THE ROTATION OF THERMALS!
By Bruce Abell bruce_abell@bigpond.com.au
When, back in the 1920's, glider pilots flying their gliders on the slopes of the Wasserkuppe found that they could gain height away from the lift generated on the face of the slope realized they were flying in columns of warm air that were rising off patches of ground that had been heated by the Sun's rays. The realization of this then led to the development of glider flying over flat country and the subsequent cross-country goal and return tasks that are now an integral part of today's glider flying.
I became interested in gliders many years ago and obtained a license to carry out minor repairs to wooden aircraft and a Senior Pilots Rating before having to give it away to raise and educate four children. However, I had been badly bitten by the Gliding Bug and, when my younger son took an interest in model aircraft, I took to building and flying Radio Controlled Model Gliders and this fascination has lasted to this day.
Naturally enough, in the process of these activities, I became interested in Thermal activity and made a lot of observations of their form, shape and behavior.
It was once believed that a Thermal was just a column of rising warm air but it is now known that it's form is much more complex than that and is actually more in the form of a rotating dough-nut, so let's look at how this is formed.
Our Thermal usually is generated by the Sun warming a piece of bare ground, metal roofing, bitumen road or similar area and this, in turn, warms the air in contact with this area. As we know, warm air rises, so when this patch of air becomes sufficiently warmer (less dense) than the surrounding air, it commences to rise . Now, Nature abhors a vacuum, so this rising patch of air has to be replaced and,consequently, the surrounding cooler air is drawn in to replace it, thus we now have a bubble of warm air breaking away and commencing to rise.
Figure 1.
However, the air replacing it is being drawn in from all around this bubble and is then drawn up through the center of it, thereby causing the dough-nut shape. This column of air rising through the center starts the dough-nut rim rotating and this, in turn, drags the center column back down the outside of the dough-nut where it is once again drawn up through the center. See Figure 2 for clarification.
Figure 2.
This dough-nut is, however , rising as a unit and now comes under the influence of what is called the Coriolis Force and this will, under dead calm conditions, cause our dough-nut of warm air to steadily spin. This spin will be anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. Our Thermal is now in pretty much it's final form and rising steadily.
So we now have a dough-nut shaped mass of warm air that is spinning slowly in the horizontal plane and, at the same time, rotating from the inside, over the top, to the outside and back up the center. A look at Figure 3 will make this clearer.
Figure 3.
When flying full-size gliders, I found that sometimes the aircraft would fight me and want to get out of the Thermal, while at other times the flight was very smooth and I finally came to the conclusion that, if I was flying WITH the direction of rotation, the Thermal would try to spit me out, whereas flying AGAINST the direction of rotation, i.e. into it, then I would find the flight was smooth and the glider tended to be drawn in towards the center of the Thermal where the rising air (read Lift!) was strongest. Under calm conditions, then, I would usually turn to Port (left) in the southern hemisphere when I thought I'd entered a Thermal but would change direction if the aircraft was fighting to get out of it.
So, if the Thermal in the southern hemisphere spins in a clockwise direction, why would it be necessary to fly anti-clockwise in order to fly against the Thermal's direction of spin?
As I said earlier, all the aforementioned directions applied only under relatively calm conditions but these conditions are fairly rare, so let's now have a close look at what can cause the Thermal to behave in this different manner.
First and foremost, if the wind is blowing at around 5 knots or more, the Thermal Bubble will break free much earlier and the resulting fully formed Thermal will be much smaller and, consequently, harder to work.
Secondly, if the wind if deflected from it's normal direction when close to where a bubble is forming, it will very often impact on the bubble in a way that will cause said bubble to spin in the direction opposite to it's normal tendency as dictated by the Coriolis Force. I think almost all of us have, at sometime or other, noticed that, by deflecting the flow of the water down the plug-hole of the bath, the direction of rotation can be changed and, once changed, will continue to rotate in the new direction. This is exactly what a strong breeze does to Thermal Bubbles!
This deflection of the breeze can be triggered by the likes of a clump of trees/bushes or an orographic feature such as a hill, a mound of dirt or clump of rocks, so always be aware that the Thermal can be spinning in either direction!
I've always been fascinated by the eagles, etc., and my pick of them all are the Pelicans and I've spent a lot of time watching them soar and work Thermals. I've noticed them do two unusual things very often and it took me a long time to work out why they did them. The first one that caught my attention was their ability to find a Thermal. I have observed them many times suddenly veer off their flight path and head in a different direction and then start climbing in a Thermal. It wasn't until I saw a Television program a few years ago that I discovered they could see into the infra-red and thus could actually see the Thermals.
The other unusual occurrence was seeing them working a Thermal and suddenly reverse direction! This one really confused me for a long time but I feel sure that I now understand what is happening here.
As our Thermal rises, it expands as more air is drawn into it and this is why, the higher our aircraft is, the easier it becomes to find and work Thermals and, consequently, the easier it becomes to center in the stronger core where the air is rising at a greater rate than the Thermal as a whole.
What I think happens is that we are in a Thermal that, as it expands, contacts another larger and stronger one and is drawn into this stronger Thermal and, consequently, is forced to change it's direction of spin to that dictated by the stronger one THAT HAPPENS TO BE SPINNING IN THE OPPOSITE DIRECTION!!
This, to me, is the only Hypothesis that satisfactorily explains why those magnificent soaring birds change direction when working a Thermal. Perhaps I'm wrong but, if so, I'd be delighted to be enlightened as to the real (?) reason.
The presence of those fluffy white clouds is usually a sign that there are Thermals, as the usual cause of the formation of these Cumulus clouds is the condensation of the moisture in the air when the warm air of the Thermal meets the colder upper air, usually at the Inversion Layer.
However, there are often Thermals present under clear blue skies and this situation occurs when the air is dry and these Thermals are often called “Blue Sky Thermals”.
When Thermals start forming, it is usually in the morning as the Sun starts to warm up the heat absorbing patches of bare ground,etc., and this activity usually starts to die around mid afternoon. However, it is not uncommon to find Thermals pumping off out of small patches of trees late in the afternoon and I feel that this is because it has taken longer for the air under these trees to warm up to the extent that it is now warmer than the surrounding air. Thus, it is advisable to always be aware of this possibility.
I hope this is not confusing but helps to understand a little bit of what is going on in the air surrounding our glider and maybe can give us that elusive edge.
Green Air!
A.B. Abell,
17 Ferguson St.,
Cessnock,
N.S.W. 2325,
Australia.
