Saturday, November 29, 2008

Field Trip

Today's student only soloed recently. The ceiling was down to 1700 feet, and the winds were gusting to 21, not exactly his kind of weather. "Let's try again tomorrow," I suggested. "What's the weather gonna be?" he asked.

What's the weather going to be?

It's one of the oldest questions. Not one of the oldest questions in aviation, but one of the oldest questions, period.

So we sat down to look at the forecasts. We teach private pilot students a little about forecasts, commercial pilots a little more, instrument pilots a lot more, and ATP students a little more. A lot of pilots I know sit through this, get enough right answers on the knowledge test, and forget it. "Who cares about the weather," I heard one ask, "As long as I have minimums I'm going." The other version is "Who cares about the weather, I'm going anyway." Never mind the minimums.

But nobody can go always. It depends on the pilot and the equipment. I used to fly a King Air to minimums regularly, but once on my day off I did a quick 180 in my no-gyro Taylorcraft when the visibility dropped to 4 miles (still VFR!); there was no way to find the horizon line between the mist and the snow-covered ground.


So I don't believe that the weather is irrelevant. We sat down to look things over. For fun, I looked at the high altitude prog(nostic) chart; all of these are available from aviationweather.gov. The one to the right is a little later, but still shows the same features. In the west, the jet stream is coming down hard from the north, with speeds approaching 150 knots. But follow that green jet stream to the Gulf of Mexico, where it intersects another green line coming from the Pacific.

"What's that?" he asked, pointing to the apparent intersection. "I don't know," I said. Then the idea hit both of us at once: Let's go over to the National Weather Service office and ask. So we drove across the field.

The door was locked, of course, but I knew they were in there, so I rung the doorbell. Someone came out and gave us a suspicious look.

"Hi" I said, "I'm a flight instructor and he's a student pilot and we have some aviation weather questions." His face lit up with a big smile. "C'mon in!"

So we sat down with a couple of forecasters and lots of high definition monitors. I showed them the prog chart. "Convergence," one said. We knew that, but I had never seen such a large angle. Hmm, they said. What we were looking at was convergence between the polar jet and the southern jet.

They pulled up all of their charts, which showed a closed high over the eastern Pacific, that is, streamlines formed complete circles around the center. I have seen closed lows; they are always a bad sign. But a closed high? But there were lows on either side, forcing the air around the corner to turn east.

We looked at other features of the chart. First, the little "home plate" by Los Angeles says "H 510," meaning that the height of the tropopause, which is where the troposphere and stratosphere meet, was about 51,000 feet. At this latitude, 51,000 feet is very high for the tropopause, even in the summer. "Trop at 51?" someone yelled from across the room. "Weird."

To the east, north of Dallas, the little box says "270", meaning that the height of the tropopause was 27,000 feet, more typical for winter. The gradient, 24,000 feet in a little over 1000 nautical miles, is very steep.

Another convergence zone appeared closer to Hawai'i. "All the tourists are belted in tight and they're not serving drinks," someone remarked. A flight to Hawai'i was going to be rough.

The same was occuring over the Gulf of Mexico. "There should be some thunderstorms forming there," a forecaster said, but there were none in the forecast. My student recited the three ingredients for thunderstorms, "Moisture. Instability. Lift.". Moisture from the Gulf; instability from the warm tropical air, and lift from the convergence. (The chart above is from later, and now predicts the thunderstorms.)




NWS no longer does individual weather briefings; we formed the joint conjecture that that was in order to keep the government from competing with Lockheed Martin Flight Service. They seemed to miss doing it; they worked hard to get the certification, and were convinced (as was I) that they would do a much better job than the LockMart briefer who merely reads the METAR and TAF over the phone, framed by lots of CYA verbiage.

On the other hand, these folks aren't pilots and don't really know what we do. To them, a couple of middle-aged men are flying a rag-and-tube taildragger. "Take some O2 if you go that high," they say, even though I know a lot of middle-aged pilots flying pressurized twins and even turboprop singles.

But we're not meteorologists, either. There's a lot to be learned on either side.
We ended up learning a lot.

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