WRAL WeatherCenter Blog

Aglow in the wrong direction?

Posted July 23, 2012

Cumulonimbus cloud looking southeast from Sumter, SC around 8 pm on Friday, July 20th.

We get quite a few questions about weather and related phenomena, sometimes asking for our help in interpreting a photo or two that the sender finds less than logical. This week, Joanne Wadleigh  sent us a couple of shots from her cell phone that she captured Friday evening, July 20th, from the vicinity of Sumter, South Carolina. She noted that she had photographed a thunderstorm off to her southeast at about 8 pm, and was surprised at a bright orange glow that seemed to backlight some clouds southeast of her, while looking toward the west she caught only blues and grays in the general direction of the sunset.  She wondered if we could take a look and explain.

To help in doing so, I collected a set of images to post here. The first two are the images she sent in, and the following three include an 8 pm radar intensity plot from the Columbia, SC NWS Radar and a pair of visible light satellite images of the southeastern U.S. from 6:45 and 7:45 pm.

The first photo is the "glowing" storm, and especially for a shot from a cell phone, is a nice image indeed! We see in the background the western side of a moderately distant thunderstorm, with a layer of pendant, bulbous mammatus clouds extending west from partway up the storm. These are in the upper middle and upper right sections of the image. In the lower portion of the image are several "scud" clouds, or stratus fractus, that are much lower and a good deal closer to her location. They are likely moving northeast and may be associated with moisture feeding into some showers forming up to the north and northeast of her location. Between the top of those clouds and the bottom of the mammatus, we see the brightest orange-gold glow, likely from the west or northwest side wall of the cumulonimbus cloud that comprises the main body of the thunderstorm.

The second photo was taken toward the west, and in that we see scattered to broken low stratocumulus clouds that are just east of some small showers in a cluster to her west and northwest, and above/behind those clouds a sheet of high cirrus clouds and perhaps the sides of some taller shower-producing clouds and finally in the upper left, an area where the clouds are thin enough to see some patches of blue sky.

The third image is a radar reflectivity plot from the Columbia radar, where you can see the cluster of showers around Columbia stretching northeast, along with the label "Shaw" which is a little northwest of where Joanne reported she  shot the photos from. You can see the storm in her photo is located about 10-15 miles or so to her southeast, while some other showers were about 15-20 miles west and northwest.

The clouds associated with those areas of rain are shown in the two satellite images, and you can see that between 6:45 and 7:45 the storm southeast of Sumter grew rapidly and became very tall, while in the area from Columbia northward and west a rather large area of extensive cloudiness remains in place. One thing that is quite notable, though, is that by 7:45 while many of the clouds in the region are becoming dimmer in the image as the sun becomes low in the western sky, the storm southeast of Sumter remains brightly lit on its west side as it towers above many of the surrounding clouds.

By now, this all gives a pretty good sense of why the photos looked as they did. A check of solar altitude and azimuth for the Sumter area at 8 pm on the 20th shows it would have been down to an elevation of about 5 degrees above the horizon by then, in a west northwest compass direction of about 291 degrees (remember this time of year the sun rises and sets notably north of due east and west, respectively). So in the west-northwest sky, the sun has lowered to a point that if you could see it directly it would be near the horizon and its light would have a distinctly gold or orange cast due to passing horizontally through the air on a long path that scatters a lot of the blue light off to the sides. This orange colored light is passing over top of some of the clouds to the west (and perhaps beneath some of the cirrus clouds in that direction) and striking the west-northwest face of the big storm in Joanne's photo. It reflects most strongly off the vertical sides of the storm cloud, hence the brightest glow partway up the side. Meanwhile, the low clouds in the foreground of that image appear a dark blue-gray, since they are in the shadow of the extensive cloud cover to the west. Direct sunlight is passing over the top of those clouds, so they are illuminated mainly by the light that is scattered downward, which leans toward the blue end of the spectrum.

Similarly, the photo taken in the other direction is dominated by blues and grays due to the clouds in that direction blocking the direct rays of the sun. Instead a good deal of the light is scattered in from above, and takes on at least somewhat of a blueish cast rather than the warm hues you might expect looking toward the west. Those would be much more evident if the fairly extensive field of clouds were not in place there.

So, Joanne, in effect the storm cloud was not backlit with an orange glow, but "frontlit," with the side of the storm acting as something like a diffuse mirror that reflected the colors of the setting sun back to you from the opposite side of the sky. Thanks for your question, and for sending the pictures!

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