cooling. Because there was no rain at the surface, this is referred to as a dry microburst.

Growing cumulus in Ontario

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Lynnkarl Karst
Location: Innerkip, Ontario
Date: 28 June 2014

On a warm summer day, cumulus clouds form as sun-heated surface air rises. As the air rises, it cools, eventually to its dewpoint temperature where the air saturated, above which condensation occurs to form a cloud. As long as the air stays warmer than its environment, it will continue to grow to be cumulus congestus clouds, as are seen in this picture over Ontario, Canada.

When the clouds block the sun, alternating rays of light and shadows are observed, referred to as crepuscular rays. As the sun continues to set, the sun’s rays have to pass through more of the atmosphere, where the shorter wavelengths of the visible spectrum (the blues) are scattered away, leaving the pinks/oranges/reds to color the clouds late in the day.

Illuminated congestus along the Gulf of Mexico


Jason Taylor
Location: Mississippi Gulf Coast
Date: 26 June 2014

Warm, moist air from the Gulf of Mexico fuels growing cumulus congestus clouds along the Mississippi Gulf Coast and beyond. This picture shows an absolutely beautiful example of this! The sun illuminates the clouds while the clouds block the sun to create upward-directed crepuscular rays.

Cumulus under Cirrus before the heat returned


Sheila Martin-Lynch
Location: Redding, CA
Date: 26 June 2014

Early on this, moist onshore flow found its way into this valley, providing moisture for shallow cumulus clouds to form. Upper-level cirrus clouds above indicated moisture associated with a disturbance moving through the area, responsible for light showers prior to this photo. But, as high pressure nudged its way back into the area, the lack of instability limited the growth of the “fair-weather” cumulus humilis clouds, eventually clearing out the sky for the return of hot, dry conditions for the weekend.