Thermal
Encyclopedia
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
. The sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
warms the ground, which in turn warms the air directly above it. Dark earth, urban areas and roadways are good sources of thermals.
The warmer air expands, becoming less dense
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
than the surrounding air mass. The mass of lighter air rises, and as it does, it cools due to its expansion at lower high-altitude pressures. It stops rising when it has cooled to the same temperature as the surrounding air. Associated with a thermal is a downward flow surrounding the thermal column. The downward moving exterior is caused by colder air being displaced at the top of the thermal.
The size
Size
The word size may refer to how big something is. In particular:* Measurement, the process or the result of determining the magnitude of a quantity, such as length or mass, relative to a unit of measurement, such as a meter or a kilogram...
and strength
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
of thermals are influenced by the properties of the lower atmosphere (the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
). Generally, when the air is cold, bubbles of warm air are formed by the ground heating the air above it and can rise like a hot air balloon. The air is then said to be unstable. If there is a warm layer of air higher up, an inversion can prevent thermals from rising high and the air is said to be stable.
Thermals are often indicated by the presence of visible cumulus
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...
clouds at the apex of the thermal. When a steady wind is present thermals and their respective cumulus clouds can align in rows oriented with wind direction, sometimes referred to as "cloud streets" by soaring
Lift (soaring)
Gliding flight is heavier-than-air flight without the use of thrust. It is employed by gliding animals and by aircraft such as gliders. The most common human application of gliding flight is in sport and recreation using aircraft designed for this purpose...
and glider
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...
pilots. Cumulus clouds are formed by the rising air in a thermal as it ascends and cools, until the water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
in the air begins to condense into visible droplets. The condensing water releases latent heat
Latent heat
Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a process that occurs without a change in temperature. A typical example is a change of state of matter, meaning a phase transition such as the melting of ice or the boiling of water. The term was...
energy allowing the air to rise higher. Very unstable air can reach the level of free convection
Level of free convection
The level of free convection is the altitude in the atmosphere where the temperature of the environment decreases faster than the moist adiabatic lapse rate of a saturated air parcel at the same level....
(LFC) and thus rise to great heights condensing large quantities of water and so forming showers or even thunderstorms.
Thermals are one of the many sources of lift
Lift (soaring)
Gliding flight is heavier-than-air flight without the use of thrust. It is employed by gliding animals and by aircraft such as gliders. The most common human application of gliding flight is in sport and recreation using aircraft designed for this purpose...
used by soaring birds and gliders to soar
Gliding
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word soaring is also used for the sport.Gliding as a sport began in the 1920s...
.
Thermals on the sun typically form hexagonal prisms (Bénard cells).
External links
- What do thermals look like? - Thermal Structure and Behavior by Wayne M. Angevine
- Time-lapse video of clouds caused by thermals forming and decaying