How thermals form!

THERMAL FORMATION

Thermals generally form when solar energy heats the ground, which heats the air above it. The bubble of warm air eventually breaks free and rises to a point where it cools to the same temperature as the surrounding air. Several factors determine the size, shape, and speed (lift) of the thermal.

Think of a thermal formation as a pot of water about to boil on a stove. The hotter the flame (sun) the faster the bubbles (thermals) form. Dark surface features, which absorb solar energy, are better at heating the air than are light colored surfaces, which reflect sunlight. Areas such as rock fields, parking lots, plowed fields, hillsides facing the sun, and areas protected from the wind are often good thermal generators.

Thermal size is determined by many factors; the size of the thermal generator, wind direction and velocity, terrain, air pressure, etc. Back to the boiling water analogy... In still water (no wind) the bubbles tend to stay attached to the ground longer and become larger before releasing. Windy conditions (stirring the water) tend to break off the thermals prematurely, forming smaller bubbles rather than large columns. Wind gradients and sheer tend to tear thermals apart, forming smaller bubbles, branching columns and distorted shapes.

When there is wind, thermal bubbles are usually blown across the ground before they hit a trigger point and start rising. Trigger points are anything, which causes the wind to be disturbed. Rises in the terrain, such as hills, ridges, and mountains, make good release points. Thermal bubbles are "sticky"; they tend to cling to the ground until they are disturbed or drift up inclines and release off of protrusions.

Some thermal generators form thermals, which release with great predictability. Knowing the frequency, longevity, and potential altitude gain of thermals from a particular generator will help you decide the best time to launch. Early in the day, thermals tend to be small, weak, and inefficient. If, for example, you launch into a thermal, which takes you up 1,000 feet, then dissipates, you have five minutes to find your next thermal (assuming you have sink rate of 200 ft./min and no sustaining ridge lift). If the thermals are only releasing every ten minutes, you will sink out before the next thermal releases. However, if you wait until the thermals become more frequent and/or provide greater altitude gains, your chances of sustaining thermal flight increase.