Introduction
A thermal column (thermal) is a vertical convective current in the lower atmosphere in which relatively warm, buoyant air ascends from the surface, carrying sensible heat upward. Thermals arise from spatial variations in solar heating of the ground: patches that warm more strongly heat the air above them, producing a density deficit that induces upward motion and initiates convective overturning.
Morphologically, a thermal appears as an upward-moving column that originates at the surface and can extend through the boundary layer toward the bases of cumulus clouds under convective conditions. Schematic cross-sections commonly emphasize the juxtaposition of vigorous updraft cores and surrounding subsidence—using contrasting colors or shading to show stronger lift and sinking regions—to illustrate the three-dimensional pattern of ascent and descent. Functionally, thermals provide a key mechanism of vertical heat and mass transport, promoting boundary-layer mixing and contributing to cloud formation; their existence and intensity are governed primarily by buoyancy, maintained while a positive surface–air temperature contrast and permissive atmospheric stratification persist.
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Thermals on Earth
Solar heating of the surface warms the air in immediate contact with the ground; this near‑surface air expands, becomes less dense than its surroundings and initiates buoyant ascent. As a rising parcel encounters lower ambient pressure it expands and cools adiabatically, and its upward motion diminishes once it reaches the same temperature (and therefore density) as the environment. A thermal thus takes the form of a buoyant, rising column accompanied by compensating subsidence of cooler air around it, produced as displaced air sinks alongside the updraft.
The size and vigor of thermals are governed primarily by the vertical stability of the troposphere: steep, unstable lapse rates favor vigorous, deep thermals, whereas a warm layer aloft (an inversion) caps vertical motion and suppresses their development. When ascending air cools to saturation at the thermal top, condensation produces isolated cumulus cloud heads; under steady wind these cells can align into linear “cloud streets” parallel to the flow. Moist processes further modify thermal behavior: condensation releases latent heat, warming the parcel relative to its surroundings and allowing ascent beyond the height expected from dry adiabatic cooling. In very unstable conditions parcels can reach the level of free convection (LFC) and develop into deep convective clouds that yield heavy showers or thunderstorms.
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Deep convective thermals pose significant hazards to aviation because of strong updrafts, downdrafts and attendant turbulence, while weaker thermals provide essential natural lift exploited by soaring birds and unpowered gliders to gain altitude and travel long distances without engines.
Thermals beyond Earth
Thermals on other Solar System bodies are convective upwellings whose observable form is controlled by the medium in which they occur and by the available particulate or volatile phases. Where Earth’s thermals commonly transport water vapor, extraterrestrial thermals take forms set by different atmospheric composition, particulate inventories, and the physical state (gas versus plasma) of the convecting fluid.
On Mars, convective updrafts commonly manifest as dust devils: rotating, columnar vortices that entrain and loft mineral dust rather than water‑rich plumes. This behavior reflects Mars’s thin, cold atmosphere dominated by CO2 and its very low humidity; with little atmospheric water available, suspended solid particles provide the primary visible tracer of convective motion.
In the solar atmosphere, convective activity analogous to thermals is expressed as cellular patterns of hot upwelling and cooler downflow in the plasma. These cells can exhibit a regular, often hexagonal arrangement reminiscent of Bénard convection cells observed in fluid‑dynamical experiments, representing organized convective prisms within the Sun’s convecting layers.
The differences among Earth, Mars, and the Sun illustrate that thermal morphology and visibility depend fundamentally on atmospheric composition, the presence and type of entrainable material (vapors versus solids), and the physical properties of the convecting medium, producing water‑vapor plumes, dust‑laden vortices, or plasma convection cells, respectively.