Which surfaces produce strong thermals, the daily thermal cycle, albedo, and how to spot trigger points in the field.
⛰️ Terrain sources
We have already mentioned that hills and other rising surfaces are good triggers. Here we will expand on that idea. High ground such as mountains or ridge tops are excellent thermal generators for a number of reasons.
First, they are heated more readily by the sun because they are in thinner atmosphere which attenuates the sun's radiation less.
Secondly, they often have slopes that directly face the sun as shown in the figure below. The figure also illustrates how a concave shaped hill heats the air more readily than a convex shape. A concave bowl also shares this property.
The third reason a mountain generates thermals exceptionally well is because the air overlying it is much cooler than that in the valley, while its surface temperature gets just as warm or warmer. Thus a thermal will originate from the mountain earlier than in a valley and be more buoyant or frequent during the warm part of the day.
Finally, a mountain top will be above the nighttime inversion layer that sets in as cool air slides down the mountain. Consequently the mountain will send off thermals earlier establishing it as a primary source for the day.
Terrain features that generate thermals: sun-facing slopes, rock outcrops and cliffs that absorb heat.
Over deserts and terrain where the surface is largely undifferentiated, the high points will be the thermal sources. The figure below illustrates this concept. The highest points are the better sources. One way to envision this is to imagine turning the terrain layout upside down and imagine a layer of water on the surface. Where it would drip off is a thermal source. You can perform this imagination trick while flying to help you locate the best possible thermal sources.
By the above analogy, flat pans and depressions would tend to be areas of sink. However, over a large flat area a depression may serve as a trigger source if it disrupts a normally steady wind flow that often exists in desert areas.
Wet ground produces weaker thermals because much of the solar energy goes into evaporation.
💧 Wet ground sources
We have learned that water is not a warm surface because of evaporation, its large heat capacity and the spread of any heating down from the surface. Thus it tends to be an area of sink and is normally avoided by pilots with a desire for altitude. However, there are some exceptions to this generality.
Water that lies in a thin layer in a warm marsh perhaps or after flooding will become nearly as warm as other more solid surfaces. In this case thermals may readily be found, but they will tend to be light, large and difficult to pinpoint to a definite source if the wet area is large. Ground that is moist after a rain is generally a poor producer of thermals because of the cooling effects of evaporation. However, the water vapor present does help this air to rise once it is lifted. On largely moist areas look for thermals over higher or well-drained areas as these areas are likely to be dryer. In the more humid regions it is the areas that tend to be dryer that generate thermals. This is the case in northern Europe and the eastern half of North America. When droughts occur in these regions the improvement in thermal production and strength is dramatic.
Large bodies of water are natural heat sinks, but even these expanses can produce thermals in some situations. When cold northern air blows across a body of water it gets heated from below and enjoys the production of large and light thermals with a smooth disposition. This state of affairs is often experienced in autumn and winter near lakes and oceans and may be referred to as lake thermals or water thermals.
Snow reflects sunlight in the day and radiates off heat at night. As a result it remains very cold even after a succession of sunny days. However it too can produce thermals when very cold air moves over a snow-covered landscape. Snow thermals like water thermals are large and smooth and not at all uncommon as winter pilots will aver.
Thermal cloud on the lake
🕒 The thermal cycle
We have learned that solar heating of the earth undergoes a daily and seasonal cycle. The diagrams below depict these cycles. The main point we learned is that maximum daily heating and thus thermal production does not occur when the sun is at its zenith, but a bit later due to a lag in the surface temperature compared to the solar radiation. The chart below indicates that maximum surface temperature and thus maximum thermal production should be expected between 2:00 and 3:00 pm (14:00 to 15:00 hours).
Surface Heating Cycles
These cycles times can be greatly altered, however, by terrain and cloud effects. For example, a west facing slope may not receive peak heating until four hours after that of a horizontal surface while an eastward facing slope may receive peak heating in the morning. A north facing slope in the northern hemisphere may only receive ample solar heating in the height of summer. This is expecially true at latitudes closer to the poles.
A layer of fog or clouds can greatly reduce surface heating and of course prevent thermals from forming. Morning clouds that dissipate later will naturally delay the normal thermal production cycle. But when the clouds do disolve the build-up of heat is rapid and thermals form quickly (unless the cloud is a thin layer that disappears slowly). Generally high cirrus clouds reduce thermal strength while spreading cumulus clouds stop thermals altogether if they remain over a wide area. Sometimes such cumulus build-up undergoes cycles as thermals rise, clouds develop, thermals get cut off, clouds dissipate, thermals reappear and the cycle repeats itself.
In general, on an annual basis thermal production goes with the sun. Peak solar heating produces peak thermal generation. Winter brings fewer and weaker thermals. This cycle is modified somewhat in temperate zones where cold fronts in spring and fall introduce unstable air from the poles and thus make these seasons the peaks for thermal soaring.
The daily cycle goes like this: morning heating starts the first stirring of the air. Light circulations develop that give way to the first thermals around 10 or 11 o'clock. Thermals then continue to build until 2 to 3 in the afternoon then they taper off and give way to late evening heat releases around 6 to 8 pm (18:00 to 20:00 hours).
There are often two thermal pauses that occur during the day:
The first happens about 1/2 hour or so after the first thermals appear. The air seems to take a deep breath, thermal activity stops then returns with vigor. This early morning pause seems to be caused by the ground reaching trigger temperature and releasing a major fusillade of thermals that bring down a large volume of cool replacement air that takes time to heat. Once this air starts producing thermals they continue on a more regular basis.
The second thermal pause appears in the evening when regular thermal production wanes. Sometimes there is a period of about 1/2 hour occurring between 4:00 and 6:00 pm (16:00 to 18:00 hours) when not much seems to be happening. After this pause thermals produced by direct solar heating are rare and they are replaced by residual heating which we look at next.
🌆 Late-afternoon thermals
Once the sun's smiling face begins to lose its bright disposition regular thermal production is reduced. Areas that turn to shadow and those that cool rapidly such as sand start producing sinking air. Other areas that were poor thermal generators or sink areas during the day come into the picture.
Forests and rock areas are particularly good places to look for an evening thermal. Fields of deep crops are also releasers of late afternoon heat. Finally, water comes into its own and displays its natural heat capacity by warming the air for hours in the evening. Deep water is assisted in its ability to produce thermals if a wind is blowing to help stir the water and bring heat up from the depths. Shallow water and all other evening sources are best in light or zero wind.
Thermals of the evening variety are never as strong, abundant, high rising or reliable as day time thermals, but weak lift is better than no lift. Besides, occasionally it is a joy to circle in a glassy bubble in a quiet sky. And sometimes we can be surprised by a rowdy little bullet released late in the day by some patch of ground storing it just for us.
In passing we should mention man-made areas such as parking lots and towns as being good places to look for evening lift. Also don't forget the fires and smoke stacks we mentioned previously.
📊 Thermal source summary
Summary table of surface types and their relative thermal-generating strength.
☀️ Albedo – how strongly a surface heats up
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.
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 easily than a more compact one.
📊 Albedo values for different soils
🔍 What to look at when assessing a surface
💧 Damp soil/Moor: Damp soil absorbs much energy without releasing it again. For a moor 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 forest: Has a relatively low albedo value, but contain much humidity. This makes them less thermally interesting than coniferous forests where there is less humidity stored.
⛰️ Sun orientation: Any surface oriented perpendicular to the sun rays will heat better than surrounding, non-perpendicular surfaces. In the Northern hemisphere this means east slopes in the morning, south slopes around noon and west slopes in the afternoon. Because the sun is higher around noon the south slopes can be shallower (on the Equator they can be horizontal) than the east- and west slopes. In the European winter only steep south-facing cliff faces produce usable thermals.
🪨 High specific heat capacity: 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.
🏜️ Desert & dry sand: Desert surfaces and dry sand have high albedo values but are very porous. Further, deserts are often in regions with strong sunlight, and the porosity plus the strength of the sunlight combine to produce strong thermals in desert regions.
🌲 Coniferous forest: Coniferous forest, and clearings therein, are good thermal generators.
🌾 Fields & meadows: Wet green fields are no good, but newly harvested they are OK. If there’s hay drying in a field it is probably good!
🥔 Crops: Grain or potato fields are good. Corn fields only get really good in the autumn.
🚜 Tilled soil: Ploughed fields are better than untreated ones.
🅿️ Asphalt & parking lots: And crowded parking lots or industrial expanses are always excellent thermal generators.
💡 Hint: 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.
Flying over the official car parking. This parking lot is an excellent thermal generator in spite of its location in the middle of a valley. However the thermal will normally be offset by the valley wind and is often located right above the official landing, to the joy of the students training there. The instructors are less enthusiastic about it …
💡 Hint: Thermals may come from any surface that is readily heated by the sun. For your 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.
⏱️ How long a thermal takes to form
The time that passes between the sun hitting a given surface and the surface releasing the first thermal could be called the thermal development delay. This time differs from surface to surface.
Rocks have long delays but store the energy for a long time; coniferous woodlands and fields have shorter delays but stop working as soon as the sun doesn’t hit them anymore.
Shade drawing in from the side triggers thermals, provided the air has been heated sufficiently, but once the thermal has released and risen the show will be over there until the shade is gone again. Depending on the surface, some time will pass before a previously shaded area releases its next thermal after the sun has come out again. Readily heated surfaces will have the next thermal ready in as little as 10 minutes whereas areas consisting of soils with a high specific heat capacity, like rocks, take notably longer.
🚀 Thermal trigger points
Warm air, being lighter than cold, has the inherent tendency to rise. However a release impulse is needed to overcome the inertia. The following is a short and incomplete list of possible impulses or triggers:
🏔️ Terrain or vegetation changes, like forest edges, ridgelines, uneven slopes.
🌡️ Temperature changes caused by snow, shade or water.
🌪️ External factors, like moving objects or even acoustic impulses.
The most obvious trigger point is always the peak, but the shoulder (if at hand) is no less important! In the Alpine spring the snow line overtakes the shoulders’ role.
A farmer prepares the hay harvest. The pilot has seen this and promptly flies there. 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.
Such a distinct clearing in the woods is a very good thermal trigger. If the pilot is high he should fly to the crest above but if that fails searching right above the clearing is a good stra- tegy.
When the main peak has a little shoulder in front, this will often be a more reliable trigger than the main peak. In this illustration the points A to C are the shoulders, and the mountain D has no shoulder. D is the best thermal generator (see arrow), E is the peak above shoulders A to C. If we’re high we fly straight towards E, knowing that should it not work we can aim for A to C. Once high in either place the way to D is secured.
🎯 Trigger points on flat terrain
🪵 Forest clearings.
⚡ Powerlines. In the mountains these are often located on shoulders and their triggering capacities may be partly explained thereby. But powerlines are also known to trigger thermals in flatlands, where there’s no terrain to explain it.
🌊 Large temperature differences, as found on lake- and river shores.
🌲 A forest edge or a tree line.
⛰️ Small hills – although these are difficult to see from an altitude.
🛤️ Railroad tracks and roads.
🚜 Active farming machinery. Harvesting is very often associated with excellent thermal development.
☁️ Cloud shadow moving over the landscape – but beware! If the cloud is big and the drift slow the shade causes all activity to cease for a while!
Once the cloud shadow is gone it takes only a couple of minutes before the next thermal is ready.
"There is a sharp bend in the road. In a terrain feature with such a small bend, thermals will trigger there, and it is possible to catch the thermal core from as low as 50m above the ground.
The shade from the cu moves over the landscape with the wind. As it progresses it triggers thermals in front of it. The red arrow shows the wind direction, and the pilot can fly with a tailwind from the cloud directly to the new thermal trigged by the cloud shadow. If the pilot has been flying on the upwind side of the big cloud (left) and the thermal has died, getting across the big shadow can be a challenge, but there’s a good chance of getting up again if we can only reach the shadow border, where the next thermal is released.
💡 Hint: If the wind is strong such tree lines or forest edges may even be soared until a thermal is released. It is however an “experts only” game, as coring thermals low down in strong drift is very tricky, and the chances of being dumped either in the lee or over unlandable woodlands are high.
Sourse: Book Understanding the Sky (Dennis Pagen) & Book Thermal flying (Bukhard Martens)
