1. Thermals: Why Gliders Stay Up Instead of Falling
The secret to long glider flights is thermals. Sunlight warms the ground, causing the air above it to rise in bubble-like masses [1]. These thermals are the most common rising air used in gliding. They can form over flat land or hills [1]. If a glider enters one of these air columns, it is lifted faster than it sinks. However, the air is transparent, and the column itself cannot be seen. The challenge is finding this invisible target.
2. How Cumulus Clouds Show Where Warm Air Is Rising
If the air is moist enough and a thermal rises high enough, water vapor in the cooled rising air turns into water droplets, forming cumulus clouds [2]. Cumulus clouds are visible evidence that air has risen to that height. Pilots look for these clouds. Not all clouds are good. Growing cumulus clouds have sharp, clear bases. Decaying cumulus clouds have fuzzy bases, and the rising and sinking air below them is small [2]. It is not guaranteed that air rises under every cloud.
Weather forecasts also provide clues. The US National Weather Service issues soaring forecasts for gliders. These include the maximum height of thermals, the height where cumulus clouds form, and the temperature and time when thermals start [7]. This helps pilots know if cloud signs will appear before they fly. What do pilots do when there are no clouds?
3. Finding Thermals on Clear Days by Reading the Ground and Birds
On days without clouds, pilots read the ground. Examples of places with many thermals include towns, freshly plowed fields, and asphalt roads [3]. However, it is often difficult to connect ground features directly to thermals [3]. The state of the ground is only a rough guide. Another clue is birds. Eagles, hawks, and vultures circling upward, or swifts eating insects in the air, are said to be signs of a thermal [4].
Birds also seem to use each other as clues. A study at a French raptor facility used GPS and cameras on vultures. It found that vultures using information from other individuals glided at high speeds between thermals [5]. This result comes from a study with specific conditions and does not prove a general ability for all birds.
4. How the Variometer Tells Pilots They Are Climbing
Clouds and birds are clues, but the decisive factor is whether the glider is actually rising. The variometer (or climb rate indicator) quickly shows this. It is frequently used while gliding [6]. The sound becomes higher when the rise is stronger. When sinking, the sound changes to a low hum [6]. Thermals and mountain waves are hard to see, so they are difficult to find without instruments [6].
Pilots watch clouds and birds with their eyes while listening to the changing sounds of the instrument with their ears. They combine visual and auditory clues to decide if they have entered a rising column.
5. How Pilots Circle in the Center of a Thermal to Gain Height
To use a thermal, pilots follow a sequence: search, enter, move to the center, and exit [2]. The most important part is after entering. They circle tightly within the thermal to stay in the center and gain altitude. Once high enough, they glide toward the next thermal [2][1].
The strength of the rise varies with conditions, but several meters per second is considered normal [3]. For example, at 3 meters per second, the glider would gain 180 meters (about 590 feet) in one minute. If thermals line up due to wind or terrain, cumulus clouds can form a line like a road, called a "cloud street." In this case, pilots can fly straight and long while maintaining lift [3]. Can machines learn this method of finding lift?
6. How an AI Glider Learned to Find Thermals by Feeling Forces on the Plane
In 2018, a research team led by Reddy taught a glider with a 2-meter wingspan to find and ride thermals on its own in the field. They used reinforcement learning, a learning method that looks for a direction where performance improves through trial and error. The results were reported in the scientific journal Nature [8].
The clues used by the AI were not clouds or the ground. They were the acceleration from upward wind estimated by the aircraft, and the force trying to tilt the plane sideways (roll torque). From several days of outdoor flight experience, it learned how to fly to ride the lift [8]. The study suggests that birds may also rely on such clues [8]. However, this experiment alone cannot determine if birds actually use them.
7. Try It Yourself: Observing the Bottoms of Cumulus Clouds
On a day with many clouds, look up and search for white, puffy cumulus clouds. Focus on the very bottom of the cloud. Compare clouds with sharp bases to those with fuzzy bases. Sharp bases are said to indicate growing clouds [2]. Look again after a few minutes; the outline may have changed.
Do not look directly at the sun. Look at the clouds from a shaded area. If birds are circling upward, record their behavior. If you want to learn more, you can open the FAA Glider Flying Handbook (Chapter 10) [2] or the Nature paper [8].