By James Cooper
Many years ago when reading Wallington’s Weather for Glider Pilots I noted the absolute concept of how thermals develop.
a) The sun heats the ground.
b) The ground heats the air in contact with the ground.
c) The air does not immediately rise but is stored for a period of time in the form of a hot air reservoir for a period of time until some function causes it to break away from the surface.
d) The depth of this layer of hot air depends upon the environment.
So, what governs the depth of the hot air reservoir? If the air is over an area of rock or road, the layer will be very thin. You can see it as a mirage when driving on a hot summer day. Over some grass, the layer is a little thicker, while in crop, the layer can build up to the depth of the crop. Finally, in bush, the layer can be deeper still.
There is another influence that will affect the depth of this layer and that is the wind. If the wind is strong it will tubulate the hot air adjacent to the ground and force it to break away and rise. If the wind is light the hot layer can build up to a greater depth until it has reason to break away. We will see what causes it to break away later.
Let us picture a day where the wind is blowing from right to left. When the wind meets the bush, it will slow down to a light breeze through the bush. This light breeze will assist in filling the bush up with hot air. In addition the sun will still be adding to the reservoir of hot air within the bush. Thus the layer of hot air will build up within the bush over a period of time. Now when hot air comes to the lee side of the bush, it may border some crop or even better, some rock. There will be a large volume of hot air flowing out of the bush that cannot be sustained over the rock - remember point d) above. Therefore this huge volume of hot air that has been stored up in the reservoir of the bush will have to escape. It will tend to do so in a continuous stream from the downwind edge of the bush. Similar situations occur where a rocky environment exists in the middle of the bush.
So now we see that the leeward side of bush will give a good thermal source. It works nearly every time for me. Now, what happens down wind?
We have a large bubble of air rising vertically, but being drifted down wind. The bubble has considerable mass and momentum and will, as it rises, create a low pressure underneath that will suck any residual hot air off the surface of the ground below. Now we can see that the layer of hot air over the rock or crop down wind is not as large in volume as the air that has just exited the bush, but it will now be persuaded to leave the grip of the earth. The thermal will be continually fed by this energy supply as it drifts downwind.
We can look further at what happens as the thermal drifts downwind and is further stoked by the hot air below from the crop that it is passing over. As the volume of air that has been stored up over the crop does not have the volume, strength and size, the thermal will be weaker and narrower.
In addition, as the thermal drifts downwind it will consume any available air to the right and left of track, and as this is a continuous process, a thermal street will be created. In addition to the right and left of the thermal for some distance, there will be little hot air available to create another thermal, since all the hot air will have been consumed, as we can see in Fig 3
To look now at the situation we have seen develop -
* We have a strong thermal reservoir in the bush.
* The thermal escapes on the lee end of the bush.
* As it rises, it draws up further fuel from the paddocks below.
* The thermal downwind of the bush is not so strong.
* The street will be very narrow perhaps, 30m across
* As the air is drawn from the sides there is little or no chance of other thermals developing for some distance either side of the thermal street. So, what do we gain as glider pilots from this knowledge?
* On a day with sufficient wind to generate streets, we will probably find lift if we fly away from the sink cross wind.
* When we find lift, if it is weak, we will need to fly upwind to the thermal source. Don’t turn if it is too narrow - you will fall out the side.
* This part of the street may be narrow, broken and turbulent, so fly with all your senses switched on straight into wind.
* Do not turn until you find the thermal bubble that is stronger and smoother. This will be the bubble that has left the lee of the bush.
*Once you have gained sufficient height, if you leave into wind you will probably run out of lift very soon as you leave the main thermal bubble. So pick up speed before you leave the lift.
Just some further comments, you may find willy willys running down the side of bush. One of our clubs in Western Australia has a runway that is located against an area of bush, which creates problems of willy willys upsetting parked gliders. I have even noticed small ground turbulence running down the border of cut and non cut grass.
The explanation of the layer of hot air explains why large rocks are not good sources. Let’s look at why.
In this case we see that the hot layer adjacent to the rock is actually sealed in and will only escape by drifting around the side of the rock, or being pushed up over the top in the case of a strong breeze.
My First Thermal
As a child I lived in a large area of woodland, Sherwood Forrest. To the back of our house was an area of cleared land measuring about 100sqm. When I was about 10 years old, I had a small figure of a man with a parachute that I would send through the air. Usually, he would drift down to the ground with the parachute generally open. On one day, however, the man's journey was rather different. The hot layer that had stored up in the woodland must have drifted into the grassy area. Although I did not understand the process at the time, I did understand that my little man was not coming down. Instead, he was lifted up and drifted off over the trees and horizon, never to be seen again. I suppose that from that moment, I was hooked.
Another interesting point to remember is that water vapour is actually lighter than air. Hydrogen has an atomic weight of about 1 and oxygen of 16. Therefore, the weight of H2O is 18 while nitrogen, which is the largest component of air, has a weight of 14 and N2 therefore has a weight of 28. It is, however, also important to remember that water requires a lot of heat to raise its temperature. So areas like salt lakes that reflect the heat and absorb much heat will not be a good source in the early part of the day. With their high water content they may be good in the evening, as they will have stored up a lot of heat energy over the day. In addition, the more humid air will be inherently more buoyant.
Although I fly in predominantly flat lands, it is worth noting how hills will help us. If a hill faces into the sun it will absorb more heat than the surrounding flat land. If the wind is blowing up the slope it will assist the now hot air to break away. I can assure you that if the wind is blowing the opposite way to the diagram it could still work. I have experienced this fact, and the logic is as follows. The lea side of the hill is sheltered by the wind, allowing a thick layer, or reservoir, of hot air built up in the shelter of the hill. As this layer builds up and drifts down wind, it will eventually break away as a large volume of air.
As we have looked at how the air is heated up, now we should look at what happens as it breaks away from the ground. Let us initially assume there is no wind.
The thermal rises and in doing so, drags in hot air from all sides. It will continue to rise and increase its cylinder length while being supplied with more hot air. This will depend on its local environment. If there is a large area of available hot air in the vicinity the thermal column will be tall. However, if there is little available air due to a thin hot layer, adjacent cloud shadows, or other thermals in the vicinity, the height will be less. Once the hot air supply has run out, the bubble will be cut off, leaving the bubble to rise. One thing to note is that the top of the thermal has a smooth contour, while at the bottom of the column it is turbulent.
When you are flying, if you find a smooth thermal it may be the top of a new bubble worth hanging on to. Alternatively, if it is rough then you may have arrived too late. We will see later, however, that it is possible to climb through the bubble, that is, climb faster than the bubble itself.
Next issue we look at thermal structure