Thermals
What they are and how they are formed
Popular Science Article | Meteorology and Free Flight | 2026
Introduction
Who has ever looked at a gliding eagle without flapping its wings for long minutes, or watched a paraglider twirl in ever-higher circles in the blue summer sky,
unknowingly witnessed one of the most fascinating and fundamental atmospheric phenomena in the physics of the atmosphere: thermal currents, commonly called
thermals.
Thermals are columns or bubbles of hot air rising from the ground to higher elevations, driven by temperature differences and buoyancy. They are the invisible engine that allows free flight without an engine – paragliding, hang gliding, glider – but they are also responsible for important weather phenomena such as the formation of cumulus clouds, thunderstorms and even tornadoes. Understanding what thermals are, how they arise, how
they develop and where to find them means better understanding how the Earth’s atmosphere works.
In this article we will explore in depth the physical origin of thermals, the mechanisms that regulate their formation, the different types, their internal structure, the daily life cycle and the geographical and seasonal factors that influence them. A journey into the invisible architecture of the air that surrounds us.
1. What are thermals
The term thermal comes from the Greek adjective thermikos, relating to heat. In meteorology, a thermal is a warmer mass of air than the surrounding air that, due to its lower density, tends to rise upwards. This phenomenon is a direct manifestation of the physical principle of buoyancy (or Archimedes’ thrust applied to fluids): a less dense body immersed in a denser medium experiences an upward force.
Air, contrary to what it might seem, is not a static fluid. It is in perpetual motion, influenced by gradients of temperature, pressure and humidity. When a portion of soil heats up faster than surrounding areas-for example, a rocky surface compared to a meadow, or a plowed field compared to a forest-it gives off heat to the air that rests on it. This air, which has become warmer and therefore less dense, is literally “pushed” upwards by the colder, heavier air that creeps underneath.
The result is an updraft that can reach vertical speeds of 1-2 m/s on moderately thermal days, up to 5-8 m/s or even higher on days of great summer convective activity. This ascending column of hot air can extend hundreds of meters in diameter and reach heights of 1000, 2000, or even over 3000 meters in the most favorable regions.
Fundamental physical principle: The buoyancy that generates thermals follows the same law that causes an object to float in water. An air mass 1°C warmer than its surroundings is about 0.3% less dense and experiences an upward push. This seemingly small difference, over large volumes of air, generates enormous ascending forces.
2. The physics behind thermal formation
2.1 Soil heating and heat conduction
The main source of energy that powers thermals is solar radiation. During daylight hours, the sun radiates the Earth’s surface with an amount of energy that varies as a function of the angle of incidence, the albedo (reflectivity) of the ground, and the transparency of the atmosphere. The soil absorbs this radiation and heats up, then releases heat to the air above by conduction and convection. Not all surfaces heat up at the same rate. The thermal behaviour of the soil
depends on its specific heat capacity and thermal conductivity. Dark surfaces such as asphalt, bare rocks, ploughed soils or fields of ripe wheat reach very high temperatures – up to 60-70°C – and give off heat to the air intensely. Conversely, surfaces such as lakes, forests, and wetlands heat up much more slowly and retain heat longer.
This soil heterogeneity is the main cause of the spatial variability of thermals: they do not form everywhere with the same intensity, but tend to arise above areas of greater thermal contrast.
2.2 The adiabatic gradient and atmospheric stability
A fundamental concept for understanding thermals is the adiabatic gradient. When a mass of air rises, expanding due to the decrease in pressure, it cools. In the absence of condensation, this cooling occurs at a rate of approximately 1°C every 100 meters of altitude (dry adiabatic gradient). If the air contains sufficient moisture and condensation occurs, cooling is slower – about 0.6°C per 100 meters – because the latent heat released by condensation partially heats the air mass (saturated adiabatic gradient).
The thermal continues to rise as long as its temperature is higher than that of the surrounding air. When it reaches the same thermal level as the atmosphere, the buoyancy ceases and the thermal dissolves or, if sufficient humidity is present, generates a pile – the classic cauliflower cloud that signals thermal activity to the paraglider pilot. Atmospheric stability describes how easily air reacts to vertical disturbances. An unstable atmosphere – in which the vertical thermal gradient is higher than the adiabatic one – favors the formation of strong, deep thermals. A stable atmosphere, on the other hand, suppresses convection and makes thermals weak or nonexistent. In temperature inversions, layers of warmer air at altitude block mixing and “plug” thermals at low altitudes.
3. How a thermal is formed: the life cycle
3.1 The accumulation phase
A thermal does not come suddenly. Before the warm air begins to rise, there is an accumulation phase during which heat accumulates in the air layers closest to the ground – the atmospheric boundary layer. The sun warms the ground, which in turn heats the layer of air immediately above by conduction. This process happens slowly, millimeter by millimeter, centimeter by centimeter. During this phase, the weak wind can “remix” the heat horizontally, preventing it from accumulating at a specific point. When the wind is absent or very weak, however, the heat is concentrated above the hottest areas, creating ideal conditions for the thermal to detach.
3.2 The detachment and the ascent
When the temperature difference between the air near the ground and the surrounding air exceeds a critical threshold, the hot air mass “detaches” from the ground like a bubble and begins to rise. This moment is called a trigger or trigger. Detachment can be favored by a wind that “sweeps” away the boundary layer or by an obstacle such as a hill that forces the air to rise. During ascent, thermal can take two main forms: una thermal bubble – a discrete, isolated mass of air that rises like a soap bubble – or a thermal column – a continuous flow that keeps itself fed from the hot zone to the ground. In the
reality, most thermals are a hybrid between the two models, with a toroidal vortex structure: the air rises in the center and falls at the edges, forming a kind of invisible doughnut in vertical movement.
3.3 The formation of the pile
When the thermal reaches the level of convective condensation (LCC), the moisture in the air condenses into microdroplets of water forming a cumulus. This is because, as it rises, the air cools to dew temperature. The mound visually represents the top of the thermal and is one of the most valuable tools for the free flight pilot: looking at the mounds, it is possible to understand where the updrafts are located. The base of the mound is relatively flat and is at the same altitude for all mounds present in the same atmospheric area, since the level of condensation depends on the temperature and humidity of the air, parameters that are quite homogeneous on a regional scale. The height of the base of the cumulus clouds is one of the key indicators used by meteorologists and pilots to assess energy
convective of the day.
4. Types of thermals
Not all thermals are created equal. Weather conditions, soil type, time of day, and season produce thermals with very different characteristics.
4.1 Dry and wet thermal
Dry thermals develop in environments with low relative humidity and do not produce clouds. They are typical of arid regions and days with dry air. They can be very intense – thermals from African or Australian desert regions are among the most powerful on the planet – but their summit is not visible without instruments. Pilots identify them using the variometer and associated turbulence. Wet thermals, on the other hand, produce mounds at their peak. They are the easiest to spot visually and are characteristic of summer days in temperate regions. When the air is very humid and thermals are powerful, cumulus clouds can develop vertically until they become cumulonimbus, with the possibility of thunderstorms.
4.2 Orographic and lowland thermal
In the mountains, thermals combine with topography to create particularly strong updrafts. Orographic thermals arise when the sun warms south-facing slopes, which in turn heat the air in contact. This phenomenon is often combined with valley breezes-winds that move up the valleys during the day-further amplifying the rise of air. Lowland thermals, on the other hand, are more uniform but often less predictable. In the absence of
orographic obstacles, the trigger zones are determined solely by the thermal contrast of the surfaces. The edges of cultivated fields, paved roads, and the roofs of industrial buildings are among the most common starting points.
4.3 Coastal thermals
In coastal areas, the contrast between the land – which heats up rapidly – and the sea – which maintains more constant temperatures – generates a very characteristic system of breezes and thermals. The sea breeze blows inland from the coast during the day, while thermals develop preferentially above land. This combination creates particularly favourable flight conditions in the hilly coastal areas of the Mediterranean.
5. The daily thermal cycle
Thermal activity follows a precise daily rhythm, directly related to solar radiation and soil heating.
In the early hours of the morning, after sunset, the soil has given up the accumulated heat and temperatures are low. The atmosphere is stable and thermals are absent or minimal. As the sun rises, the ground slowly begins to warm. In the hours around 9-10, depending on the season and latitude, the first weak and discontinuous thermals appear. Activity peaks between 12 and 3 p.m., when the sun is high and the ground has accumulated maximum heat. Thermals during this period are stronger, more continuous and reach the highest altitudes. It is the most sought-after window of time for free flight pilots. In the late afternoon, as solar radiation decreases, the thermals progressively weaken. At sunset, convective activity almost completely ceases.
time of day thermal activity typical conditions
Alba – 9:00 Absent / minimum Stable atmosphere, fresh air
9:00 – 11:00 Weak, discontinuous first bubbles termiche, cumuli bassi
11:00 – 14:00 moderate or severe Developed piles, regular thermals
14:00 – 16:00 maximum peak Powerful thermics, risk of thunderstorms
16:00 – 18:00 attenuating Weaker and irregular temperatures
Tramonto – notte absent Cooling, heat inversion
Table 1 — Typical daily cycle of thermal activity in summer at mid-latitudes.
6. Factors influencing the intensity and distribution of
thermal
6.1 The season and latitude
Thermal activity is highest in summer, when the sun is high, the days are long, and the soil accumulates large amounts of heat. In temperate zones such as central Italy, the best conditions occur from May to September. In winter, thermals are weak and limited to the central hours of the day in sunnier regions.
.At low latitudes – tropical and subtropical regions – thermals are active for much of the year, as solar radiation is intense and constant. Not surprisingly, large migrations of birds of prey and storks exploit tropical thermals to cross seas and deserts without almost flapping their wings
6.2 wind
Wind has an ambivalent role in the formation of thermals. A light wind (2-15 km/h) can help the thermals detach, breaking the layer of stagnant air near the ground and stimulating the ignition. A strong wind, on the other hand, breaks the thermals, tilts them and makes them turbulent and discontinuous. Paragliding and hang gliding pilots know well that above 25-30 km/h thermals become difficult to manage.
6.3 The humidity and cloud cover
High relative humidity lowers the level of condensation, producing low-level cumulus clouds – often visually pleasing but potentially dangerous if they evolve into cumulonimbus clouds. Low humidity allows thermals to develop in height without producing clouds, creating so-called “blue thermals” – difficult to spot but often of great intensity. Cloud cover is one of the most important limiting factors: if clouds cover the sun, the ground does not warm and thermals weaken or cease. On days with stratified cover, convective activity is practically zero.
7. Thermals in nature and sport
Thermals are not only a subject of study for meteorologists: they are a vital phenomenon for many living beings and fundamental to some human activities.
Many species of migratory birds – eagles, storks, hawks, vultures – skillfully use thermals to travel thousands of kilometers while consuming very little energy. Flying over classic trigger points-sunny slopes, arid plains, cities-they let themselves be carried aloft by updrafts, then glide toward the next thermal. This behavior, called thermal-dependent gliding flight, is one of nature’s masterpieces of energy efficiency.
For paragliding, hang gliding, and glider pilots, thermals are the lifeblood of flight. Knowing how to read the terrain to anticipate where the next thermal will be born, recognize it by the behavior of the wind at altitude, enter it precisely and make the most of it to gain altitude: these are the skills that distinguish an experienced pilot from a beginner. Cross-country races – distance – are based entirely on the ability to navigate from thermal to thermal covering hundreds of kilometers. Thermals are also important in operational meteorology: their intensity and distribution influence the formation of thunderstorms, hailstorms, and even tornadoes. Numerical time prediction models pay great attention to the parameterization of convection, which is largely driven by thermal activity.
8. How to recognize and predict thermals
Predicting where and when thermals will form is an art that combines theoretical knowledge, field experience, and interpretation of meteorological data.
Atmospheric soundings-vertical measurements of temperature, humidity, and wind taken with radiosondes-are the most valuable tool for analyzing the structure of the atmosphere and predicting the intensity of thermals. Radiosound analysis allows us to calculate CAPE (Convective Available Potential Energy), an index of the amount of energy available for convection: high values indicate days with powerful thermals and a risk of thunderstorms. In the field, the most useful visual signals are: the presence of well-formed mounds with a flat base and vertical development, the behavior of wind manga and windsocks, the flight of birds of prey that circulate at altitude, dust and wind whirlwinds on the ground – called “dust devils” – which signal the triggering of intense thermals. Ground observation is also essential: a dark, rocky hill exposed to the sun is almost certainly a good trigger area. Summary: Thermals are the result of the complex interaction between solar radiation, heterogeneous soil, atmospheric humidity, and the vertical thermal structure of the atmosphere. Understanding this phenomenon means not only knowing how to fly better, but also appreciating the extraordinary complexity of the atmosphere that envelops us every day.
Conclusions
Thermals are much more than just a meteorological phenomenon: they are one of the fundamental mechanisms by which the Earth’s atmosphere transports heat from the ground to higher altitudes, regulating the climate and fueling large weather systems. From the formation of cumulus clouds to major storm surges, from bird migration to human free flight, thermals are present, invisible yet powerful, everywhere around us. Studying thermals means opening a window into the physics of the atmosphere: thermodynamics, fluid dynamics, observational and numerical meteorology meet in this seemingly simple but extraordinarily complex phenomenon. Those who learn to “read” thermals – whether they are paragliding pilots, ornithologists or meteorologists – acquire a
deeper and more intimate understanding of the environment around us.
At a time when understanding the climate system is increasingly crucial, studying and monitoring atmospheric convection-of which thermals are the most immediate and visible expression-represents an important contribution to understanding our planet.
