Thermals are rising masses of air that rise because they are warmer than the rest of the surrounding air mass. This warming is generally the result of points on the ground such as dark plowed earth or asphalt paving that are being heated by sunlight. Thermals generally arise at increasing rates as the day moves from dawn to mid-afternoon, then subside at the ground cools toward sunset. They usually develop in some sort of interval that leads to “waves” of thermal activity that propagate downwind at a predictable periodicity that changes with the time of day and the wind speed. They can be detected with sensitive temperature-measuring devices that show warming air, with devices such as thin streamers on tall poles or soap bubble generators that show rising air, or by observing the flight of birds that use them to soar or the flights of other models already in the air. Thermals have both a horizontal and a vertical structure; a thermal detected at the ground may or may not mean that there is a thermal at that point which extends all the way to the flight altitude of a model that flies over that point at that time.
Flight performance in all duration events is heavily influenced by the degree to which models exploit and ride in thermals. A flier who can regularly detect and then launch reliable models into thermals will almost always do better than a flier who does not have this skill.
The article below is an excellent tutorial on thermals, on how to detect them, and on how to use them in rocketry duration event flying.
Another excellent resource is the DVD “Secrets of Thermal Soaring” available at http://www.radiocarbonart.com
|Detecting Thermals (Gassaway & Mizoi)||May 29, 2014, 6:33 pm||422 KB|