Hang gliding and paragliding techniques: Thermal Flying part 2: Thermal generators and triggers

Monday 22 October, 2007

Thermal flying part 2 - thermal generators and triggers

A series of choice cuts from Burkhard Martens’ new technique book on everything you need to know about cross country paragliding and hang gliding flying

I OFTEN SEE pilots searching around for thermals in what I consider unlikely places, and pilots also often ask me why I have flown to a particular spot to search.

Whenever the latter happens I have to stop myself from simply saying something like, “It looked good.” In fact, what makes a specific point in nature ‘look better’ than all the potential locations around it is a complex matter, and coming to a conclusion happens only via numerous deliberations.

In the beginner these are all conscious, but with experience become increasingly subconscious until we cannot even say what caused us to fly there anymore!

Whilst learning we need to consider the following things:

Thermal flying part 2, fig 1.

A very clearly marked thermal trigger point, the saddle above Schnalstal in the Italian Alps. Generally the ridgeline will trigger the thermal, but with a bowl like this facing right into the sun the areas right or left, depending on the wind direction, are also very good. In the middle of the bowl we can expect increased sink values.

We have learnt that thermals happen because the sun heats the ground, and the ground heats the overlying air. Now let us take that one step further.

Albedo value, a measure for the ’heat-ability’ of the soil
The albedo value indicates how much of the sun’s rays are reflected by a given material. The higher the albedo value, the worse for thermal development because all the energy is reflected and not enough lingers to heat up the soil.

Dry grain fields: Extremely low
Asphalt: Extremely Low
Black soil: Very Low
Damp sand: Very Low
Coniferous forest: Very Low
Vegetation-free soil: Low
Grass: Low
Deciduous forest: Low
Desert / water: Medium
Dry sand: High
Snow: Very high

But the albedo value alone isn’t the whole story. If the soil is soaked with water, energy must first be spent on evaporation before the heating can get under way. This process uses up a lot of energy, which then isn’t available for thermal generation.

Finally, a porous soil containing lots of air heats more readily than a more compact one.

Factors to consider when evaluating the thermal generating properties of any given surfaces
Damp soil absorbs much energy without releasing it again. For damp moorland to generate thermals we must wait until late in the day, when the surroundings have begun to cool down. The moorland will cool slower and sometimes allow us to linger in light lift over places where we are not accustomed to finding lift.

Deciduous forests have a relatively low albedo value, but contain much humidity. This makes them less thermally interesting than coniferous forests where there is less humidity stored.

Any surface oriented perpendicular to the sun’s rays will heat better than surrounding, non-perpendicular surfaces. In the Northern hemisphere this means east-facing slopes in the morning, south-facing slopes around noon and west-facing slopes in the afternoon.

Because the sun is higher around noon the south-facing slopes can be shallower (on the Equator they can be horizontal) than the east- and west-facing slopes. In the European winter only steep south-facing cliff faces produce usable thermals.

Surfaces with a high specific heat capacity (like rocks) take longer to heat, but once warm they will continue to produce thermals even during short overcast periods. East facing vertical cliffs are the first to produce thermals in the morning, not because of the specific heat capacity but because they have been facing into the sun for the longest time.

Thermal flying part 2, fig 2.

A farmer prepares the hay harvest. The pilot has seen this and promptly flies over. The location is good, not only because of the low albedo value of grass fields, but also because the farmer in his tractor triggers all the accumulated hot air with his driving around.

When a parking lot is full of cars it becomes even better, as more hot air may be trapped among all the parked vehicles. Thermals originating from full parking lots are generally both stronger and wider and thus easier to core.

Thermals may come from any surface that is readily heated by the sun. To build a mental picture try to imagine walking over the ground where you’re flying. Wherever you feel the air getting warmer you can expect thermals to originate, whenever it gets cooler it is less interesting. This means that cool, shady and wet areas will always hinder thermal development.

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