From: HAHLLUND@finabo.abo.fi (Hans Ahllund Inf)
Subject: re:re:scratching
Organization: Abo Akademi University, Finland
Date: Mon, 5 Oct 1992 11:26:00 GMT
X-News-Reader: VMS NEWS 1.23

Ron Gaynier writes....
*I have often heard people talk about climbing to the top of a rising bubble
*of air, I don't think this is possible. Remember that a glider is normally 
*sinking through the air mass even when thermaling. What causes us to climb is
*the fact that the air mass is rising faster than the glider is sinking and
*the net affect is an increase in altitude. If we could climb faster than the
*air mass is rising then we wouldn't need thermals at all. If you stay in
*a rising bubble of air you will eventually come out the bottom as it rises
*faster than you do.

My reply.......... is the following ....
--------------------------------------------
One bad picture describes a bubble better than a thousand good donut-
descriptions.

    + is rising air                   side-view
    - sinking air

                    --+++++++++++++++++++++++++++--
                   ---+++++++++++++++++++++++++++---
                 ----+++++++++++++++++++++++++++++---
               -----++++++++++++++++++++++++++++++----
              -----++++++++++++++++++++++++++++++++----
               -----++++++++++++++++++++++++++++++----
               ----------+++++++++++++++++-----------
                --------------+++++++---------------
                 ---------------++-----------------
                ------------------------------------

This hole package of air is rising in relation to the surrounding airmass
due to the fact that it is warmer than the air that surrounds it. It will
stay warmer until it reaches the inversion. There the upward-movement
will stop. If it doesn't brake through the inversion as a few very hot
bubbles sometimes do...

But, as we all know, air doesn't want to mix with other air of different
temperature, that is why air is such a good insulator in clothes for 
example. So , when the bubble is rising through the surrounding airmass,
there will become friction on the sides of the bubble. That creates an
internal rotating movement inside the bubble. In the core the lift is 
strongest and on the edge of the bubble the sink is strongest.

It there is wind, as there often is. The shape of the bubble will alter
in such a way that it becomes "almond-shaped" when you look at it from
above.
           top-view                     

          ----------------
       --+++++++++---------
      ---+++++++++++++++++-        > wind direction >>>>>>>>>>>>>> 
       --+++++++++---------
        -------------------
This happens especially at low altitudes. It happens because wind (near the
ground) increases in speed with altitude. This is quite hard to picture
at first, but  just think that the stronger wing on the upper side of
the bubble forces the sink to be blown downwind.
This fenomenon will occur in the part of the troposphare that is called
the friction-layer. This us usually up to 500-1000 meters above ground.
After that the thermals will become nice and smooth.
So, this is the primary reason for thermals of being  so tight and hard
to center in when you are near the ground. I will spare you the techniques
of flying in this kind of thermal...


                                    Hans_Bubble_head.