7 November 2012

Here are clouds we never see at home.

Nacreous clouds are named for their iridescence.  Like nacre, the mother of pearl substance that lines oyster shells and pearls, they reflect all the colors of the rainbow.

They’re also called polar stratospheric clouds (PSCs) because they form in the lower stratosphere at 49,000 to 82,000 feet in the presence of super low temperatures, -108^oF and colder.  These conditions are only found in the polar regions during winter, mostly in Antarctica.

To give you an idea of how rare these clouds are, consider that they form in an extremely dry part of the earth’s atmosphere way above most human activity. Commercial jets fly at 23,000 to 41,000 feet; these clouds are much higher.  It’s so dry up there it’s a wonder that they form at all.

The prettiest nacreous clouds contain only water droplets that glow profusely when the sun lights them from below.  These clouds are benign but others are not.  PSC clouds that form from nitric acid + water cause chemical reactions that deplete stratospheric ozone and make a hole in the ozone layer that protects Earth from the sun’s ultraviolet light.

As we head toward winter in the Northern Hemisphere, it’s summer in Antarctica so it will be many months before there are nacreous clouds again.  These were photographed in August 2009.

(Nacreous clouds in Antarctica over the NASA Radome, photo by Alan Light on Wikimedia Commons. Click on the image to see the original)

24 October 2012

“Radiation fog” sounds scary but it’s actually the kind of fog we get in Pennsylvania at this time of year.  We often see it in the early morning below our hawk watch sites.  (Can you see the bird in this picture?)

It forms when winds are calm overnight while the land cools.  The land’s thermal radiation lowers the air temperature and condenses moisture into fog that usually evaporates in the morning.

In hilly country it’s called “valley fog” and is more pronounced because the topography traps cold air in the valleys with warm air overlaying it in a temperature inversion.

Like fog, inversions are also common in southwestern Pennsylvania in fall and winter.  They can be deadly when air pollution is involved.  The famous Donora Smog occurred during a five day inversion in 1948, October 27 to 31.  It killed 20 people and 800 animals immediately, sickened 7,000, raised the mortality rate in Donora for at least a decade, and lowered property values (who would want to live there after that!).

Inversions still occur but our air is cleaner. 

Nowadays we take for granted that our laws will protect us from air pollution. Unfortunately the laws could be weakened because companies complain it costs money to avoid killing or sickening us.

Without protection from air pollution, the fog would be scary after all.

(photo from Shutterstock)

Tonight the sky will be clear in Pittsburgh, great for viewing the peak of the Orionid meteor shower — if you can get away from city lights.

Interestingly, the shower is caused by the dust from Halley’s comet that enters our atmosphere as Earth passes the site where Halley’s passed before.

Read more about this celestial event in the Los Angeles Times where they’ve provided links to NASA’s live streaming video.

(photo of Halley’s comet in 1910, in the public domain on Wikimedia Commons.  Click on the image to see the original.)

We’re in for some interesting weather though it probably won’t look as dramatic as the cold front pictured above.

Last night the National Weather Service Pittsburgh forecast discussion said, “Showers becoming likely daytime Thursday with the passage of a mature occluding cold front. NAM model profiles show the cold frontal passage can also be accompanied by wind gusts up to 30 mph.”

I had never heard of an occluding front let alone a mature one (obviously, I haven’t been paying attention), so I had to look it up.

Occluded means blocked or stopped up.  An occluding cold front is one that overtakes a warm front, jamming it in a wedge between the cold air ahead of the warm front and the new cold air mass overtaking it.  The warm air has nowhere to go but up.  Cold air floods in and the warm air rides atop it like a cork on water.

It looks like this — before and after — as the cold front approaches from the left, catches up to the warm front and forces it up.  (Technically this drawing shows a “cold occlusion.”)

The practical result is that we had cold air early this week, warm air today (the warm front), and cold air tomorrow.  The weather map shows the actual occlusion will track north of us.

The forecast also said, “As what often occurs with these maturing systems there can be a dry slot passage Thursday night before the ensuing cold upper low passes eastward through the upper Ohio Valley Friday.”  So it will be dry on Friday.

The cork will rise tomorrow.

(photo from Wikimedia Commons of a cold front moving rapidly along the Rappahannock River. Occlusion diagram from Wikimedia Commons. Click on the images to see the originals.)

p.s. 10/17, 6:23 pm, Thursday’s forecast more includes the possibility of a severe thunderstorm & Friday has a chance of showers. Things change all the time!

From the ground auroras look like sheets of light.  From high above the Earth they look like shining green rivers.

Click on the image above to watch an aurora over the Indian Ocean as seen from the International Space Station.

Way cool, huh?

(image from NASA)

These rare Kelvin-Helmholtz clouds look like waves on the ocean.

In fact they are breaking waves generated by the same fluid dynamics that creates wind-driven waves on water.

Both are caused by Kelvin-Helmholtz instability which occurs at the boundary where two fluids flow by each other at different speeds or densities.  The air above these clouds is moving faster left-to-right than the air below them.  The boundary is very turbulent and becomes more so when the waves break.

Kelvin-Helmholz instability can be described mathematically and its effect plotted over time.  This silent video by VanjaZ shows a yellow fluid on top flowing faster than the black fluid on the bottom.  Talk about turbulence!

We rarely see K-H clouds because the atmosphere has to be just right to make them stand alone. The curling waves disappear in seconds, wiped out by chaos as soon as they break.

(photo by GrahamUK on Wikimedia Commons; video by VanjaZ on YouTube)

The National Climate Data Center has 300,000 images of tropical cyclones (hurricanes) from 30 years of satellite observations.  Unfortunately the method for categorizing them has changed over time and from place to place.

Is a cyclone labeled “Category 3” in 1988 the same intensity as a Category 3 today?  Maybe not.

The database needs to be standardized but reclassifying this many storms is an impossible task for NCDC staff.  How can they solve this problem?  Crowdsource it!   Once you know the color scheme, anyone can easily recognize patterns and pick similar images.

And so CycloneCenter.org was born.

Pictured above is Hurricane Gilbert from 1988.  It has the classic cyclone swirl and an obvious eye in the middle.  The intensity is also shown in color.  Dark blue clouds are the very tallest, then red, orange, yellow, with pink-gray the lowest.  Gilbert is one intense storm!

Now you’re ready to try your own storm.  Here’s what you’ll find at CycloneCenter.org:

1. The very first time you visit:  Watch the demo and click on the “?” Help symbols.  If you want, you can create a login so you get credit for your storms.
2. Occasionally the first step presents you with two images and asks you to click on the more intense storm.
3. For every storm:  A single image is presented on the left.  Pick its pattern:  Eye, Embedded center, Curved band, Shear, Other.  Click the “?” Help buttons to get used to the patterns.
4. Now pick the image that most closely matches your storm.
5. Repeat for #3 and #4 for five more time-lapse images of the same storm.

Don’t worry if your first attempt seems clumsy.  There is no right answer.  Everyone can do it.  All of us can help.

Read more about the project here or go directly to CycloneCenter.org to try your eye on a hurricane.

(image of Hurricane Gilbert, 1988, from the Cyclone Center)