THERMAL DIMENSIONS AND CHARACTERISTICS
Thermals rise because their warm air is less dense than the surrounding air; the same reason a balloon rises. The greater the difference in temperatures between the thermal air and the surrounding air the faster the thermal rises. Generally, air decreases in temperature with altitude, usually a few degrees with every thousand feet.
I have taken off in 100 degrees at 2,000 ft. and found myself freezing cold at 14,000 feet a few minutes later. I have also cored strong thermals only to have them suddenly stop after a few hundred feet of lift. This is usually due to inversion a phenomenon where cooler air is trapped under a layer of warm air. Usually thermals stop when they hit this layer of warm air. Occasionally extremely powerful thermals will punch through the inversion layer and regain momentum in the colder air above. It is the temperature gradient, which determines the strength of the thermal lift.
If you do much flying around cumulus clouds, you will eventually experience "cloud suck." It will happen something like this:
When you catch the thermal low, it will be narrow and booming. You will have to fight to stay in it. When you center the core, you will be going up at 800ft./min. As you settle back and start to relax, the thermal widens and your rate of climb slows. Above, you notice a cumulus cloud. Suddenly, your rate of climb will rapidly increase, getting stronger and stronger as you are sucked upward. Now it's time to make a run for the edge of the thermal so you do not get sucked up into the cloud. That's cloud suck. It happens because of temperature changes, which occur when the moisture in the thermal condenses into a visible cloud, creating a strong, sometimes dangerous temperature gradient. Cumulus clouds are simply the visible tops of thermals. The bottom of the cloud is where the rising moisture reaches air, which is cold enough to cause it to condense and become visible. Vertical winds inside a cumulonimbus cloud can reach over 100 mph, and can take you above 30,000 feet.
OK, lift is good, so what's the catch? Sink is the catch. Rising air causes turbulence at the boundary layer where it passes the surrounding air. Nearly everyone has seen film of an atomic explosion's mushroom cloud. As the warm air rises, it forms a turbulent doughnut shaped ring of sinking air surrounding the thermal. Only a few feet sometimes separate strong lift and sink. It is this turbulence which causes the majority of canopy collapses associated with thermal flying. There are also areas of sink where cooler air rushes down to fill the void left by the rising thermals. The stronger the thermal, the stronger the associated sink.
Some thermals are so mild that the gentle beeping of your vario will be your only clue of having entered one. Others will humble you with their power. The following describes what you might expect to experience in a moderately strong thermal.
Entering a strong thermal head-on will likely produce the following series of events:
- You are flying at trim speed with a slight amount of brake on and a sink rate of about
- 200 ft./min. The canopy is directly above you.
- As you enter the turbulent boundary layer at the edge of the thermal, you will usually encounter brief sink more severe on the down-wind side than on the up-wind side.
- The canopy surges forward, your sink rate increases, and you apply an appropriate amount of brake to control the canopy surge.
- Not too much brake though, because… Next you encounter the lift of the thermal itself. Your body swings forward, putting the canopy behind you. Your vario will start showing lift.
- You let off the breaks an appropriate amount to allow the canopy to come forward then stabilize it above you. Start counting off the seconds as you pass through the thermal. This will tell you where the thermal core is.
- As you exit the thermal, you will go from lift to sink at the boundary layer.
- This is referred to as "going over the falls," and will usually produce a stronger surge than when you entered the thermal.
- Again, use appropriate braking to control the surge and prevent a frontal collapse.
Entering a strong thermal at an angle will likely produce the following series of events:
- You are flying at trim speed with a slight amount of brake on and a sink rate of about 200 ft./min. The canopy is directly above you.
- For this example, the thermal is to your left. As you approach, your left wing enters the turbulent boundary layer before the right wing does.
- The left wing surges forward. Your left wing enters the thermal and your right wing remains in the turbulence. You feel lift from your left risers but there is little lift on the right.
- You may get an asymmetrical collapse on your right wing. Again, as you exit the thermal, you will go over the falls, but it will not be symmetrical.
With experience, you will learn to do "active flying" where the wing will be an extension of your body. With it you will feel the contours of the air. You will know what is happening around you and how to use the turbulence to your advantage.
|
