We have previously introduced the concept of instability and lapse rate. It may be a good idea to review that material at this time. We will build on the basic ideas and gain more insight into the workings of our flying environment as we learn the lift history of thermals.
🌱 How a thermal is born
A thermal is a collection of air rising through the general air mass because it is lighter than its surroundings. Thermals come in a wide variety of shapes, sizes and strength. If we want to be most effective at hitching a ride on these free elevators we must begin our study with their birth. The sun heats the earth's surface on an almost daily basis. This heat is passed directly to the air above the surface as we found out previously. If heating is slow the warm air may rise in a light, continuous plume. In a faster heating process a bubble may form that remains on the ground for a period of time before it releases in a sudden rush. As an illustration of how this works, watch a pan of water coming to a boil. At first the bottom of the water is heated you will see convection currents – plumes – begin to rise. Then, as heating gets more intense bubbles begin to form on the bottom, breaking away to rush to the top surface. As the heating continues larger bubbles form in the hotter areas. A couple other things to notice in our model is the presence of downward moving currents in the early heating process, the general spherical shape of the bubbles and the overall mixing of the water. These features also occur in the air. The figure below shows the effects of surface heating:
- In the first case, light heating causes a slow circulation plume. This may occur in the morning, when a layer of cloud partially obscures the sun or in the evening when warm ground cover slowly releases stored heat.
