The following texts will give a brief introduction to thermals in the context of paragliding. Understanding the prerequisites for thermals to build up and finally being triggered will enable a pilot to make efficient use of thermal maps, skyways and hotspots in the preparation of a flight as well as during a flight.
What Is A Thermal
Thermals can be briefly summarized as uprising air that is used especially by paragliding pilots to gain height and fly cross country or simply extend the flight time at a local site. Flying from thermal to thermal, professional pilots can reach cross country distances of up to 400 kilometers.
While thermals are triggered by the texture, shape and topography of the underlying terrain, the basic principle is always the same. For air to rise the so called air package close to the ground needs to heat up (energy intake) until reaching a temperature advantage big enough to allow this air package to rise . In short: the hotter the air gets the less dense it becomes with respect to the surrounding air and eventually it will take off and rise.
thermal map and hotspot markers in XC Vario
What Makes A Thermal
The prerequisite for any thermal is a 'hotter air package'. Just how well an air package will heat up depends on a few factors of the underlying terrain or surface. With the sun being the only source of energy intake the most important factors are in theory quite obvious:
deciding between different surfaces the one where the sun strikes the ground closest to perpendicular has more energy intake and should be preferred. Mountains and Flatlands will differ here.
the energy intake will heat up the ground and the air package on top fast, only if it is not wasted for evaporating water. For thermals dry ground is better than moist ground.
for the surface to heat up the sun's energy must not be reflected as it is the case with white snow. How well a surface heats up is characterized by the albedo. High albedo like in white snow means most of the sun's energy is reflected while a dark surface absorbs / takes in most of the energy.
Identifying these obvious requirements of course is not always as straightforward when paragliding. But with practice good thermal areas can be identified quick. It is important to point out that although rules of thumb exist one always needs to look at the bigger than just focusing on a rule of thumb. The overall rule of thumb for thermals is the question of which area will heat up best when compared to the surrounding areas.
What Triggers A Thermal
A 'hotter air package' with respect to the surrounding air is merely a prerequisite for a thermal. Just as everything in life, one event triggers another event. And with potential thermals it is the same. Every 'hotter air package' has a trigger to become usable thermal for paragliding.
Let's compare the potential thermal to a couch potato. A couch potato is usually well nourished (energy intake) and has a lot of potential energy. Yet it takes some trigger, like his wife disturbing the silence, to get him of the couch and do something. It is the same with a heated 'hotter air package' on the ground. Air that is heated up can stay attached to the ground and it feels quite comfy there. You can think of it like this (very simplified): If this hotter air would go up, new air would have to fill its current place and depending on the surrounding area this needs again energy. So, unless something triggers the hotter air to rise it can stay quite long on the ground.
Now, what could be a trigger for a thermal?
Again, in theory it is quite easy! A trigger is potentially everything where the homogeneity of the surface in question, meaning where the potential thermal is suspected, is disrupted. This will anything that separates different surfaces or 'sticks out' in the current surface. Identifying this disruption again is not as straightforward when paragliding but with a little practice one will be able to favour to identify potential triggers.
Sticking to the couch potato example, a tractor turning hay around noon or later would disrupt the hotter air sitting on and between the hay. Other triggers are:
A tractor mowing a field
A field with some trees on one side and the wind pushing the hot air against these trees
A pond within fields, a smaller elevation, the edge between two fields that are not alike
A clearing within the forest
Shadows of clouds in the flatlands
Boundaries between snow and soil in the mountains (albedo)
Types Of Thermals
A simplified and condensed view for a model that can be helpful for many pilots.
Clouds And Thermals
Thermals Are With The Sun
Where To Look For Thermals
paragliding without thermals while soaring the dunes of Walvis Bay in Namibia
Mountains vs. Flatlands
Other Sources Of Upwind
Understanding 'What makes a thermal', 'What triggers a thermal' and 'Types of thermals' is beneficial to judge what thermal maps in paragliding are suitable for and what not. Using the terminology from the kk7 Thermal Maps site, the project distinguishes between three applications which will be briefly described. Also check out the information on the website!
Understanding The Sky - Wind, Flying Conditions, How thermals form and behave
Thermal Flying - Volare in termica, le VOL THERMIQUE, Thermal Flying, Cross Country Flying