Thermals

XC paragliders seek thermals, while the majority of speedflyers avoid them like the plague. If the majority of speedflyers had a paragliding background, this would not be the case.

In the mountains, thermals indicate the presence of valley wind. Before flying, be sure to familiarise yourself with the topography of the valley that you are flying in, and the expected direction of any valley wind after thermic activity. On or near the coast, you can expect a sea breeze to develop following thermic activity, in much the same way as mountain valley wind.

Flying a speedwing in thermic conditions is dangerous without proper consideration, awareness, active piloting skills, and well-trained positive reflexes to collapses. Variable air greatly increases the chance of collapses, and collapses are more likely to occur without active piloting. This is particularly true while flying with a reduced angle of attack. If you wish to fly safely during times of thermic activity, it is essential to maintain proper caution and awareness at all times. This practice is unsuitable for pilots lacking paragliding experience. Paragliding is the only way to safely practice active piloting and train positive reflexes to collapses.

With that said, flying a speedwing in thermic conditions offers some advantages over calm conditions:

• Thermals result in lift at takeoffs above terrain that has been exposed to the sun, making takeoffs much less demanding.
• Valley wind and sea breeze offers headwind for landing, thus making landings easier with reduced ground speed at stall point.
• Lifty conditions allow pilots to make longer glides, accessing areas that can be inaccessible in calm air.

Bear in mind that turbulence can be expected in lee of all terrain and objects with surface exposed to strong wind.

When to Expect Thermic Activity

The sun is the driving force behind thermic activity. As such, you can expect weaker thermic activity in the morning, evening, and on overcast days. The terrain is heated by the sun throughout the day. Thermals are strongest in the afternoon, while the sun is still high in the sky. Residual heat, especially southwest-facing rocks, will continue to release weaker thermals into the evening.

In addition, thermic activity tends to be weaker on days when the sunny side of a mountain is shaded by wind pushing any cumulus cloud over terrain normally exposed to the sun. For example, in the Northern hemisphere, a North wind will push cumulus cloud above the southern side, therefore reducing thermic activity on the southern aspect.

Additional factors contributing to thermic activity:

• Season: Thermic activity tends to be weakest in the winter, and strongest in mid summer. In the spring, thermals tend to have smaller diameter cores than in the summer.
• Terrain aspect: The more time terrain has spent exposed to sun, the stronger the thermic activity.
• Temperature gradient: The greater the temperature gradient over an altitude range, the stronger the thermic activity.
• Periods of dry weather: Rain water requires energy to evaporate, and therefore rain inhibits the production and strength of thermals. Therefore, periods of dry weather contribute to stronger thermic activity.

There are many web applications that offer thermal forecasts for aviation. Windy Premium includes a thermal forecast, which can be sufficient for limited use. Services such as Burnair and XCTherm offer better European (ICON-EU) thermal forecasts, ideal for pilots also doing XC.

Where to Expect Thermic Activity

You can expect no thermal activity above water. You can expect the strongest thermic activity above and below typical thermal trigger points, including:

• Rocks
• Dry grass
• Buildings
• Asphalt

The topology of terrain is a good indicator for likely thermal triggers. Thermals tend to run uphill, before disconnecting from trigger points. Imagine that you covered a 3D model of the terrain in honey, and then turned it upside down. The points that the honey drips from would likely be thermal triggers.

Additionally, after disconnecting from the trigger, a thermal is pushed in the direction of the prevailing wind.

Indications of Thermic Activity

Cumulus clouds indicate thermic activity. While a cumulus cloud has a flat base, the thermal is active. Once the base of the cloud becomes uneven, the thermal is dead.

If you observe the bottom surface of deciduous leaves from above, this is a sure sign of rising air. The bottom surface tends to be lighter in colour, producing a shimmering effect while the leaves flicker in the air flow.

In mountain valleys, a valley breeze that flows up the valley indicates thermic activity. Conversely in the morning, a down-valley breeze is common, while air is still sinking after thermic activity during the previous day. In German, this katabatic flow is called "bergwind", which translates to "mountain wind".

Indications of Ending Thermal Activity

When active, cumulus clouds have a flat, smooth base. When dying, the base becomes uneven, even more so when dead.

One can observe ending thermal activity at the takeoff, as the cycles gradually become weaker. Before turning to katabatic conditions, there will normally be a period of very calm conditions. Be aware that lack of thermal activity will expose you to prevailing winds. As such, I recommend that you choose favourable aspects for the prevailing wind, as well as for expected thermal activity.