Thermal Maps

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.

A heterogenous collection of homogeneous fields while paragliding Porterville South Africa.  Imagine flying closer to the ground. Can you decide for three spots that are likely to be potential triggers for thermals? Check with the thermal map by clicking the buttons beneath the image. Reference the river bed to find your way in the images!

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.

Standing at a mountain start site early, you will notice every now and then that the wind picks up a little and slows down again. Congratulations, this is a thermal bubble! With more time and sun the breaks between these bubbles usually get smaller until a constant flow of thermal air is in place. Good pilots will monitor this carefully to decide when it is safe for them to start.

This pattern can also be seen at winch tow places or in the flatlands. Now and then the wind picks up. A thermal starting will mean air being sucked towards the thermal. So in case the wind sock starts to indicate a zero wind or even a little wind from behind, the thermal most likely is in front of you.

Thermal Bubbles can be thought of as kick off thermal due to not enough hot air mass being available to continuously feed this initial thermal rising. 

A stationary or continuous thermal starts to exist if the air mass rising will continuously drag further hot air which then is also rising and continuously feeding this kick off thermal. For constant thermals basically the ground or mountain slopes need to heat up enough with respect to the overall air mass to provide enough air mass with a temperature difference big enough. Stationary or  continuous thermal and will be the latest around noon and ending in the evening. Of course noon and evening are relative with regard to time zones, seasons and where to look for thermals.

Clouds And Thermals

A cloud is the result of air masses that have been rising up to the point where the formerly less dense air mass has the same density as the surrounding air. At this point the main vertical movement will come to an end. Given that the air is not as dry as to take in the thermals' moisture  a(nother) cloud begins to form.

Clouds build up and fall apart, the moisture aka water vapor in the air mass will condensate and a cloud begins to form. Condensation means the moisture changes its physical state into a more favorable meaning less energy intense (due to physics) state. The gaseous water vapor will become liquid water. Along with this process further heat (since energy is being freed) and will be released by this condensation. This will work like an afterburner for the cloud boosting cloud growth.

Thermals Are With The Sun

Remember that for an air mass to rise, this mass needs to be warmer than the surrounding air. For air to rise it needs to heat up. To heat up the sun needs to warm the surface. This means, best chances for thermals are in places where the sun hits the ground as vertical as possible at that time of day, simple as that.

Where To Look For Thermals

During the day the sun will roughly “rise in the east” and “down in the west” (not taking into account differences between northern and southern hemisphere and time within the year).

Taking this into account thermals will start on east southeast slopes in the morning, move to southern oriented slopes around noon, switch to more southwest slopes in the afternoon and finally end in the evening on west oriented mountain slopes. Remembering that the ideal angle of the sun should be vertical for optimum energy intake, flying steeper slopes  in the morning, less steep slopes around noon, steeper slopes again in the afternoon and really steep slopes in the evening. Why really steep slopes in the evening as opposed to (regular) steep slopes in the morning - the surrounding air is warmer in the evening

paragliding without thermals while soaring the dunes of Walvis Bay in Namibia

Mountains vs. Flatlands

Thermals in the mountains originate most of the time at the same location for a given time of day and year. This is due to the fact that the warm air crawls up the (mostly sun facing) slope till it reaches a trigger where the thermal starts. The slope will act like a boilerplate further heating up the air mass. The shape of the mountains and the landscapes won’t change all of a sudden so hotspots will be stationary.

Thermals in the flatlands originate at very different locations for any given time of day and year. Thermal spots or hotspots in the flatlands are not as easy to predict as they are often triggered by moving objects or the trigger depends on the direction of the wind. On top a relevant trigger is the shadow of moving clouds which in turn makes a thermal become a cloud first and later act as a trigger and aggregating cloud acting like a vacuum cleaner on the surface.

Other Sources Of Upwind

Ridges and mountain slopes facing the (valley) wind will push the air upwards. Especially the latter especially can be used as a safe harbor if thermal activity is low due to clouds covering the sky. Two winds facing each other will push the air upwards - a convergence.

Thermal Maps 

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! 

Skyways Overlay

Is basically a heatmap of overlaid flight tracks mostly from but not limited to paragliding. In areas that are highly frequented by paragliders this will give you an idea

• where pilots were flying most of the time,

• which routes were taken by pilots that flew cross country (XC)

All in all this visualization contains all the raw information with regard to location (horizontally). Visualizing all this information comes with some drawbacks. Areas close to paragliding takeoffs on mountain, hills or ridges will always point out, as will areas where paragliders winched or toed into the sky making them overrepresented. Thus, skyways can assist with potential routes to take but are less suited for identifying thermal spots during flight.

thermal hotspots in XC Vario