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!
On Saturday while hiking with KTA in the Quehanna Wild Area we encountered an area of low vegetation and waist-high stumps. The only trees were those growing on top of stumps like the one pictured above. These were not live sprouts from the old stumps. They were all different species.
It was an oddly barren place where tree regeneration was prevented unless the seedlings were nurtured in the core of a stump. Here’s a Wikimedia photo by Ruhrfisch, taken in the same area.
The stumps were white pines, felled a hundred years ago. State Foresters wondered how old the trees were when cut so they studied stumps with intact rings and discovered that they were all the same age – 200 years old. Something had caused the area to regrow from scratch around 1700.
And they were all cut down at once at the turn of the last century. Loggers clear-cut the entire state, each tree felled by two men with a cross-cut saw. When they were done Pennsylvania looked like this (Tioga County, 1914):
It took a long time to recover from this damage. The clear-cuts were ravaged by fires and erosion. During the Great Depression some areas were replanted by the Civilian Conservation Corps. In other places the land is still challenged.
Southwestern Pennsylvania’s waterways are scenic but in many places the water is bad. This photo of the notch where Stony Run meets the Conemaugh River is a case in point. See the orange tinge on the river bottom? That’s bad water from abandoned mine drainage.
How prevalent is bad water in our area?
PittsburghTODAY recently published a map of non-attaining waterways in southwestern Pennsylvania. Using Department of Environmental Protection data, the yellow lines show where water quality is compromised by abandoned mine drainage, agricultural runoff, sewage, and other causes. The good water is blue.
Even in this thumbnail it’s easy to see that most of Allegheny County has bad surface water while most of Greene County is good. The white space in the middle of Allegheny County is the City of Pittsburgh where the streams were buried as the city was built. Click on the image to see the large map at PittsburghTODAY and drill in for a close-up.
The region’s bad water affects both our quality of life and the natural world. Where water’s impaired aquatic life is poor, there are fewer fish, fewer birds, fewer mammals and bad water for us to drink.
So why is a lot of the map yellow? It’s the legacy of coal.
During the heyday of deep mining in the early 1900′s Pennsylvania had weak or non-existent environmental laws and the state did not collect money from industry for clean up of the inevitable abandoned mine drainage. Pennsylvania eventually enacted laws to prevent new damage but there’s no money to turn all of the yellow lines into blue.
One would hope that Pennsylvania learned from this history but in my opinion (not necessarily the opinion of WQED) our state has not. Though damage is predictable from new industrial threats like Marcellus shale, the state still begins with weak laws, suffers new damage, then changes the laws after the damage is done. (Click here for an example.)
The winter flock is building. Hundreds gathered last evening near Bigelow Boulevard at Craig Street. As sunlight faded in the western sky they left to roost … where?
This morning Tony Bledsoe dodged the “rain” from their roost in the trees near Clapp Hall. His guess at the size of the flock? 500. And this is just the beginning. By November they’ll build to a crescendo of crows.
Where do they gather at dusk? Leave a comment with the news … or tweet me the location of Pittsburgh’s crow flock @KStJBirdblog (hashtag #pghcrows)
(photo of hooded crows in Denmark, by Jens Rost via Wikimedia Commons. Click on the photo to see the original.)
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.