Sunday, December 20, 2009

A Thermal Made Visible

For as long as I can remember I have occasionally seen flocks of seagulls gathering over our town - up to about 200 at a time, wheeling in circles in a group a few hundred feet tall. Later on I remember seeing the ones at the top heading out in a line, usually toward the ocean. I thought it looked like a thread being pulled from a spool - the flock eventually turned into a long line of gulls and disappeared. I never really noticed that they entered at the bottom and left from the top - I just figured it was some sort of holding pattern as they arrived from feeding all over town.

That was before I knew about thermals.

After I learned to soar, I realized that the gulls in those flock never flap - they're soaring in some thermal over a parking lot or dark roof or something. They're obviously "tanking up" before heading out on a long flight. The dimensions of the flock - horizontal and vertical - probably define the size and shape of the thermal.

So far, so obvious.

This is one of the few times that a thermal is actually visible. The other time that thermals are visible is when they're filled with dust and we call them a "dust devil". Usually those are tall and thin and often moving laterally, but are pretty much vertical columns of dust.

Yesterday I saw another seagull-filled thermal as I was driving on the freeway. The day before we had had "Santa Ana winds" - a local name for a foehn wind. This day the wind had settled down to a breeze - but was still probably between 5 and 10 knots. As I passed by the flock, I could see that the column of gulls was skewed at a 45-degree angle to the vertical. They were still thermaling, but were being carried downwind at the same rate they were rising.

We glider pilots all know that thermals are skewed by the wind - but we very rarely get to see it. The hundreds of gulls made the distortion perfectly clear. What a great teaching tool that would be. Now if only we can arrange for flock of seagulls during ground school...

Saturday, December 12, 2009

A short flight under lowering skies

Last Saturday, the weather looked there might be a bit of an opportunity to soar. A cold front was moving in but was well to the north. We could see some lenticular clouds about 15-20 miles away that indicated winds coming our way. If the winds ahead of the front were strong enough, ridge soaring might work. If the sky remained clear, and the wind was NOT strong, some thermal soaring might be possible. A special event going on at the airport meant that we couldn't launch until about 1:00, and this time of the year the sun goes behind the hills about 3:00, so the window would be narrow.

In the morning some of us spent some time talking with a pilot who's flown at Elsinore quite a bit, and observing the sky. The wind was coming in from the ocean side, causing some cumulus clouds to build on the west side of the coastal hills. If they came far enough east we might be able to get up underneath them. As the first launch time approached, they were getting closer and some had concave bottoms indicating lift.

I was about third in line to launch. As we got up to 3000' AGL over the hills, about the typical altitude to release, we were nearly up to the base of the clouds, and the clouds were much grayer and more widespread than they had been just a half hour before. I found a little lift as I released, and a little zero-sink for a while. But the clouds were within about 1000' of the top of the ridge, which left little room for hunting for any lift. Before long I had to drop off the ridge onto the side of the hill, and from the on found nothing but moderate sink. I knew that was a possibility, because if the wind was still coming over the hills from the ocean, it'd be spilling down into the valley.

With a few minutes of slight lift and zero-sink, and then some moderate sink, I ended up with just an 18-minute flight. When I got out of the cockpit after landing, the cloud cover was about 65-75% and the temperature had dropped about 10 degrees. If there was a window of usable lift, I had missed it. The only other private pilot who took off that day got skunked too.

That's OK - I was glad to fly for even a short time, as I'm still focusing on learning the area, getting used to the runways and operations, so I can start planning to take my Commercial practical test at this location.