Fool Me Twice...
The atmosphere continues to make me feel like an idiot. I had two students scheduled this morning. The weather room is right by the door, and I printed the METAR and TAF before joining Dan in the hangar for his preflight. (I had called and asked the line crew to bring the 172 inside to pre-heat.) "Weather looks good," he said. "I don't believe it," I replied. I have been flying here for over 15 years, so while the official TAF called for good visibility and unlimited ceilings, my internal TAF called for fog. I had even driven through some on the way to the airport.
So we stood in the hangar and talked weather. I reminded him that the TAF was only valid for a 5 mile radius. Was that fog 5 miles away? "Must have been," he said. Hmm...
He had been away for Thanksgiving so I took him upstairs for a little review of the basics. "What makes an airplane fly?" "What makes an airplane turn?" "What is a stall?" Like I said, the basics. Then we went over the plan for the flight (short- and soft-field takeoffs and landings) and headed down to the plane.
"Looking bad," the lineman said as we passed him on the stairs. We headed straight to the door.
Fog. Dense fog. Visibility below half a mile (1000 meters). Can't do pattern work fog.
So we went back inside and worked on real-world flight planning, using computer tools like FltPlan.com and skyvector.com, comparing our results to those a charter pilot had left behind.
Dennis, the next student called. "I just called the ASOS," he said, "and the visibility is 1/2."
"Don't worry, it'll burn off at 11." Again, this is my personal TAF. And at 11:05 the fog lifted and the visibility report went up to 3 miles. "Let's go!"
The first pattern was kind of rough, which was surprising at the time because I normally think of foggy conditions as smooth. He went around, and I flew a pattern. It was flyable, barely. "I'm not doing any solo practice in this," he said.
But he persisted and flew a couple of more good patterns before we called it quits. "Back so soon?" the airport bums asked. "We were really getting beat up."
So why was it so bumpy? I think the main problem was that the fog was just lifting, and there were little pockets of energy, some from condensation and some from vaporization. This heat energy became mechanical energy, which we felt as turbulence. As evidence, notice that there was still some moist air below is in the pattern. As that vaporized it shot little bursts of energy up at us, and each one became a bump. They weren't organized enough to form thermals, just little unpredictable bumps.
We had high pressure and some of the pockets of moisture finally sank to the surface. One of the airport bums stopped me on my way out. "That wind sock is all over the place! That's why you got beat up."