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Wednesday, October 28, 2015

Weather pictures speak a thousand words, Part 1: The air flowing like water

During the private pilot ground school I teach at the local college, we spend two full weeks on weather. Some people's eyes glaze over at the technical discussion of things like heat exchange processes, fronts, air masses, standard pseudoadiabatic lapse rates, and the other things that go into the atmospheric dynamo that creates the weather.

Personally, I love weather, which will probably come as no surprise to those of you who have seen all of the posts I've written on it throughout the last several years. One of the things that I enjoy most about flying is that you're not just talking about some abstract concept: you're actually up in the atmosphere dealing with weather on every single flight. That, after all, is why we spend so much time on it in class!

This time, instead of the typical post analyzing a certain weather phenomenon, I'm just going to show pictures of the processes in action. Lots and lots of pretty pictures. In fact, so many pictures I need to split this one into more than one post!

Air Masses

Weather comes from air moving. It's that simple. Everything else is details. The air starts moving because the equator heats up more than the poles because the sun hits the equator more directly. After that, the warm air tries to flow toward the colder poles. It doesn't make it there due to things like the coriolis effect, but its attempt it what sets the weather process in motion.

I said the air tries to flow from the equator to the poles. That's because one of the other important things to keep in mind is that air behaves like a liquid.

Yes, this is a picture of actual water. However, you'll notice that you'll see the same sort of behavior in many of the pictures to come.

Imagine dropping a pebble into a very gently flowing stream. It's easy to visualize what would happen: you would get ripples that would follow the current.

Now imagine that instead of dropping a pebble downward, you threw it up into the sky. That wouldn't do much, but if you heat a parcel of air (like, for example, by having a big smokestack with a lot of hot air rising out of it), you can do almost the same thing, as you can see in the next two pictures.

Now imagine if you had the same stream, but on the stream bed were some ridges. As the stream flows along, the water at the bottom gets pushed upward when it hits the ridges. This would cause some small waves or bulges on the surface. In Life on the Mississippi, Mark Twain recounts how he learned the art of reading the Mississippi River. One of the things an experienced riverboat pilot could do was to be able to read what's under the surface just by seeing what the surface looked like.

Air does the same thing and has the same telltale signs. I spend much of my flying time going across and along the Blue Ridge Mountains. I have numerous pictures of "gravity waves", which is the technical term for ripples in the air that happen when air hits something like the ridges of the Blue Ridge.

First, here's what the mountains themselves look like:

Now here's what happens when air flows across those ridges:

If the cloud remains connected between waves, the gravity wave "humps" are still there on top:

Here's an excellent cap cloud:

If the conditions are just right, the waves can actually crest! These are Kelvin-Helmholz waves, and can be seen where two fluids meet at different velocities. They can even be seen clearly and beautifully in the atmospheric bands of Jupiter, but on Earth they're easy to spot along muddy riverbanks where the slower water near the bank encounters the faster current in mid-river, or when a breeze blows over the ocean, or (like what's happening in this picture) a faster layer of air rides over a slower one:

Continuing with the stream metaphor: what would happen if the stream hit an obstacle that it was too shallow to go over? It would back up and be dammed, right? Or if you live near water, you've probably seen a breakwall, which is a man-made obstacle placed in front of waves to cut down on erosion.

If the conditions are right (meaning stable air and lots of moisture in that air), the air will do the same thing water would do: it will hit the obstacle and either try to crash over it or get backed up behind it. The next picture is what happens when it flows over it. Recall that as the air rises, it cools. Once it cools to its dewpoint, it dumps its moisture and creates a nice cloud that traces the ridgeline almost perfectly:

The next picture was taken outside the airport entrance at State College, PA, early in the morning. I was standing there for about 20 minutes watching the veeeeerrrrrryyyy slooooowww process of the air hitting the mountain like water hitting a breakwall and slowly splashing up and over it. And by "slowly" I mean about half an hour for one "wave"!

The next two pictures show the same thing happening a couple of months later:

Here is the air getting dammed when it hits the ridge. The sharply defined line is it stopping when it hits the ridge and getting backed up like water in a reservoir. If you look closely you can see the mountains it is hitting:

To cap off part 1, here's some very stable air trapping a cloud in between two ridges, with the skyline of New York City in the distance:

In part 2, there are more pretty pictures. This time, it's about weather hazards: icing and thunderstorms. If you've ever wondered what rain looks like from above, head over there now!

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The author is an airline pilot, flight instructor, and adjunct college professor teaching aviation ground schools. He holds an ATP certificate with a DHC-8 type rating, as well as CFI, CFII, MEI, AGI, and IGI certificates, and is a Master-level participant in the FAA's WINGS program and a former FAASafety Team representative. He is on Facebook as Larry the Flying Guy, has a Larry the Flying Guy YouTube channel, and is on Twitter as @Lairspeed.

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