Gary Brock answers my questions. 2000/12/7.
· By Andre Dreyer, 2004/5/1: Convergence from Hemet to Yucaipa.
Jérôme : Hi Gary, I read this message from Len : Monday 12/4/00. Yesterday at Marshall was more like flying Elsinore ! The convergence line was extending back and forth from the 215 to behind Marshall peak. It was more of a horizontal search for lift than actual thermalling. Below launch things were really ratty, but after you got up, it was a generally smooth convergence for about 1 1/2 hours. Gary got up to almost 8K, but I had to flap like crazy just to make 7.
Gary : Hello Jerome, Len and a few of us were lucky enough to go to the wrong place last Sunday, 2000/12/3, and managed to enjoy a great day of flying thanks largely to a bit of convergence.
Jérôme : How do you know where to find a convergence ?
Gary : Convergence can set-up in a variety of ways with the most common being produced by two convergent air masses; as in the case last Sunday, we had weak Santa Ana's and a weak cold front just coming on-shore. These two air masses were traveling in opposite directions, and just happened to meet up over the San Bernardino mountains. This occurrence really isn't that uncommon during this time of the year, and was much more obvious this fall.
If you remember we had some exquisite CB's over the San Gabriel and San Bernardino mountains this fall. These ominous clouds were a direct product of convergence; and although they were quite beautiful, I certainly wouldn't want to be anywhere near them in a paraglider. In fact, I remember giving them a wide berth when coming back into SNA in the corporate aircraft.
An obvious indicator of convergence at Marshall is a conflicting report of winds from the windtalker, and what is happening (wind wise) at the top of Marshall. When I got to the top of Marshall on Sunday, the windtalker was reporting an average wind of 3 mph out of the north; but the winds on the top of Marshall were 10 - 15 out of the west-southwest, with an occasional gust to 30. At this point the light bulb went on. Another indicator was all the birds were cruising in back of Marshall, when on a normal day they would be a little out front or directly over the top. Of course once they catch a "normal" thermal they do drift with it, but on Sunday, they were catching lift way north of where they would normally cruise, and were initially going straight up.
Orographic obstacles, such as mountain ranges are a great place for convergence to set-up. You can witness this almost daily at Elsinore. The mountain serves as an separating point between the conflicting air masses, and allows each air mass to accumulate energy. As the day progresses and the earth's surface heats and releases energy in the form of heat into the atmosphere; each air mass is allowed to collect energy and momentum. Because the mountain range doesn't allow the mixing of the air masses, each air mass has a distinct flavor if you will. In the LA basin these air masses are typically marine and desert. As the valley floor heats up and eventually releases energy, the surrounding air is sucked in to replace the heat / energy laden air. This becomes obvious at Elsinore when the winds switch to onshore.
I have found that the area where the two air masses meet / collide, (convergence zone), is usually linearly shaped on the general line of the obstacle where the two masses were initially separated. This line varies horizontally with variations in the obstacles' altitude. i.e. - where the obstacles' elevation is lower, the air mass has less resistance. So the line tends to move horizontally away from the obstacle, and horizontally towards higher obstacle elevations.
When flying in convergence, you may find that the air has a different texture than a thermally day. I have found that the turbulence associated with sheer, (this occurs at the edge of the two conflicting air masses), is generally more dramatic / dynamic than you would find on a normal thermally day. Also, when flying thermals in convergence, you will find that the thermals drift with the surrounding air mass - I experienced this Sunday. The thermals were drifting slightly to the north for the first 1500' or so, then markedly started drifting to the south as I continued to climb. This is a great indicator of convergence.
Jérôme : Do you maximize your tailwind (ground speed) when you can't find lift, hoping that will lead you to the convergence ?
Gary : As in any environment where you are trying to find lift; it is best to sample as much air as possible in the hope of finding lift. When in lift fly at minimum sink When in non-dynamic air that has no apparent motion either up or down, fly at best glide speed, and when in sink use some speed system to escape the sinking air as soon as possible. Use of the speed system should be based on the rate of descent, i.e. - high sink rate, more speed system - slightly higher sink rate than normal, less speed system. Of course this measure will vary with the average strength of thermals, and the presence of headwinds or tailwinds.
Jérôme : Are there visual clues ?
Gary : The most apparent clues are a marked contrast or difference in the look of the air masses. About two weeks ago I was flying in from the East Coast, and had the opportunity to view a classic convergence from altitude during the descent into Orange County Airport. There was a hazy marine layer over the northwest portion of the LA basin, and a beautiful clear air mass over the southeast end of the LA basin. The convergence zone started about 1/4 mile west of the Elsinore launch, and eventually ended up right over the top of Marshall. You can't imagine how badly I wanted to fly that line. You could see dusties going off right at the edge of the line. It was gorgeous. Some other apparent indicators : noticeable temperature change in air; change in the direction of drift angle and ground speed, and when flying from one air mass to another, you will commonly encounter a noticeable surge of the wing fore and / or aft.
Hope this helps you out a bit. Thanks, Gary Brock.