Under the winter sun

The Winter Sun

The Swiss climate is characterized by a pronounced seasonality; by warm summers, cold and rainy winters and the changeable transition seasons in between. This seasonality is reflected in the duration and strength of thermals: thermals are generally rarer in winter, smaller, weaker and more short-lived and usually only occur in places that are optimal in terms of various criteria. Solar radiation is the primal force of weather events. Worldwide, of course, but we will not look beyond our own borders and concentrate on the northern side of the Alps in Switzerland. The sun always shines with the same intensity, but it does not always rise at the same height: in winter it hits our latitudes at a much sharper angle than in summer, the result of which is that less energy/heat is absorbed. As a rough estimate, the winter solstice on December 21st at noon results in half the radiation output compared to the summer solstice on June 21st, i.e. half as much energy! Now we must also consider the length of the day, i.e. the number of hours that the sun shines: in winter, this is again around half as many as in summer (winter means, in a model, December 21st, summer June 21st, these are the two extremes. With each additional day away from these dates, the values ​​become closer again). Half the power, which only lasts half as long: in winter, therefore, only a quarter of the summer radiation power reaches us over the course of the day (½ x ½ = ¼)! Or, to be more specific, because we receive four times less energy, it is cold and wintery.

It is not surprising that this lack of energy also affects the development of thermals. Solar radiation that hits a surface heats it and thus also the air above this surface until it is a few degrees Celsius warmer (= less dense/heavy) than the surrounding air and rises/flows upwards = thermals. In winter, when the main influencing factor is now halved, it is important to choose the other factor that plays a key role in the development of thermals if we want to extend our flight, namely the surface that is illuminated, including its humidity, vegetation, thermal properties, inclination and exposure. To make matters worse, the sunshine that we absolutely depend on in winter is often accompanied by a stable high pressure system, grey below, blue above. The already cold (and therefore dense and heavy) winter air is also dry (and therefore even heavier) and is prevented from rising by the high pressure. That may be impressive on paper, but we should by no means be discouraged by it - winter thermals do exist! - let's go and find them!

The image table shows the winter and summer solstice in numbers using the example of the city of Lucerne. (Source: www.timeanddate.de)

Exposure

Even in winter, the sun's meridian is exactly in the south; the sun "hits" us from there at its highest and therefore strongest point (around 12:30 Swiss time), before it descends to set in the SW. Therefore, many W/SW slopes, which I know from summer to be reliable sources of thermals, do not receive any sun at all in winter, or only sparsely and very late. So what works best in summer is an absolute requirement in winter: a slope exposure of S-SSW (180-200°). 

Slope gradient

If solar radiation hits a surface at a right angle (90°), this results in maximum energy input. In midwinter, the sun reaches us at an angle of 20° at midday, which is far from the right-angled ideal. To compensate for this, we need steep/sloping slopes and walls (in this way, the radiation output can reach summer levels for a short time (sun's highest point + 2 hours)). We behave in the same way when tanning in winter; for the best complexion, you should position yourself in a deck chair and not lie flat on the ground (every pale face knows how sensitive the nose is; even in the countryside, backs and peaks are more intensively exposed to the sun than basins and hollows). The closer to the winter solstice (December 21st), the steeper the slope (the deck chair) for optimal radiation! Such slopes, which also have to be south-facing, are naturally found in the mountains, the Alps and less or not at all in the foothills of the Alps and in the lowlands. Due to this lack of slope, lowland thermals are the exception in winter. The general rule is: the higher and steeper, the better. Air density (or air pressure as a cause) also plays a key role in this credo: the higher, the less dense the air (fewer particles in the same volume), the faster it heats up - thermals therefore form faster/earlier at higher altitudes. The steeper the slope, the more radiation the individual fir tree receives. In his insightful article on snow thermals, Lucian Haas aptly compares this to the rising tiers of a theater, which give all spectators a clear view of the stage. Now, however, it is important to put this "the higher and steeper, the better" into perspective: 1. The tree line is on the northern side of the Alps at around 1800 m above sea level. 2. A humus thickness that is sufficient for a dense tree population can form up to a gradient of around 45°, everything above that remains bare and at most covered with herbs. 3. How high up can I get to take-off points in winter comfortably and safely, and from there into the air?

A look at the practical side provides clarity: the dark areas of the forest and the tree line as a separation edge. Winter thermals in Davos. (Image: Ueli Neuenschwander)

Soil Moisture

The rule is: the drier, the better. At altitudes where, as described above, the air is less dense and the southern slopes are steep enough for thermal development, there is snow in the model winter. If the snow melts or precipitation falls as rain, the ground is moist. The solar energy that is introduced is then used to evaporate the rain/meltwater and not to warm the ground. Due to the reduced daily radiation balance, evaporation/drying takes four times longer than in summer. It can therefore be assumed that even a south-facing slope that has not thawed out is still too moist for days to be able to warm up thermally. Vertical rock faces hardly get wet and dry quickly. They therefore meet the criterion of low moisture. But do they heat up sufficiently and quickly enough, as do pine needles? The latter do not absorb moisture and any residual moisture drips off or evaporates quickly on the dark, waxy surface. The needles heat up and the thermal radiation (this is long-wave radiation, which, unlike short-wave solar radiation, is not reflected by the snow and therefore leads to an energy input) causes the snow to melt and slide off the branches - fir trees shed their leaves very quickly after snowfall. Mild temperatures and wind accelerate this process even further.

A pine forest in time-lapse (20 min time span between the individual images): February 3, 2022 on the Weissboden above the Schächental at 1720 m above sea level, southern exposure, approx. 35° slope. About 20 cm of fresh snow fell overnight. The time-lapse covers the four hours, from the first ray of sunshine at 8:30 a.m. to midday at 12:30 p.m. The spruce trees were mostly free of snow, but the sun was gone. (Source: www.roundshot.com)

Heat capacity and conductivity

The rule is: the more airy, the better. Rock has a high heat capacity, so it stores heat for a long time and then gradually releases it to the environment. However, it takes a long time for it to warm up (this is known as thermal dead time); in winter, in many places, it probably takes too long, because half the energy and half the length of the day are not enough to heat rock thermally. Therefore, surfaces with poor heat capacity and conductivity are an advantage for us in winter: anything that heats up quickly and cools down quickly again. This is usually organic material with a high air and low water content, such as wood, pine needles, dry earth/fields, etc. Traditionally, fields are only found on a large scale in lowlands and not in the Alps. In addition to the lack of inclination towards the sun described above, in winter they are also usually in cold air below our Mediterranean model inversion at 1000-1500 m above sea level, which further reduces/scatters the radiation.  

Yes, what else?!

In the course of the article we have now increasingly looked from macro to micro, from the whole to the detail. And to the core that holds the details together at the core and answers the question posed at the beginning. Winter thermals are almost ready, just a few ingredients and prerequisites are missing! Conifers have a very low albedo (0.2-0.05); therefore they are able to absorb a lot of the incoming sunlight (80-95%), which is then largely converted into heat and used to develop thermals. If the subsoil of the fir forest is covered in snow, the forest is also irradiated from the ground by the reflected, diffuse radiation and the energy input is increased. In winter, forest surfaces have a temperature that is 1-2° Celsius higher than their surroundings (for comparison: a dry alpine pasture in summer quickly reaches several times this value). In summer it is the other way around, forest surfaces are then up to 2° Celsius cooler than their surroundings; the leaves "sweat" water and this transpiration has the same effect as in mammals: evaporation takes energy/heat from the air (we ignore the energy balance of photosynthesis). This explains why forests are not very thermally interesting in the summer months: they cool down continuously and many other surfaces therefore simply get warmer. The leaf structure of conifers, adapted to cold climates and long winters, allows this transpiration to almost stop in winter, which protects the tree from drying out (frost and snow, the roots are drying out) and dropping its needles. Not so with deciduous trees, they have to shed their leaves to stop the loss of water. So now: because fir trees hold their breath in winter and the incoming solar energy does NOT have to serve evaporation, it has a warming effect. Not much, but when everything else is dormant under the blanket of snow, this minimal temperature advantage of the coniferous forest is enough to create updrafts.

Forest communities on the northern side of the Alps near the tree line consist mainly of spruce trees and are not only impressive in terms of their uniformity in terms of species: the individual trees also reach the same height. Trees that are too small eke out a shadowy existence until they die, while those that stray upwards are punished by the wind. Therefore, the treetops of a forest on a steep south-facing slope form a fairly flat slideway, a cushion on which the air warmed by the trees slowly slides up the slope, thus continuously increasing its heat advantage until it breaks off.

This is where you should look!

Conclusion: In winter we find thermals over (preferably) 45° steep, south-facing spruce forests close to the tree line. In order to be able to enter the thermals at a certain height, we are dependent on winter launch sites at around 2000 m above sea level. It should be assumed that winter thermals suitable for paragliders can develop between around 12:00 and 14:30 Swiss winter time.

Trying is better than studying, so let's move from theory to practice and to a selection of flying areas with winter thermals.

Thermal flying in winter is not reserved for top pilots with high-class paragliders, but for all those who are willing to put up with cold fingers and toes and try it! Jakobshorn Davos (Photo: Ueli Neuenschwander).